Clinical Approach to Genetic Testing in Amyloid Cardiomyopathy: From Mechanism to Effective Therapies

Abstract

Purpose of review:

To highlight the evolving understanding of genetic variants, utility of genetic testing, and the selection of novel therapies for cardiac amyloidosis.

Recent findings:

The last decade has seen considerable progress in cardiac amyloidosis recognition given advancement in cardiac imaging techniques and widespread availability of genetic testing. A significant shift in the understanding of a genetic basis for amyloidosis has led to the development of disease-modifying therapeutic strategies that improve survival.

Summary:

The systemic amyloidoses are disorders caused by extracellular deposition of misfolded amyloid fibrils in various organs. Immunoglobulin light-chain (AL) or transthyretin (ATTR) amyloidosis are the most common types associated with cardiac manifestations. Genetic testing plays a central role in the identification of genotypes that are associated with different clinical phenotypes and influence prognosis. Given the emergence of effective therapies, a systematic approach to the diagnosis of cardiac amyloidosis, with elucidation of genotype when indicated, is essential to select appropriate treatment.

Introduction

Cardiac amyloidosis (CA) is a restrictive cardiomyopathy characterized by extracellular deposition of insoluble and misfolded protein leading to progressive myocardial dysfunction.[1] Classification of CA, or amyloid type, is based upon the precursor protein that misfolds: Light-chain (AL amyloidosis) characterized by misfolded immunoglobulin light-chains resulting from a plasma cell dyscrasia [2], or transthyretin (ATTR amyloidosis) which results from deposition of misfolded transthyretin protein. TTR (formerly known as prealbumin) is synthesized by the liver and functions as a transporter of thyroxine and vitamin A (retinol) in plasma. ATTR-CA is further sub-classified based on the sequence of the TTR gene as wild-type ATTR (ATTRwt) in which the gene sequence is normal, or hereditary or variant-type ATTR (ATTRv) in which the gene sequence bears a pathological single nucleotide polymorphism[3].

CA was historically considered to be a rare disease, however recent advancements in non-invasive diagnostic modalities have rendered this disease more detectable in an earlier phase that permits effective administration of available amyloid-specific therapies [4]. A thorough understanding of the gene variants of CA is important as each genotype confers a specific clinical presentation, age of onset, organ involvement, and prognosis. In this review, we explore the current understanding of molecular pathogenesis, role of genetic testing, and the selection of available disease modifying therapies for CA from the clinician’s perspective.

Pathogenesis of transthyretin amyloidosis

Transthyretin, formerly named prealbumin, is a 127-amino acid protein encoded by 4-exons residing on chromosome 18q12. It is mainly synthesized in the liver, with lesser amounts produced by the retinal pigment endothelium of eye and choroid plexus of cerebrospinal fluid. TTR is a transporter protein that carries thyroxine hormone and retinol-binding protein [5].

The main factors contributing to TTR instability are gene mutation and/or aging, leading to dissociation of the homo-tetramer into its monomers, which can rapidly misfold and self-assemble into amyloid fibrils. Furthermore, amyloid fibril composition plays a major role in pathogenesis thereby determining the phenotypic presentation of ATTR amyloidosis. ATTR deposits have been classified into two distinct entities characterized by the length of protein fibrils. Type A fibrils consist of a mixture of truncated and full length ATTR fibrils, whereas type B fibrils consist of full-length fibrils primarily. In a Swedish study of 33 patients with Val30Met familial amyloidosis with polyneuropathy that examined the amyloid composition in fat biopsies, type B fibrils were associated with early disease onset without cardiac involvement and a strong affinity for Congo red stain, whereas type A fibrils were associated with late disease onset and cardiac phenotype with weaker Congo red staining[6]. Therefore, genotype alone does not determine fibril composition.

Overview of genotypes and associated phenotypes

ATTR amyloidosis

The common ATTR genotypes and their associated phenotypes are summarized in Table 1.

