New prospectives on treatment opportunities in RASopathies

Abstract

The RASopathies are a group of clinically defined developmental syndromes caused by germline variants of the RAS/mitogen-activated protein (MAPK) cascade. The prototypic RASopathy is Noonan syndrome, which has phenotypic overlap with related disorders such as cardiofaciocutaneous syndrome, Costello syndrome, Noonan syndrome with multiple lentigines, and others. In this state-of-the-art review, we summarize current knowledge on unmet therapeutic needs in these diseases and novel treatment approaches informed by insights from RAS/MAPK-associated cancer therapies, in particular through inhibition of MEK1/2 and mTOR in patients with severe disease manifestations. We explore the possibilities of integrating a larger arsenal of molecules currently under development into future care plans. Lastly, we describe both medical and ethical challenges and opportunities for future clinical trials in the field.

1 |. INTRODUCTION

The RASopathies are a group of clinically defined developmental syndromes caused by germline mutations of the RAS/mitogen-activated protein (MAPK) cascade. The prototypic RASopathy is Noonan syndrome (NS), which is characterized by short stature, dysmorphic facial features, congenital heart disease, lymphatic disease, learning difficulties/intellectual disability, bleeding diathesis and skeletal malformations. Related disorders include cardiofaciocutaneous syndrome (CFC), Costello syndrome (CS), Legius syndrome, and NS with multiple lentigines (NSML). Typically, RASopathies are caused by activating genetic variants in the RAS/MAPK pathway, whereas NSML alleles exert their effects through an activation of the PI3K/AKT/mTOR pathway (Figure 1). RASopathies manifest with structural heart defects, specifically pulmonic stenosis, in the majority of cases. A subset of patients presents with severe hypertrophic cardiomyopathy (HCM) and lymphatic malformations, which all carry significant morbidity and mortality (Hickey et al., 2011; Wilkinson et al., 2012). Also, patients with RASopathies have a 10.5 fold higher risk than control for both childhood leukemia and solid tumors (Kratz et al., 2015).

FIGURE 1.

Patient-specific treatment concept and small molecule inhibition of RAS-MAPK pathway. Schematic depiction of the main signaling pathways in RASopathies, with gain-of-function alleles signaling culminating in increased ERK activation (right), and kinase-inactivating allele associated with NSML culminating in increased mTOR activation (left). Drugs that inhibit RAS activation include those that target RAS directly (e.g., sotorasib), inhibit RAS membrane localization (e.g., tipifarnib), prevent the activity of SHP2 (e.g., RMC-4550), inhibit the interaction of RAS with its exchange factor SOS1 (e.g., BI 1701963), or block the kinase activity of receptor tyrosine kinases (e.g., dasatinib). Please see the text for more details. Yellow font indicates preclinical tool compounds; white font indicates a drug in clinical development. Several have been FDA approved for the treatment of cancer, including sotorasib, trametinib and copanlisib, among others, and could potentially be used for the treatment of RASopathies. TTM, tetrathiomolybate. Figure created in BioRender Source: Adapted from Hebron, Hernandez, and Yohe (2022)

To date, beta-blockers remain the first line of treatment in patients with RASopathies and significant HCM (Ostman-Smith, 2014; Wolf et al., 2022). Empirical evidence points to the need of high doses (i.e., propranolol greater than 4.5 mg/kg/d) for maximum benefit in this patient population. Additional choices include L-type calcium channel blockers, disopyramide, and diuretics (Calcagni et al., 2018). For HCM in general, these medications can provide symptomatic benefits but do not reduce cardiac hypertrophy or the incidence of sudden cardiac death. For RASopathy-associated HCM, none has been rigorously tested, so efficacy for symptomatic relief is not established. Surgical options include myomectomy and, rarely, alcohol ablation (Calcagni et al., 2018; Wolf et al., 2022). Outcomes in surgical series of patients with NS are characterized by low mortality but significant morbidity, due in particular to the high risk of complete heart block in biventricular reconstruction (S. Chen et al., 2022; Hemmati et al., 2019; Laredo et al., 2018; Poterucha et al., 2015). Orthotopic heart transplantation may become necessary in RASopathy patients, but has a high mortality of 33% (McCallen et al., 2019).

Extracardiac manifestations of RASopathies also cause a significant burden of disease. Lymphovascular disease in particular is often already present prenatally and a predictor for lymphatic disease during infancy (Cox et al., 2022; Roberts, Allanson, Tartaglia, & Gelb, 2013; Sleutjes, Kleimeier, Leenders, Klein, & Draaisma, 2022; Sparks et al., 2020; Swarts et al., 2022). The lifetime prevalence of lymphatic disease is estimated at up to 37%, with age-dependent preferential manifestations: the most prevalent fetal anomalies are increased nuchal translucency and chylothorax (9% and 6%, respectively). In infants, lymphedema of the extremities (19%) and chylothorax (7%) predominate. In children and adults, lymphedema of the extremities is the most prevalent manifestation (13% and 29%, respectively; Swarts et al., 2022).

Taken together, these observations highlight the limitations of the current therapeutic arsenal for NS and related disorders and lay bare the unmet needs in the field. While the approaches described above aim at hemodynamic relief, they are not fundamentally mechanistic and fail to exploit disturbed RAS/MAPK signaling.

2 |. NEW TREATMENT OPPORTUNITIES FOR RASOPATHIES

Over the last decade, significant progress has been made in the treatment of cancers driven by gain-of-function mutations in the RAS cascade. Nineteen percent of cancer patients harbor a mutation in one of the three genes encoding RAS proteins alone, 8% harbor a mutation in BRAF, with additional mutations in other canonical RAS/MAPK components, making this cascade one of the most common targets for oncogenesis in humans (Davies et al., 2002; Prior, Hood, & Hartley, 2020). MEK1/2 occupy a crucial location in the RAS/MAPK signaling pathway and are, therefore, important targets for drug development. Over the last decade, numerous MEK inhibitors have been developed, and several of them have now been approved for oncological indications (Moore, Rosenberg, McCormick, & Malek, 2020; Zhao & Adjei, 2014). Of note, for most oncological indications, MEK inhibitors exhibit only limited efficacy as single agents and are usually combined with other anti-cancer therapeutics (Blumenschein et al., 2015; Cleary et al., 2021; Dummer et al., 2017; Kenney et al., 2021). Among various causes, this is due to the fact that inhibition of MEK1/2 can lead to the disruption of a negative feedback loop to the RAF proteins (Morrison, 2012).

Haploinsufficiency of NF1, as typically seen in pathogenic variants causing neurofibromatosis type 1, leads to a hyperactivation of the RAS-MAPK signaling cascade. In 2020, the MEK inhibitor (MEKi) selumetinib was approved as a single agent for the treatment of pediatric patients 2 years of age and older with neurofibromatosis type 1 who have symptomatic, inoperable plexiform neurofibromas (Casey et al., 2021). The overall response rate was 66%, and 82% of responding patients had a duration of response of more than 12 months. The safety profile was judged acceptable, with serious adverse events occurring in 24% of patients, mostly consisting of known MEKi class effects affecting the gastrointestinal and dermatological systems (Gross et al., 2020).

Specific treatment strategies were developed for MEKis in diverse cancer indications and have allowed the field to draw parallels and distinguish important differences with potential RASopathy-related treatment issues. Several small molecules have now been first used in adults for RAS-related cancers and then in children. The experience from this therapeutic extrapolation in hematological and oncological malignancies is encouraging to date and provides impetus to enlarge the therapeutic arsenal for disorders due to similar perturbations, such as RASopathies. In addition, the mutational spectra in RASopathies and cancer have been well studied over the last decades. Genetic studies show that the germline pathogenic variants found in RASopathies overlap minimally with the somatic mutations observed in cancer. Biochemically, this usually corresponds to a stronger activation of the cascade in cancer as compared to RASopathies, making the latter ones attractive targets for MEKi as monotherapy. Another important difference between germline RASopathies and cancer is a phenomenon called RAS addiction. This is a process in which cancers become dependent on the continued activity of oncogenes to maintain their malignant phenotype. This knowledge can be taken into account for pharmacodynamic considerations when translating therapeutic approaches from oncological applications to germline RASopathies.

3 |. INSIGHT FROM ANIMAL MODELS

Several animal models of RASopathies have been developed. Given the gain-of-function nature of RASopathy alleles, knock-in mice carrying specific disease-causing variants are of particular relevance for this review. Several mouse models carrying such genetic variants have been engineered that recapitulate aspects of RASopathy phenotypes, including short stature, altered craniofacial development and hematologic anomalies. While no mouse model fully mimics the human phenotype of obstructive HCM, a number of these models manifest at least some degree of cardiac hypertrophy (RAF1 L613V, RIT1 M90I, RIT1 A57G, KRAS V14I, HRAS G12S, KRAS T58I; Castel et al., 2019; Hernandez-Porras et al., 2014; Oba et al., 2018; Takahara et al., 2019; Wong et al., 2020; Wu et al., 2011). In two of them, modulation of the RAS/MAPK cascade using the MEK inhibitor PD0325901 ameliorated the cardiac phenotype: in the RAF1 L613V mouse, treatment was started at 1 month of age and pursued for 6 weeks, resulting in prevention or reversal of cardiac hypertrophy (Wu et al., 2011). In the KRAS V14I mouse, responses varied according to age at which treatment was started. Beneficial effects on cardiac hypertrophy and survival were more pronounced with earlier treatment, and persisted after cessation of treatment (Hernandez-Porras et al., 2014). In a mouse model of NSML with the PTPN11 Y279C allele, treatment with the AKT inhibitor ARQ 092 at 12 weeks of age reduced cardiac hypertrophy and normalized heart size after 4 weeks of treatment (J. Wang et al., 2017). Similar results with reversal of cardiac hypertrophy were obtained in this model using rapamycin to inhibit mTOR (Marin et al., 2011). Low-dose dasatinib, a Bcl-Abl and Src kinase family inhibitor, ameliorates HCM in this same model (Yi et al., 2022). In aggregate, the findings summarized above provide the rationale for implementing patient-specific, mechanistic therapies to improve the most critical manifestations of RASopathies.

4 |. MEETING AN UNMET NEED: FIRST EXPERIENCES FROM COMPASSIONATE USE OF MECHANISTIC THERAPIES

The first extrapolation of mouse studies to human pathology occurred with the administration of the mTOR inhibitor (mTORi) everolimus in a young infant with NSML and severe HCM (Figure 1; Hahn et al., 2015). Exploiting the experimental evidence that these missense substitutions activate the PI3K/AKT/mTOR pathway rather than the RAS/MAPK/ERK cascade, a critically ill neonate with a PTPN11 Q510E allele and HCM received everolimus from the age of 24 weeks onwards (Hahn et al., 2015). In this case, significant clinical improvement was seen while awaiting heart transplantation, which occurred at 36 weeks (i.e., after 12 weeks of treatment). However, there was still severe HCM at the time of surgery, and histologic examination of the myocardium showed the typical findings observed in HCM. Whether the everolimus treatment resulted in the reduced heart failure and whether or not the HCM might have diminished with a longer treatment course is unclear.

When confronted with two infants with RIT1 pathogenic variants and severe HCM with heart failure, we (G.A. and B.D.G.) postulated a class effect for MEKis in RASopathies (i.e., an FDA-approved MEKi would be as effective as PD0325901) and that regression of HCM would be possible with such treatment in humans (Figure 1; Andelfinger et al., 2019). For both patients, conventional therapies had been exhausted, and disease progression made an unfavorable outcome highly likely. Therefore, both infants were started at the age of 3 months on the MEKi trametinib, which was FDA approved for melanoma. Due to the lack of data about pharmacokinetics and side-effect profile in infants for this drug, strict surveillance for potential side effects was implemented, including monitoring for retinal damage (which has been reported in adults with cancer treated with trametinib). Second, we consulted oncologists, pediatric oncologist, pharmacologist and pharmacists with expertise in small molecular kinase inhibitors and drug monitoring and related all known information to the parents. Also, appropriate clinical ethics approval and informed consent from the parents was also obtained. This consent also considered the possibility that novel, unknown effects could occur during the treatment. Third, a pre-defined time threshold of 3 months was set by which point clinical improvement had to be present or treatment would be discontinued. Fourth, exploiting the insight from biochemical study of RASopathy mutations, a dosing regimen below oncological dosing was chosen empirically. This key postulate, which takes into account cellular state rather than disease severity, is depicted in Figure 2. In essence, we predict that the higher biochemical signaling strength of cancer-associated RAS mutations will require higher doses of MEK inhibition; conversely, for RASopathy patients, lower doses should suffice. Whether this quantitative concept developed from biochemical studies will also hold true in practice needs to be determined in future clinical trials.