Table 1:

Common gene variants associated with hereditary ATTR amyloidosis

	Val122Ile	Thr60Ala	Val30Met	IIe68Leu	Leu111Met
Prevalence	3–4% of African Americans	1% North-west Republic of Ireland	Most common world-wide mutation, varies depending on country, can approach 1:1000 in endemic areas	Unknown	Unknown
Presenting age (median)	>65 years	>60 years	30–40 years in endemic areas, 50–60 years in non-endemic areas.	>60 years	>30 years
Male: Female	3:1	unknown	2:1	Male predominance in affected patients, but not in mutation carriers	unknown
Ethnicity	African/Afro-Caribbean	Caucasian	Any	Caucasian	Danish
Geographical distribution	USA, Caribbean, Africa	USA, Ireland, Germany, England	Sweden, France, Portugal, Japan	Central-northern Italy	Denmark
Cardiac Phenotype	Cardiac involvement always present	Cardiac involvement in 42% of patients	Cardiac involvement is rare, more in late-onset cases (conduction disease more common)	Cardiac involvement nearly always present	Cardiac involvement always present
Extra-cardiac manifestations	PN (10%) and CTS	AN is common, PN is less common	PN and AN are common	CTS is common (42%), PN is less common (<10%)	CTS is common

AN, autonomic neuropathy; CM, cardiomyopathy; CTS, carpal tunnel syndrome; PN, peripheral neuropathy; THAOS, Transthyretin Amyloid Outcome Survey.

ATTRwt amyloidosis, previously known as senile cardiac or senile systemic amyloidosis, is the most common type of amyloidosis in the United States and likely world-wide. While it is understood to be caused by age-related misfolding of TTR, the precise mechanisms underlying pathogenesis remain unclear[3]. ATTRwt amyloidosis primary affects elderly, male patients of Caucasian origin with a median age of 74 years at time of diagnosis[7, 8]. The clinical phenotype is characterized by cardiac involvement leading to restrictive cardiomyopathy. Recent data suggest that ATTRwt amyloidosis may be present in about 10–15% of elderly patients with heart failure and preserved ejection fraction[9] and patients undergoing aortic valve replacement[10]. Soft tissue involvement is usually common with increased incidence of bilateral carpal tunnel syndrome, spinal stenosis, and biceps tendon rupture[11, 12], whereas (in contrast to ATTRv amyloidosis) peripheral and autonomic neuropathy are uncommon, perhaps affecting only about 10% of patients[13].

ATTRv amyloidosis follows an autosomal dominant inheritance pattern that manifests clinically as a length-dependent, small fiber sensory peripheral neuropathy, autonomic neuropathy, cardiomyopathy, or mixed phenotype[14]. The variation in clinical spectrum is related to many factors most notably including the specific TTR gene variant, geographical distribution and migration pattern, and age of onset[15, 16]. In ATTRv amyloidosis, the mechanism of amyloidogenesis is better understood as the specific disease variant induces thermodynamic instability in the TTR tetramer that favors disassociation and misfolding.

There are over 140 TTR gene variants that have been identified with the majority occurring in exons 2 and 3 [5]. The nomenclature convention for TTR variants stipulates the wild-type amino acid, followed by the location of substitution, followed by the substituted amino acid. Some genetic reports also include the 20 amino acid signal-peptide in the count of residues. The Val122Ile (ATTR V122I or pV142I) variant, in which isoleucine is substituted for valine at position 122, is the most common amyloidogenic variant in the United States. It predominantly affects persons of West African origin with a prevalence rate of approximately 3–4% in self-reported Black individuals[17]. Upon extrapolation from US Census data, an estimated 1.5 million Americans are carriers of TTR Val122Ile though clinical penetrance of this allele is not known. Data suggest age-dependent phenotypic expression (> 60 years) and male predilection (80%)[18]. Retrospective analysis of the Atherosclerosis Risk in Communities (ARIC) study which included 124 Val122Ile carriers and 3732 non-carriers suggested a low phenotypic penetrance rate of 20%, but this study utilized a relatively insensitive echocardiographic wall thickness criterion to define disease[19]. That stated, data from the Cardiovascular Health Study (CHS) study has shown higher frequency of heart failure symptoms after age of 65 in Val122Ile carriers[20]. Similarly, Damrauer et al. recently observed increased frequency of heart failure among African or Hispanic/Latinos with Val122Ile (odds ratio of 1.7) compared to age, race, and sex matched wild-type controls[21]. Taken together, these data suggest that Val122Ile can be conceived of a novel risk factor for HF development, likely mediated by amyloid deposition. Peripheral neuropathy is thought to be uncommon (10%) in Val122Ile [22]. Compared to ATTRwt amyloidosis, ATTRv Val122Ile likely is typified by a more severe phenotype. Maurer et al compared 91 patients with Val122Ile and 189 patients with ATTRwt enrolled in The THAOS (Transthyretin Amyloid Outcome Survey) registry, reporting higher NYHA class and B-type natriuretic peptide levels among variant carriers, though overall survival was not significantly different between groups[23]. In addition, a large cohort study including 869 patients from the UK National Amyloidosis Center reported worse median survival among ATTR Val122Ile patients in all three defined biomarker stages in comparison to ATTRwt patients[24]. These studies are obviously confounded by ascertainment and referral bias.