FIGURE 2.

Rationale for low-dose MEK inhibition in RASopathies. Left column: normal signaling strength in a healthy individual as depicted by an intermediate gray level in all cells of the body (top and middle row). Middle column: Homogenously increased ERK signaling strength in a germline RASopathy (top row), uniformly decreased signaling strength after mild MEK inhibition (middle row). For the purpose of illustrating an effect on hypertrophic cardiomyopathy, the heart is highlighted as a target organ. Right column: Very strongly increased ERK signaling strength in a somatic mutation of a tumor, with normal ERK signaling strength in the unaffected cells of the body (top row), inhomogenously decreased ERK signaling after strong MEK inhibition (middle row). Bottom row: Gradient indicating degree of ERK signaling strength. For more detailed discussion of mechanisms and caveats see text

In both children, rapid and sustained improvement of cardiac status was observed in the following sequences: clinical status within the first week, neurohumoral improvement with decrease of amino-terminal-pro-brain natriuretic peptide (NT-pro-BNP) within 4–6 weeks, and echocardiographic improvement at 6–12 weeks (Figures 3 and 4). In both patients, subvalvular and valvular obstruction of the outflow tracts occurred (Figure 3). Of note, in one patient, resolution of ventricular arrhythmia and lymphatic drainage was also noted (Andelfinger et al., 2019). Based upon the observation that most of the attrition in RASopathy with severe HCM occurs during the first 2 years of life and adding a security window, we initially limited duration of treatment for these first two patients to 2½ years. Signs of a relapse were present 1 month after treatment cessation, with increases in NT-pro-BNP and wall thicknesses in both patients, so they were treated for one more year with MEKi. A second attempt to wean at this time was successful, and after more almost 2 years off therapy, both patients continue to do well, with normal cardiac status and without known side effects of MEKi (Figures 3 and 4).

FIGURE 3.

Effect of MEKi on (sub)valvular obstruction. Response to treatment with trametinib in a patient with biventricular outflow tract obstruction due to a RIT1 S35T mutation. Evolution from birth (first row, mild valvular pulmonary stenosis, gradient 22 mmHg), start of treatment at 3 months of age (second row, severe, mainly infundibular obstruction, 82 mmHg), after 9 months of treatment (third row, mild valvular pulmonary stenosis, 30 mmHg), after 2 years of treatment (bottom row, no gradient)

FIGURE 4.

Effect of MEKi on HCM. All pictures are end-diastolic frames in short axis view from the same patient as in Figure 3. (a) Marked concentric hypertropy at 3 months of age, (b and c) improvement after 3 months and 1 year of treatment, respectively, (d) normal ventricular morphology 1 year after treatment cessation

Subsequent to the publication of the mechanistic treatment of two infants with NS-related HCM with trametinib, several other publications described experiences with MEKi for severe manifestations in RASopathies (Dori et al., 2020; Li et al., 2019; Meisner, Bradley, & Russell, 2021; Mussa et al., 2021; Nakano et al., 2022). Excluding our initial study, the published evidence now reports amelioration in three additional cases with cardiovascular issues (Lioncino et al., 2022; Meisner et al., 2021; Mussa et al., 2021). In two of these cases, the leading symptom was intractable multifocal atrial arrhythmia, which improved within 48 hours of introduction of a MEKi without appreciable conduction anomalies (Lioncino et al., 2022; Meisner et al., 2021). In the third case, which described a multimorbid premature newborn, the cardiopulmonary status improved to a point that allowed extubation, however, acute decompensation after a surgical procedure for hydrocephalus resulted in an intractable relapse of HCM despite continuation of the MEKi. In this patient, concomitant pulmonary vascular disease was suspected clinically and confirmed at autopsy. It is, therefore, conceivable that MEK inhibition cannot reverse pulmonary vascular disease to the same degree or in all patients with NS and pulmonary involvement (Mussa et al., 2021). A total of six patients with NS and severe lymphovascular disease have now been reported as well in addition to a report of two patients with ARAF pathogenic variants and central conducting lymphatic anomaly (this latter condition is extremely rare and has not usually been categorized as a RASopathy) (Dori et al., 2020; D. Li et al., 2019; Lioncino et al., 2022; Nakano et al., 2022). In all of these patients, significant improvement or resolution of lymphovascular disease was reported, with initial response starting at around 2 days to 1 month after treatment initiation to sustained response at 12 months after treatment initiation (example in Figure 5).

FIGURE 5.

Effect of MEKi on lymphovascular anomalies. Magnetic resonance lymphangiography (a) before and (b) after treatment with trametinib showing near complete resolution of left-sided lung perfusion (arrow) and disappearance of large left-sided ducts Source: Reproduced with permission from Dori et al. (2020)

We note that these reports now cover both pathways known to be involved in RASopathy, across a wide variety of genotypes, and specific time windows for response (Figure 6). We, therefore, conclude that these results allow extrapolation from experience in oncology and animal models to treatment strategies in RASopathies, despite some differences of phenotypic manifestation of RASopathies in genetically engineered mouse models. These differences are as follows: (a) while the HCM observed in mouse models can be eccentric, mouse models of RASopathies do not show obstruction of the outflow as found in humans and (b) while prenatal edema is frequently noted in RASopathy mouse models, there are no adequate mouse models of postnatal lymphovascular disease.

FIGURE 6.

Temporal pattern of response to MEKi. Blue gradient arrows depict gradual onset and duration of response to MEKi for different organ systems. Adverse effects can occur at any time. Based upon cited literature and unpublished data

In summary, it is noteworthy that the majority of the reports of targeted inhibition of the RASopathy-related signaling events with MEKis or mTORis in severely or critically ill patients with cardiac or lymphovascular involvement is positive. For some patients, this is true even in situations where one can reasonably anticipate that conventional therapies would have failed. Also, no death attributable to medication has been reported, even for very sick children. Overall, toxicity seems to be reasonably well tolerated and manageable in most cases. Efforts to collect larger series of these infants and children are currently ongoing in the field.

5 |. ONE SIZE WILL NOT FIT ALL: WIDENING OPTIONS TO MODULATE RAS/MAPK SIGNALING

As noted above, many novel agents designed for the treatment of cancers driven by alterations in the RAS/MAPK pathway have been developed recently. These agents could represent treatment options for patients with RASopathies with the exception of NSML (Hebron et al., 2022). The novel agents fall into three classes, outlined in more detail below: inhibitors of RAS activation, direct RAS inhibitors, and inhibitors of the MAPK pathway (Moore et al., 2020). In large part, these agents have not yet been tested in animal models of RASopathies but have the potential to be more efficacious or cause fewer side effects than MEK inhibitors in these models (Figure 1).

5.1 |. Inhibitors of RAS activation

Several classes of drugs can be categorized as inhibitors of RAS activation (Table 1). These include prenylation inhibitors, SHP2 inhibitors, and SOS1 inhibitors. C-terminal prenylation is required for RAS membrane localization and activation. Both the HMG CoA-reductase inhibitors (e.g., atorvastatin, lovastatin) and the N-bisphosphonates (e.g., alendronate, pamidronate) inhibit the synthesis of substrates used in the prenylation reaction. Either HMG CoA-reductase inhibitors or N-bisphosphonates, then, might prevent RAS prenylation and, thus, activation in patients with RASopathies (Baranyi, Buday, & Hegedus, 2020). These drugs are well studied and relatively well tolerated when used to treat hypercholesterolemia and osteoporosis in adults and children, and, thus, represent potential long-term treatment options for patients with RASopathies. Importantly, lovastatin reverses learning defects in a mouse model of NS, Ptpn11^D61G/+ (Lee et al., 2014). Based on these results, a phase 3 clinical trial of simvastatin for the treatment of growth and bone abnormalities in children with NS is currently underway (NCT02713945). Neither the HMG CoA-reductase inhibitors nor the N-bisphosphonates have been evaluated in other preclinical RASopathy models.

TABLE 1.