The second most common variant observed in the US is Thr60Ala (pT80A). First described in an Irish family in 1986[25], ATTRv T60A amyloidosis has been estimated to affect 1% of the population in the northwest region of the Republic of Ireland[26] and was reported in approximately 20% of THAOS registry patients in the US[23]. Like Val122Ile, ATTRv Thr60Ala amyloidosis exhibits a male predominance of 3:1 and typically manifests at later age with median age at onset of symptoms of 63 years, though symptom onset in the 5^th decade of life has been observed. Clinical phenotype is usually mixed with cardiac involvement and autonomic neuropathy in about 42% of patients, while peripheral neuropathy is less common[27].

Val30Met (pV50M) is the most common world-wide mutation outside of the US[28], endemic in Japan, Portugal and Sweden[23]. ATTRv Val30Met amyloidosis is a primarily a neurological disease with onset usually between the ages of 30 and 40 years in Portugal and endemic areas of Japan with high clinical penetrance, while “late onset” can occur after 50 years in non-endemic areas including the Swedish region along with lower penetrance rate. Furthermore, the sex distribution is almost equal in endemic areas, while it is more common in males in non-endemic areas [29]. In comparison to the other aforementioned TTR variants, restrictive cardiomyopathy is usually uncommon with this mutation, and it typically manifests as a progressive small fiber neuropathy affecting peripheral and autonomic nerves[15]. Interestingly, non-endemic patients had lower frequency of autonomic symptoms than endemic patients[29]. The clinical phenotype of ATTRv Val30Met amyloidosis varies substantially depending on the age of onset, as late-onset cases tend to have more cardiac involvement than early onset cases. In a study of 81 patients with ATTR Val30Met amyloidosis in Northern Sweden, late onset cases had higher interventricular septal thickness and worse longitudinal strain measurements than early onset cases, indicating significant cardiac involvement in patients with delayed onset amyloid disease [16].

AL Amyloidosis

The estimated incidence of AL amyloidosis in the US is about 1 per 100,000 or 2500–5000 new cases annually[30]. Approximately 5–10% of AL amyloidosis patients have underlying multiple myeloma, and a similar percentage of multiple myeloma patients may develop AL amyloidosis. AL amyloidosis patients tend to have less than 20% plasma cell involvement of the bone marrow [2].

Clinical spectrum of AL amyloidosis is heterogenous and depends upon organ involvement. The kidney is commonly affected, as approximately 40% of cases usually present with nephrotic range proteinuria, and about 20% of patients eventually progress to end stage renal disease[31]. Cardiac involvement is also common, affecting about 60–80% of patients depending on who involvement is defined, and manifests in advanced stages as a restrictive cardiomyopathy with congestive heart failure symptoms and signs[32]. Soft tissue involvement is frequently observed in AL amyloidosis, with a combination of macroglossia and periorbital ecchymosis being almost pathognomonic[33]. Peripheral neuropathy occurs in up to 17 to 35% of patients, which typically presents with distal symmetrical sensory loss, with motor weakness developing at later stages [34]. Other organ systems that can be affected in AL amyloidosis include the liver and gastrointestinal tract. Hepatomegaly with or without splenomegaly can suggest hepatic involvement clinically manifesting as cholestasis with elevated liver enzymes. Clinical apparent gastrointestinal disease is uncommon, usually manifesting as bleeding, gastroparesis, malabsorption, and intestinal obstruction related to gut dysmotility[35].