Compounds of interest in RASopathies

Drug	Mechanism of action	Indications	Typical adverse events	Comments	References
Dasatinib	Tyrosine kinase inhibitor (BCR-Abl, Src, c-Kit, ephrin receptors, others)	Philadelphia-chromosome positive CML and ALL (a) Noonan syndrome-associated hypertrophic cardiomyopathy (o)	Fluid retention/edema, hemorrhage, musculoskeletal pain, myelosuppression, pleural effusion	Tested in PTPN11 D61G and PTPN11 Y279C Noonan syndrome mouse models	(Yi et al., 2016; Yi et al., 2022)
BI 1701963	SOS1::panKRAS inhibitors	Solid tumors with KRAS mutation (t)	n/a	Binds to the catalytic domain of SOS1, conferring pan-KRAS inhibition	(Parikh et al., 2022)
Adagrasib	KRAS G12C inhibitor, allele-specific, locks KRAS G12C in inactive state	KRAS G12C-mutated cancers (t)	Diarrhea, nausea, vomiting, fatigue, alanine transaminase increase, blood creatinine increase, aspartate aminotransferase increase, decreased appetite	KRAS G12C not found as germline mutation	(Kessler et al., 2020; Nagasaka et al., 2020; Parikh et al., 2022)
ASO/SSO	Anti-sense oligonucleotides and splice-switching oligonucleotides	In principle, all KRAS mutant alleles (t)	n/a	To date, 13 NDAs granted by FDA for oligonucleotide therapies, none in cancer	(Crooke, Baker, Crooke, & Liang, 2021; Kamerkar et al., 2017; Ross et al., 2017; D. Wang et al., 2022)
BI 2852	KRAS G12D inhibitor	KRAS G12D-mutated cancers (t)	n/a	KRAS G12D not found as germline mutation.	(Kessler et al., 2019)
Sotorasib	KRAS G12C inhibitor, allele-specific, locks KRAS G12C in inactive state	KRAS G12C-mutated cancers (a)	Diarrhea, musculoskeletal pain, nausea, fatigue, hepatotoxicity, cough, decreased lymphocytes, decreased hemoglobin, increased aspartate aminotransferase, increased alanine aminotransferase, decreased calcium, increased alkaline phosphatase	First FDA-approved oral KRAS G12C inhibitor KRAS G12C not found as germline mutation	(Kessler et al., 2020; Nagasaka et al., 2020; Parikh et al., 2022)
Alpelisib	α-Specific PI3K inhibitor	Metastatic breast cancer (a), PIK3CA-related overgrowth spectrum (PROS) who require systemic therapy (a)	Diarrhea, stomatitis, hyperglycemia, cutaneous adverse reactions, pneumonitis	Inhibits p110α approximately 50 times more than other PI3K isoforms	(Fritsch et al., 2014; Ross et al., 2017; Venot et al., 2018)
Copanlisib	α- and δ-specific PI3K inhibitor	Relapsed follicular lymphoma (a), marginal zone lymphoma (o)	Serum enzyme elevations, liver failure	Inhibits all four class I isoforms, tenfold higher for p110α and p110δ	(Krause, Hassenruck, & Hallek, 2018)
Idelalisib	Preferential PI3Kδ inhibitor	Relapsed/refractory chronic lymphocytic leukemia (CLL, in combination with rituximab) (a), relapsed follicular B-cell non-Hodgkin lymphoma (a), relapsed small lymphocytic leukemia (a)	Hepatic toxicities, severe diarrhea, colitis, pneumonitis, infections, and intestinal perforation	Affinity to PI3Kδ>PI3Kβ>PI3Kα in a ratio of 4:2:1	(Lannutti et al., 2011; Yang, Modi, Newcomb, Queva, & Gandhi, 2015)
Samotolisib	Triple PI3K, mTOR, and DNA-dependent protein kinase, catalytic subunit (PRKDC) inhibitor	Relapsed or refractory advanced solid tumors, non-Hodgkin lymphoma, or histiocytic disorders with TSC or PI3K/MTOR mutations (all t)	Leukopenia, neutropenia, thrombocytopenia, nausea, fatigue, diarrhea	Inhibits all PI3K isoforms (α, β, δ, γ)	(Smith et al., 2016)
Capivasertib	Pan-AKT inhibitor	Relapsed or refractory B-cell non-Hodgkin lymphoma (t), triple negative breast cancer, HR+/HER2− breast cancer, metastatic castration resistant prostate cancer, prostate cancer with PTEN loss, endometrial cancer, meningioma, cancers with AKT changes, recurrent ovarian cancer (all t)	Diarrhea, nausea, hyperglycemia, vomiting, rash	Binds to and inhibits all AKT isoforms	(B. R. Davies et al., 2012; Uko, Guner, Matesic, & Bowen, 2020)
Ipatasertib	Pan-activated AKT inhibitor	Breast cancer, solid tumors, solid tumors with AKT mutations, non-small cell lung cancer, triple-negative breast cancer, endometrial cancer, fallopian tube carcinoma, ovarian carcinoma, prostate cancer, glioblastoma, gastric cancer, gastroesophageal junction cancer, cancer of unknown primary site (all t)	Diarrhea, fatigue, vomiting, neutropenia	Exhibits differential activity in cells with and without activated Akt signaling, distinct from phosphoinositide 3-kinase (PI3K) inhibitors	(Lin et al., 2013; Uko et al., 2020)
Miransertib	Pan-AKT inhibitor	PROS (c) and (t)	Dry mouth, mucositis, pharyngitis, sinus tachycardia, headache, pain	Currently no active trials in oncology Tested in PTPN11 Y279C mouse model	(Forde et al., 2021; Kobialka et al., 2022; Leoni et al., 2019; Ours, Sapp, Hodges, de Moya, & Biesecker, 2021; Yu et al., 2015)
MK-2206	Pan-AKT inhibitor	Non-small cell lung cancer, solid tumors, ovarian sarcoma, recurrent Fallopian tube and ovarian carcinoma, recurrent primary peritoneal carcinoma, oral and salivary gland carcinoma, squamous cell carcinoma of the nasopharynx, colorectal carcinoma, endometrial carcinoma, breast cancer, lymphoma, gastric carcinoma, pancreatic and neuroendocrine tumors, renal cell carcinoma, prostate cancer (all t)	Diarrhea, rash, nausea, fatigue, hyperglycemia	Inhibits AKT membrane localization	(Uko et al., 2020)
Everolimus	mTOR inhibitor	Immunosuppression (a), tuberous sclerosis complex (TSC) associated seizures (a) and tumors (a,c), advanced renal cell carcinoma (a), subependymal giant cell astrocytoma associated with TSC (a)	Stomatitis, infections, asthenia, fatigue, cough, diarrhea, upper respiratory tract infection, sinusitis, otitis media, fever	Allosteric mTOR inhibitor Selective for mTORC1 without activation of mTORC2 Compassionate use in one case of NSML with HCM described	(Y. Chen & Zhou, 2020; Hahn et al., 2015; Witzig et al., 2015)
Sirolimus	mTOR inhibitor	Immunosuppression (a), lymphangioleiomyomatosis (a), bone sarcoma (o), tuberous sclerosis complex (TSC) (o), chronic non-infectious uveitis (o), pachyonychia congenital (o), angiofibromas (o), beta-thalassemia (o), pulmonary arterial hypertension (o), sickle cell disease (o), familial adenomatous polyposis (o)	Peripheral edema, hypertriglyceridemia, hypertension, hypercholesterolemia, increased creatinine, stomatitis, abdominal pain, nausea, diarrhea, headache, dizziness, fever, urinary tract infection, anemia, arthralgia, pain, thrombocytopenia, nasopharyngitis, acne, chest pain, upper respiratory tract infection, myalgia	Allosteric mTOR inhibitor Compassionate use in one case of lymphatic malformation with KRAS G12D mutation	(Y. Chen & Zhou, 2020; Sideris, Tng, & Chee, 2021)
Temsirolimus	mTOR inhibitor	Advanced renal cell carcinoma (a), lymphoma, approximately 1,000 clinical studies with a variety of malignancies	Hypersensitivity reactions, oral ulcers, diarrhea, nausea, poor appetite, fatigue, peripheral edema, rash, anemia	Intravenous application only	(Y. Chen & Zhou, 2020)
JAB-3068	SHP2 inhibitor	Non-small cell lung cancer, head and neck cancer, esophageal cancer, advanced solid tumors, other metastatic solid tumors (t), esophageal cancer (o)	No data
RLY-1971	Allosteric SHP2 inhibitor	Advanced or solid metastatic tumors (t)	No data	Binds and stabilizes SHP2 in its inactive conformation	(Williams et al., 2022)
RMC-4550	Allosteric SHP2 inhibitor	Shows in vitro efficacy against lung adenocarcinoma and BRAF-, NF1- and RAS-driven cancer cell lines	No data		(Adamopoulos et al., 2021; Nichols et al., 2018; R. R. Wang, Liu, Zhou, Ma, & Wang, 2020; Zhou et al., 2022)