In comparison to ATTR amyloidosis, cardiac involvement in AL patients often is noted at younger age and does not demonstrate a male predilection. Similarly, AL amyloidosis is associated with a lesser degree of left ventricular wall thickening, lower ventricular mass, and lower electrocardiographic voltage. This is in contrast to the often profound and debilitating heart failure that ensues. The onset of disease is less gradual (months rather than years) than with ATTR and the underlying pathophysiology includes both cytotoxic and infiltrative components[36]. Major heart involvement is the most important prognostic parameter in AL amyloidosis and directs the therapeutic options selected. Historically, patients with untreated AL-CA have an overall median survival of 6 months, but contemporary treatment strategies have improved median overall survival to > 5 years, with many patients with AL-CA living 10 or more years after diagnosis[30].

As in ATTR, light-chain genetics may also inform treatment selection. The chromosomal translocation t(11;14), observed in up to 40–60% of patients with AL amyloidosis, juxtaposes the immunoglobulin heavy chain locus to the oncogene CCND1. This translocation has been associated with plasma cell disease resistant to the first-line chemotherapeutic agent bortezomib and worse survival overall. In systemic AL amyloidosis, the plasma cell clone is usually small (median plasmacytosis, 10%), and presents t(11;14) and gain 1(q21) in ~50% and 20% of clones, respectively, whereas high-risk aberrations as seen in myeloma are uncommon. Patients whose plasma cell clones harbor t(11;14) have a worse outcome with bortezomib and immunomodulatory drugs (IMiDs), whereas gain 1(q21) is associated with poorer results with oral melphalan[37].

Diagnosis of cardiac amyloidosis

Cardiac amyloidosis recognition is challenging and requires a high index of clinical suspicion. It should be suspected in any patient with congestive heart failure and unexplained left ventricular wall thickening, or low-flow, low-gradient aortic stenosis [10]. Classic electrocardiogram findings in CA include low QRS voltages in the setting of increased left ventricular (LV) wall thickness, and pseudo-infarction pattern which can be seen in up to 50% of cases of AL amyloidosis leading to initial misdiagnosis of acute coronary syndrome [38]. Echocardiography is a widely available diagnostic imaging method commonly employed for initial evaluation of patients with heart failure. Common features include symmetrically increased LV wall thickness (>12mm), thickening of the free wall of the right ventricle, and evidence of increased filling pressures. Impaired diastolic function (pseudo-normalization or restrictive filling) related to excessive amyloid infiltration, and relative apical sparing pattern of LV longitudinal strain are also frequently observed in CA[30]. Cardiac magnetic resonance (CMR) is also a valuable tool in evaluating CA and has led to an increased awareness and recognition of the disease. CMR typically demonstrates the classic pattern of diffuse sub-endocardial late gadolinium enhancement in a non-coronary artery territory distribution. Diffuse transmural or patchy late enhancement with poor myocardial signal suppression and increased extracellular volume fraction are also common [39].

That stated, neither echocardiography nor CMR can effectively differentiate between AL and ATTR amyloidosis, nor can they diagnose CA without confirmatory biopsy. Bone-seeking tracers like ^99mTc-pyrophosphate (PYP) in the US and Tc^99m-3,3- diphosphono-1,2-propanodicarboxylic acid (DPD) in Europe are radiotracers that can diagnose CA without the need for tissue biopsy in the proper clinical context. ^99mTc-PYP is now considered to be diagnostic for ATTR cardiomyopahty with a specificity and positive predictive value of 100% if there is grade 2 to 3 cardiac uptake (and/or a heart/contralateral ratio >1.5 at 1-hour post injection), in the context of normal plasma dyscrasia evaluation [40]. Endomyocardial biopsy remains the gold standard for ATTR cardiomyopathy diagnosis and is indicated in situations where the imaging and plasma cell disorder testing are equivocal [41].

Role of genetic testing in cardiac amyloidosis

TTR genetic testing is recommended for all patients with an established diagnosis of ATTR-CA regardless of age to differentiate between ATTRwt and ATTRv given differences in clinical manifestations, prognosis, treatment selection, and importance of screening family members. It is important to note the commercially available direct-to-consumer genetic testing systems (such as 23 and Me) report out the presence of the 3 most common TTR risk alleles (V122I, V30M, T60A) for additional cost. Newly identified gene carriers have been identified by this approach, many of whom may not have expected this result nor were prepared to process its significance.

While there are no standard guidelines established for early diagnosis of those at risk (i.e. first-degree relatives of allele carriers) or monitoring of asymptomatic carriers, experts have developed consensus documents to assist clinicians in pre-symptomatic genetic testing (adapted into Figure 1) [42–44]. In general, pre-symptomatic genetic testing is beneficial when treatment and/or prophylactic measures are available that can alter the progression to symptomatic disease[45]. The recent development of effective treatment options for ATTR amyloidosis has changed the perception of genetic testing among physicians and families, as early diagnosis and close monitoring can be now expected to translate into improved outcomes. That stated, multiple factors should be considered prior to testing, such as the psychological impact of testing upon patient and family members and the cost of testing (which is now negligible or free of charge). Furthermore, pre-symptomatic testing should always proceed with the consent and full-understanding of the patient and should be avoided in minors[46]. Establishment of a multidisciplinary team, including medical geneticist, ATTR amyloidosis expert, and mental health professional, is optimal to maximize the proper selection, timing, and ultimately, efficacy of testing, without leading to undue anxiety and psychological stress to family members[42].

Figure 1:

Genetic testing and approach to asymptomatic carriers

A proposed work-flow is outlined to guide clinicians when caring for asymptomatic, first-degree relatives of known amyloidogenic TTR allele carriers. Three phases are described that describe the context of testing and considerations. Note the multi-disciplinary team approach to provide both decision support as well as a thorough assessment of ATTR phenotypic expression. Abbreviations: PYP – Tc99m-pyrophosphate imaging (or equivalent), CMR – cardiac magnetic resonance imaging, EMG – electromyography, BNP – Brain-type natriuretic peptide.

Once a patient carrying a pathological TTR gene variant is identified, the predicted age of disease onset (PADO) should be determined. Defining the PADO takes into consideration multiple factors such as the specific variant, typical age at disease onset, and severity of established ATTR amyloidosis disease among family members, if known. The PADO can also be helpful to frame the timing of testing for relatives of allele carriers. For example, as the Val122Ile allele does not phenotypically express prior to age 60 years in nearly all circumstances (except perhaps homozygosity), knowledge of allele carrier status is generally not clinically relevant in relatives who are < 50 years of age. One expert consensus group has suggested to begin monitoring (or testing) 10 years prior to the PADO to establish baseline, then annual follow-up with increase in frequency as carriers approach their PADO, especially for genotypes that are well-known to have rapidly progressive course[43]. While no specific guidelines yet exist, we recommend baseline testing for known carriers of a pathological TTR variant to include: echocardiography with longitudinal strain imaging, electrocardiography, and blood testing for natriuretic peptides, cardiac troponin, renal function, and prealbumin. In addition, we recommend a baseline advanced cardiac imaging test that can include either a nuclear-scintigraphy scan (^99mTc-PYP or DPD) and/or cardiovascular magnetic resonance (CMR) scan with late gadolinium enhancement (and if possible, parametric imaging including native myocardial T1 and extracellular volume fraction measurement).

Establishment of an effective approach for monitoring is crucial to detect changes in the very initial stages of the disease that may prompt early initiation of treatment. During follow-up visits, clinical evaluation should include assessment for sensorimotor neuropathy changed from baseline, using modified Neuropathy Impairment Score+7 (mNIS+7; range, 0 to 304, with higher scores indicating more impairment) [47], autonomic neuropathy or neurogenic/sexual dysfunction, cardiac involvement, or renal/ocular involvement. It is recommended to avoid invasive tests such as biopsies, and to limit the number of evaluations performed each year to avoid emotional distress[44]. The interval and frequency for followup testing can be individuated based upon age, family history, variant, and prior testing results. An expert panel has proposed criteria for clinical diagnosis of symptomatic ATTR amyloidosis assuming confounding comorbidities have been appropriately excluded: the presence of at least one quantified/objective sign or symptom definitely related to onset of ATTR amyloidosis; or at least one symptoms/sign likely related to ATTR disease plus 1 abnormal test finding; or absence of symptoms plus 2 abnormal test findings[43].