TNO155	Allosteric SHP2 inhibitor	EGFR- and KRAS G12C mutant non-small cell lung cancer, squamous cell carcinoma, gastrointestinal stromal tumors, colorectal cancer, BRAF V600 positive colorectal cancer	Increased creatine kinase, peripheral edema, diarrhea, acneiform dermatitis		(LaMarche et al., 2020)
Alendronate	Inhibits farnesyl diphosphate synthase	Currently no ongoing clinical studies in RAS/MAPK related pathologies	Abdominal pain, acid regurgitation, constipation, diarrhea, dyspepsia, musculoskeletal pain, nausea	Inhibits RAS farnesylation and thus membrane targeting of RAS	(Luckman et al., 1998)
Lovastatin	HMG-CoA reductase inhibitor	Growth and bone abnormalities in Noonan syndrome (t)	Blurred vision, constipation, dyspepsia, abdominal pain, diarrhea, flatulence, nausea, myalgia, muscle spasms, dizziness, headache, skin rash	Inhibits RAS farnesylation and thus membrane targeting of RAS Lovastatin ameliorates cognitive defects in Nf1 and Ptpn11 mice, but not HRAS mice. Findings not replicated in children with NF1	(Krab et al., 2008; Lee et al., 2014; W. Li et al., 2005; Payne et al., 2016; Schreiber et al., 2017)
Tipifarnib	Farnesyltransferase inhibitor	NF1-associated progressive plexiform neurofibromas (t), HRAS-mutated head and neck squamous cell carcinoma (b), solid tumors with HRAS gene alterations, leukemia, relapsed/refractory T cell lymphoma, relapsed/ refractory myeloma, glioblastoma, astrocytoma, oligodendroglioma, recurrent breast cancer, metastatic malignant melanoma, advanced pancreatic cancer, bladder cancer (all t)	CK elevation; gastrointestinal toxicities (abdominal pain, diarrhea, constipation, nausea, vomiting, anorexia, oral mucositis, sore throat, and dry mouth), acneiform rash, maculopapular rash	Inhibits RAS farnesylation and thus membrane targeting of RAS Possible HRAS preference, since HRAS is uniquely dependent on farnesyl transferase for prenylation (NRAS and KRAS can bypass via geranylgeranyl transferase)	(End et al., 2001; Kessler et al., 2019; Lee et al., 2014; Sebti & Hamilton, 2000; Sepp-Lorenzino et al., 1995)
Tovorafenib	Type II pan-RAF inhibitor	Pediatric low-grade glioma (b), malignant glioma (o), Langerhans cell histiocytosis, low-grade glioma, craniopharyngioma (all t)	Increased serum creatine kinase, rash, hair color change	High brain penetrance. Little paradoxical MEK activation.	(Sun et al., 2017)
Belvarafenib	Type II pan-RAF inhibitor	NRAS-mutated advanced melanoma, solid tumors (all t)	Rash, acneiform dermatitis, pyrexia	Stabilizes RAF in its inactive conformation and inhibits RAF dimerization. Little paradoxical MEK activation	(Adamopoulos et al., 2021; Moore et al., 2020; Yen et al., 2021)
LY3009120	Type II pan-RAF inhibitor	Solid tumors, melanoma, non-small cell lung cancer, colorectal cancer (all t)	Fatigue, nausea, acneiform dermatitis, maculopapular rash, arthralgia, myalgia	Inhibits all RAF isoforms, inhibits BRAF and CRAF homodimers and heterodimers. Little paradoxical MEK activation	(Adamopoulos et al., 2021; Lai et al., 2022; Moore et al., 2020; Peng et al., 2015; Sullivan et al., 2020)
LXH254	Selective RAF inhibitor	Solid tumors, non-small cell lung cancer, ovarian cancer, melanoma, colorectal cancer	Acneiform dermatitis, maculopapular rash, fatigue, nausea, myalgia, increased lipase	Potent inhibitor of BRAF and CRAF, less activity against ARAF	(Janku et al., 2018; Monaco et al., 2021; Ramurthy et al., 2020)
Binimetinib	MEK inhibitor	BRAF V600E or V600K mutation-positive unresectable or metastatic melanoma (a) Non-small cell lung cancer with KRAS gene mutation, craniopharyngioma, BRAF mutant cancer, hairy cell leukemia, gastrointestinal stromal tumor, pancreatic cancer, colorectal cancer, biliary tract cancers, melanoma, pancreatic carcinoma, solid tumors with RAS alterations, breast cancer, ovarian cancer, fallopian tube cancer (all t)	Fatigue, nausea, diarrhea, constipation, vomiting, abdominal pain, acneiform dermatitis, retinopathy	Used in combination with encorafenib	(Dummer et al., 2018)
Cobimetinib	MEK inhibitor	BRAF V600E or V600K mutation-positive unresectable or metastatic melanoma (a) Extracranial arteriovenous malformations (t) Histiocytosis, non-small cell lung cancer, solid tumors, recurrent acute myeloid leukemia, squamous cell carcinoma, adenocarcinoma, pancreatic cancer, ovarian cancer, biliary tract carcinoma, thyroid carcinoma (all t)	Diarrhea, photosensitivity, nausea, fever, vomiting, acneiform dermatitis, impaired vision, retinopathy, retinal detachment	Used in combination with vemurafenib Positive responses reported in plexiform neurofibroma in neurofibromatosis type 1 and a case of dysembryoplastic neuroepithelial tumor in Noonan syndrome	(Larkin et al., 2014; Trippett et al., 2022)
Mirdametinib	MEK inhibitor	NF1-associated plexiform neurofibromas (f) Breast cancer, low grade glioma, advanced or refractory solid tumors (all t)	Rash, fatigue, nausea, vomiting	Oncological use planned with RAF dimer inhibitor	(de Blank et al., 2022; Klesse et al., 2020; Weiss et al., 2021)
Selumetinib	MEK inhibitor	NF1-associated plexiform neurofibromas (a) Melanoma, Kaposi’s Sarcoma, blood cancers, solid tumors, breast cancer, ovarian cancer, endometrial cancer, non-squamous cell lung cancer, non-small cell lung cancer, sarcoma, pancreatic cancer, biliary tract cancer, colorectal cancer, gastric adenocarcinoma, pancreatic carcinoma, hepatocellular carcinoma, bladder cancer, vestibular schwannoma, thyroid cancer	Vomiting, rash, abdominal pain, diarrhea, nausea, dry skin, fatigue, musculoskeletal pain, stomatitis, blurred vision, retinal pigment and epithelial detachment, paronychia	One compassionate use case in Noonan syndrome associated bleeding described	(Casey et al., 2021; Chakraborty et al., 2022; de Blank et al., 2022; Gross et al., 2020; Kenney et al., 2021)
Trametinib	MEK inhibitor	BRAF V600E or V600K mutation-positive unresectable or metastatic melanoma (a), metatstatic non-small lung cancer with BRAF V600E mutation (a), anaplastic thyroid cancer with V600E mutation (a), metastatic or unresectable tumors with BRAFV600E mutation (a) Extracardiac arterio-venous malformation (t) Non-small cell lung cancer, melanoma, solid tumors, pancreatic cancer, squamous cell cancer, cervical cancer, thyroid cancer, juvenile myelomonocytic leukemia, acute myeloid leukemia, lymphoma, hemangioendothelioma, meningioma, breast cancer, colorectal cancer, prostate cancer, renal cancer, craniopharyngioma, astrocytoma, Erdheim-Chester disease (all t) Rosacea (topical)	Diarrhea, constipation, acneiform rash, serous retinopathy, retinal vein occlusion, uveitis, interstitial lung disease, hemorrhage, venous thromboembolism, hypertension	Oncological use in combination with dabrafenib Positive responses in germline RASopathy-associated pathologies (see main article)	(de Blank et al., 2022; Kondyli et al., 2018; Moore et al., 2020; Robert et al., 2015; Roskoski, 2018)
Tetrathiomolybdate (TTM)	Copper chelator	Wilson’s disease (a in Europe) Non-small cell lung cancer, prostate cancer, breast cancer, esophageal carcinoma, biliary tract cancer, breast cancer, colorectal cancer, multiple myeloma (t)	Anemia, leukopenia, thrombocytopenia	Restricts copper binding of MEK1/2, which is required for MEK1-dependent phosphorylation of ERK	(Brady et al., 2014; Turski et al., 2012; Xu, Casio, Range, Sosa, & Counter, 2018)
Ulixertinib	ERK inhibitor	BRAF mutant melanoma (o)			(Germann et al., 2017)