Disease modifying therapies in ATTR based on genetics

Liver transplantation in ATTRv amyloidosis conferred an early gene-therapy approach to replace the variant TTR protein with wild-type. While effective in ATTRv amyloidosis and polyneuropathy, the strategy proved less effective when cardiac involvement was present at the time of transplant owing to disease progression[48]. Contemporary pharmacological strategies have rendered liver transplant obsolete and furthermore, liver transplant is not a viable treatment for ATTRwt amyloidosis. The currently available pharmacological therapies for ATTR amyloidosis can be categorized based on the mechanism of action (Table 2). In 2021, selection of treatment strategies depends entirely upon TTR genotype and organ manifestation (cardiomyopathy vs. neuropathy).

Table 2:

Pharmacological therapies for ATTR amyloidosis

Therapy	Mechanism of action	Route/Dose	Indications	Adverse effects
Stabilizers
•Tafamidis meglumine	Binds and stabilizes TTR, preventing misfolding	Oral, 20 or 80 mg daily	Cardiomyopathy	Potential GI distress but clinical trial suggests 80mg dose similar to placebo
•Tafamidis free salt		61 mg daily
•Diflunisal	NSAID, binds and stabilizes TTR, preventing misfolding	Oral, 250mg bid	Neuropathy Cardiomyopathy	GI bleeding Fluid retention Renal dysfunction
•AG-10 (acoramidis)	Binds and stabilizes TTR, preventing misfolding	Oral, 400 or 800 mg bid	Not currently available, in clinical trial	Unknown
Silencers
•Patisiran	Small interfering RNA that causes degradation of TTR mRNA	IV, 0.3mg/kg every 3 weeks	Neuropathy	Infusion-related reaction Vitamin A deficiency
•Inotersen	Anti-sense oligonucleotide that causes degradation of TTR mRNA	SC, 284mg every week	Neuropathy	Thrombocytopenia and glomerulonephritis, require close monitoring. Infusion-related reaction Vitamin A deficiency

NSAID, non-steroidal anti-inflammatory drug; BID, twice daily; GI, gastrointestinal; IV, intravenous; TID, three times daily; SC, subcutaneous; mRNA, messenger RNA

TTR protein stabilizers

Tafamidis (Pfizer) is an orally administered small molecule that binds with high affinity and selectivity to thyroxine-binding sites, slowing degradation, misfolding, and amyloid fibril formation. Tafamidis was approved by FDA in May 2019 for ATTR-CA irrespective of genotype and remains in 2021 the only agent approved for ATTRwt amyloidosis or for ATTRv amyloidosis with cardiomyopathy only (as is the case with the Val122Ile variant). The ATTR-ACT trial, comprising 441 patients with ATTR-CA who received tafamidis meglumine (20 or 80 mg) versus placebo, demonstrated that tafamidis was associated with reduction in all-cause mortality and cardiovascular-related hospitalizations with attenuation of the decline in functional capacity and quality of life [49]. While tafamidis has a minimal side effect profile, administration is limited principally by excessive cost to both patients and the health-care system (sometimes termed financial toxicity), though co-pay assistance programs are available.

Diflunisal is a low cost, nonsteroidal anti-inflammatory drug (NSAID) that has been repurposed as a TTR protein stabilizer. While not FDA approved for ATTR-CA, the drug can be used off label with caution and careful monitoring in selected patients with preserved renal function (eGFR > 45 ml/min) and compensated heart failure. Like tafamidis, diflunisal binds to the thyroxine binding sites, leading to kinetic stabilization of circulating TTR tetramers[50]. While effective in ATTR polyneuropathy[51], there are limited data regarding its efficacy in ATTR-CA. In a single-center retrospective study of 120 patients with ATTR-CA, diflunisal was associated with a survival benefit similar to tafamidis[52]. In addition, diflunisal therapy appears to attenuate the worsening of cardiac biomarkers and decline in echocardiographic parameters of disease including longitudinal strain [53]. For carriers or pathological TTR alleles without overt cardiac or neurological disease and normal renal function, diflunisal offers a viable treatment option that may slow or even avert the development of the ATTR clinical phenotype, with minimal toxicity.

Acoramidis or AG10 (Eidos Therapeutics) is an orally available TTR-stabilizer that binds TTR at a different binding site than tafamidis or diflunisal. Safety and tolerability of this agent was demonstrated [54], justifying a phase 3 clinical trial, ATTRIBUTE-CM (clinical trials.gov identifier NCT03860935), currently underway to determine efficacy in ATTR amyloidosis.

TTR protein silencers

Patisiran (Alnylam Pharmaceuticals) is an intravenously infused small interfering RNA (siRNA) that inhibits TTR synthesis by degradation of the messenger RNA (mRNA), resulting in a 90% reduction in circulating TTR protein. The drug is approved for ATTRv amyloidosis and polyneuropathy (with or without cardiomyopathy). The APOLLO trial randomized 225 patients with ATTRv amyloidosis and polyneuropathy to receive patisiran versus placebo, showing that patisiran improved a wide spectrum of neurological symptoms (determined by mNIS+7 score) [14]. In a pre-specified sub-study of patients with cardiac involvement, patisiran decreased mean left ventricular wall thickness, improved global longitudinal strain, and reduced NTpro-BNP at 18 months follow-up[55]. A recent study suggests that patisiran may induce regression of cardiac amyloid deposits in a subset of patients as determined by CMR extracellular volume fraction[56]. Patisiran is currently being evaluated in ATTR cardiac amyloidosis, irrespective of genotype (APOLLO-B, NCT03997383), as is another RNAi therapeutic vutrisiran (HELIOS-B, NCT04153149, Alnylam).

Inotersen (Akcea Therapuetics/Ionis) is a sub-cutaneously administered 2′-O-methoxyethyl–modified antisense oligonucleotide (ASO) that causes degradation of TTR mRNA which results in a reduction of circulating TTR protein. Like patisiran, the drug is approved for ATTRv amyloidosis and polyneuropathy (with or without cardiomyopathy). The 15-month phase 3 randomized clinical trial NEURO-TTR compared inotersen to placebo in 172 subjects with ATTRv polyneuropathy and found that inotersen improved the course of neurologic disease and quality of life[57]. In a small, open label study of 8 patients with ATTRv and 7 with ATTRwt, inotersen demonstrated stabilization of disease as measured by left ventricular wall thickness, left ventricular mass, and echocardiographic global systolic strain at 12-month follow-up[58].

Inotersen is being evaluated in an open label study of ATTR CA (NCT03702829), as is a related ASO therapeutic (ligand conjugated antisense or LICA, Cardio-TTRansform trial, NCT04136171, Akcea Therapeutics).

Conclusions

Cardiac amyloidosis is an important cause of congestive heart failure that is increasingly recognized. Genetic testing is a critical component of the evaluation and management of ATTR amyloidosis while also affording insight into AL amyloidosis organ involvement and prognosis. The development of advanced imaging techniques and increased availability of inexpensive genetic testing has increased disease awareness. Early recognition of disease expression, as well as identification of allele carriers at risk for ATTR amyloidosis affords an opportunity for early intervention that can slow disease progression and improve outcomes.

Key points.

• Cardiac amyloidosis is an important cause of heart failure, associated with significant morbidity and mortality if untreated.

• Genetic testing is a critical component of the evaluation of ATTR amyloidosis, determining prognosis and treatment. Genetic testing in AL amyloidosis is also important as it affords insight into prognosis and treatment efficacy.

• Management of asymptomatic but pathological TTR allele carriers is challenging and is best accomplished through a multidisciplinary team approach, including a medical geneticist, mental health professional, and amyloidosis expert.

• Early diagnosis of cardiac amyloidosis is crucial as novel disease-modifying therapies are now available and are most effective when administered in early stages of disease.