Note: Summary of selected compounds that are in clinical development in oncology or RAS-MAPK driven diseases which could be of potential interest for future application in germline RASopathies. Indications and adverse effects are not exhaustive. Abbreviations: (a) approved indication(s)/mutation(s), (b) breakthrough therapy designation, (c) compassionate use, (f) fast track designation. (o) orphan drug designation, (t) targeted indication(s)/mutation(s).

The farnesyl transferase inhibitors (FTIs, such as tipifarnib and lonafarnib), directly inhibit the enzyme responsible for a particular type of RAS prenylation, namely farnesylation. NRAS and KRAS bypass the requirement for farnesylation by undergoing geranyl geranylation, an alternative prenylation that allows for membrane localization. However, HRAS, the RAS isoform affected in CS, is unable to be alternatively modified, so its membrane localization and cellular function are suppressed by FTIs (Cox, Der, & Philips, 2015). FTIs, then, may represent a potential therapeutic for CS, but not the KRAS- or NRAS-altered RASopathies.

Two classes of targeted agents that prevent RAS activation recently have been identified: SOS1 inhibitors and SHP-2 inhibitors (SHP-2 is encoded by PTPN11). SOS1 inhibitors (e.g., BI 1701963 and RMC-5845) that inhibit the interaction between RAS proteins and its exchange factor SOS1 to inhibit RAS activation are in clinical development for RAS-mutated malignancies (Hillig et al., 2019). SOS inhibitors have not yet been evaluated in models of RASopathies but could be expected to have efficacy for any genotype that requires the RAS activation cycle to be intact to achieve aberrant signaling through the MAPK pathway. Allosteric inhibitors of SHP-2, which hold the phosphatase in the auto-inhibited conformation, are also in clinical trials in malignancies (Yuan, Bu, Zhou, Yang, & Zhang, 2020). These inhibitors, including RMC-4630, TNO155, and RLY-1971, are unable to bind to SHP-2 mutants in which the auto-inhibited conformation is destabilized (Padua et al., 2018); therefore, these inhibitors are unlikely to be active for most PTPN11 alleles in NS, which do perturb SHP-2 auto-inhibition. This limitation may be prevented in a novel approach that targets the N-terminal Src homology domain of SHP2, allowing mutant SHP2 to be targeted (Bobone et al., 2021). Through their inhibitory role on the nexus of RAS exchange, RMC-4630 and related SHP-2 inhibitors may be effective in RASopathies caused by variants in genes other than PTPN11, although this hypothesis has not yet been tested.

5.2 |. Direct RAS inhibitors

Direct inhibitors of KRAS^G12C (e.g., AMG 510, MRTX849, JNJ-74699157, and GDC-6036) function by binding covalently to KRAS^G12C-GDP, locking it in the inactive state. These inhibitors are currently in clinical development for KRAS^G12C-mutated solid tumors (Moore et al., 2020), and AMG 510 has been FDA approved for the treatment of KRAS^G12C-mutated non-small cell lung carcinoma based on results from the CodeBreak 100/101 trial (Hong et al., 2020). These inhibitors are an exciting advance in the RAS field, as it was previously thought that direct RAS inhibition was impossible. However, the KRAS^G12C inhibitors are not likely to have utility in individuals with RASopathies because the KRAS^G12C variant is uncommon in the RASopathies. Drugs that specifically inhibit other KRAS mutants, such as KRAS^G12D (MRTX1133, KRpep-2D) (Lim et al., 2021; Wang et al., 2022) and KRAS^G12S (Zhang, Guiley, & Shokat, 2022) are being developed and may have clinical utility in patients with KRAS-mutant NS or CFC in the future.

Strategies to target KRAS in a mutation-independent manner are also being developed. BI 2825, for example, binds to a grove on the surface of both active and inactive KRAS (Kessler et al., 2019). Additional preclinical tool compounds, RMC-6236 and RMC-6291, are pan-RAS inhibitors that bind to a complex of RAS-GTP (RAS ON) and the ubiquitously expressed molecular chaperone cyclophilin A (Koltun et al., 2022; Nichols et al., 2022). Agents that modify the expression of RAS, such as antisense (Ross et al., 2017) and splice switching (Hartung et al., 2016) oligonucleotides, have also been developed. Clinical candidates based on any of these mutation-independent agents represent exciting potential therapeutics for RASopathies of several genotypes.

5.3 |. RAS/MAPK pathway inhibitors

As discussed above, MEKis are in clinical evaluation for the treatment of RASopathy-related hypertrophic cardiomyopathy. Agents in addition to MEKis also inhibit signaling through the RAS/MAPK pathway. For example, the MEK1/2 kinases require the heavy metal copper for their kinase activity. Copper chelators, which are well tolerated clinically, are efficacious in preclinical models of RAS/MAPK-driven malignancies and might represent a novel method by which to decrease MAPK signaling in individuals with RASopathies (Baldari, Di Rocco, & Toietta, 2020). Importantly, the tool compound U0126, which functions as a MEKi in part due to its ability to chelate copper, is effective in several RASopathy models (Jaffre et al., 2019; Josowitz et al., 2016; Nakamura, Gulick, Pratt, & Robbins, 2009; Rodriguez-Viciana et al., 2006; Senawong et al., 2008).

Pan-RAF inhibitors that are in clinical development, such as LY3009120, LXH254, DAY101 (also known as MLN2480 or TAK-580), and belvarafenib, are able to inhibit MAPK signaling in the absence of BRAF^V600E, while BRAF^V600E-specific inhibitors, such as vemurafenib and dabrafenib, cause paradoxical activation of the MAPK pathway in the absence of BRAF^V600E (Durrant & Morrison, 2018). BRAF^V600E alleles have not been associated with RASopathies to date; therefore, the pan-RAF inhibitors may have more clinical utility than BRAF^V600E inhibitors in patients with RASopathies. ERK inhibitors, such as ulixertinib and LY3214496, are also in clinical development and may be of benefit to individuals with RASopathies (Liu, Yang, Geng, & Huang, 2018). Because ERK activation is noted in most RASopathy models, ERK inhibitors merit testing in RASopathy models of diverse genotypes.

6 |. LESSONS LEARNED FROM INITIAL COMPASSIONATE USE: CONSIDERATIONS FOR NEW TREATMENTS

As investigators, affected families, and biopharmaceutical companies coalesce around the exciting development of mechanistic therapeutics for the RASopathies, be those through drug repurposing or novel drug development, there are several issues and concepts that might be useful to consider, which will be addressed here.

As the RASopathies are traits that affect multiple organs and tissues, a critical issue is determining the best therapeutic target(s). As described above, the activity in repurposing MEKis has focused on severe manifestations for which conventional approaches are either non-existent or highly morbid (e.g., severe HCM in infants, severe lymphovascular dysfunction). While rational, this has the downside of applicability to a limited percentage of affected individuals, which may not result in maximal utility and may present barriers to successful completion of clinical trials. On the other hand, targeting a common issue in the RASopathies—consider short stature as an example—would potentially benefit more affected individuals but might raise greater concerns about safety, both due to the relative health impact of that issue and the availability of an FDA-approved therapy. A potential solution to this dilemma is to try a novel drug in a severe, rare aspect of the RASopathies and use the insights about efficacy and adverse events from that experience to determine the advisability of subsequent trials for commoner, less morbid issues.

Precedent from animal studies is clearly in favor of such an approach since several extracardiac phenotypes of RASopathies have been ameliorated using targeted approaches in mouse models. Examples include attenuation of the craniofacial phenotype and improved growth in the RAF1 L613V mouse model when MEKi is started within the first 4 weeks of life (Wu et al., 2011). In a Drosophila model of gain-of-function mutations of PTPN11, neuronal development is normal, but homeostasis is altered (Lee et al., 2014). Blocking SHP-2 resolved these issues. Mouse models for the common PTPN11 mutations N308D and D61G showed hippocampal-dependent spatial learning impairments and deficits in synaptic strengths (Lee et al., 2014). Treatment with the MEKi SL327 and the HMG-CoA reductase inhibitor lovastatin, which also inhibits prenylation and thus RAS membrane association, improved synaptic strength and learning in these two models. While often not classified as a RASopathy per se, mutations in SYNGAP1 are a frequent cause of non-syndromic developmental delay through hyperphosphorylation of the MEK and ERK, with ensuing synaptic dysfunction. Treatment of a mouse model of SYNGAP1 deficiency with the MEKi PD-0325901 resulted in normalization of basal synaptic activity, but their long-term potentiation implicated in learning (Kopanitsa et al., 2018). These results point to the possibility that specific phenotypes of SYNGAP1 deficiency may be improved through treatment with MEKi.

A related issue for clinical trials will be inclusion criteria with respect to RASopathy genotype. Consider this thought experiment: there is evidence from pre-clinical studies to suggest that mechanistic therapy might improve neurocognitive function in the RASopathies. With a candidate therapy in hand, would one undertake a clinical trial with patient with genotypes associated with more severe neurocognitive impairments (i.e., Costello and CFC syndromes) or mild impairments (i.e., NS)? Relatedly, as mechanistic therapy advances beyond the most severe, early-onset forms of HCM, which are almost entirely observed in babies with NS and NSML, would all RASopathy-associated HCM be eligible? Can a prospective randomized trial be justified at all if additional data from compassionate care emerge and underpin the case for strong beneficial effects of MEKi/mTORi? Clearly, issues of equity and non-discrimination will come to the fore, and the community will need to carefully avoid decisions arising from unintentional biases. Innovative strategies for trial design in small populations are now available (Friede et al., 2018; Hee et al., 2017; Ursino & Stallard, 2021). One example is delayed start of treatment in one group versus the other, and comparison of patient groups according to the time point at which treatment was started. This could help avoid a situation in which patients on a placebo arm are denied the potential benefits of MEKi/mTORi.

In considering repurposing mechanistic therapies for the RASopathies, there are a number of important considerations. For repurposed drugs, there will generally be prior experience in one or more other clinical contexts. For the MEKis, that has meant use in patients with cancer, often adults. Aside from the obvious potential differences in drug metabolism based on age, there is another major dividing line: patients with cancer have gain of function for the RAS/MAPK pathway only in their cancer due to a somatic mutation, whereas patients with RASopathies have germline variants. Also of note, modeling of CFC-associated MEK variants in fruit fly embryos has shown that the complexities of the regulation of signal transduction pathways results in gain-of-function alleles engendering increased signaling in some tissues but decreased signaling in others (Goyal et al., 2017). This may be due to feedback loops in which some activating variants may lead to strong negative feedback resulting in reduced MAPK signaling in certain other tissues. Taken together, predicting the side-effect profile of any drug targeting RAS signaling developed in patients with cancers may or may not prove accurate for the RASopathy context. Optimistically, some drug side effects may never occur in patients with RASopathies. Realistically, patients with RASopathies will experience some of those side effects as already apparent from the dermatological issues observed in patients with NS treated with MEKis and could even encounter novel ones never observed in older patients with somatic RAS pathway mutations.

Two additional considerations might also impact the side effects associated with mechanistic therapies for the RASopathies: dosing and treatment duration. For dosing, the comparison to cancer treatment is germane. To be effective, the goal in oncologic therapeutics is to shut down RAS signaling in order to induce apoptosis in cancer tissues that are “RAS addicted.” The therapeutic goal in the RASopathies is quite different: to normalize RAS signaling levels. For repurposing therapeutics that directly target the canonical RAS/MAPK pathway, that might be achieved through dose reduction (as has been done with MEKis to date, described above). However, this concept also suggests that therapeutics that alter the RAS/MAPK signaling network, perhaps targeting more peripheral or regulatory proteins, which would not be strategically useful for cancer, might be effective for the RASopathies. If such an approach also had fewer side effects, it might facilitate use for non-life-threatening aspects of the RASopathies.

Another substantive difference between mechanistic RAS/MAPK therapy in cancer versus the RASopathies is treatment duration. For cancer, treatment, generally undertaken with the maximally tolerated dosing, is relatively brief, consistent with the goal of inducing apoptosis in the tumor and necessitated by toxicities. Treatment duration in the RASopathies will almost certainly need to be longer than for cancer and will likely vary with the indication. This issue has already arisen with the use of MEKis for NS-associated HCM as described above. Trial and error may be needed to determine the appropriate duration for this indication. On the other hand, hypothetically effective treatment for neurocognitive dysfunction would most likely necessitate life-long use as the therapeutic purpose is to restore normal neuronal homeostasis, something that would be lost shortly after stopping an effective medicine.

In summary, the RASopathy community stands on the brink of developing meaningful mechanistic therapies for various aspects of these traits. As discussed here, there are several thorny issues to consider and questions to be asked as we proceed. By being deliberate, we should be able to reach wide consensus and avoid many of the potential pitfalls.

DATA AVAILABILITY STATEMENT

This manuscript does not contain data that would require public availability.