Enchondromatosis and growth plate development

Abstract

Purpose of review

Enchondroma is a common cartilage benign tumor that develops from dysregulation of chondrocyte terminal differentiation during growth plate development. Here we provide an overview of recent progress in understanding causative mutations for enchondroma, dysregulated signaling and metabolic pathways in enchondroma, and the progression from enchondroma to malignant chondrosarcoma.

Recent findings:

Several signaling pathways that regulate chondrocyte differentiation are dysregulated in enchondromas. Somatic mutations in the metabolic enzymes isocitrate dehydrogenase 1 and 2 (IDH1/2) are the most common findings in enchondromas. Mechanisms including metabolic regulation, epigenetic regulation, and altered signaling pathways play a role in enchondroma formation and progression.

Summary:

Multiple pathways regulate growth plate development in a coordinated manner. Deregulation of the process can result in chondrocytes failing to undergo differentiation and the development of enchondroma.

Introduction:

Long bones develop via endochondral ossification, a process where bone forms from a cartilage template. During this process, chondrocytes in the growth plate differentiate through different stages and finally undergo apoptosis or transdifferentiate into osteoblasts (1, 2). In human, growth plates usually close after puberty and the chondrocytes are no longer present. Precise regulation of chondrocyte differentiation during endochondral ossification is critical for proper development of long bones (1). Enchondromas are benign cartilaginous neoplasms that develop within the medullary space of bone (3, 4). Persistence of growth plate chondrocytes in bone leads to the development of enchondromas (5, 6) (Fig 1).

Fig 1: X-ray photos of patient with enchondromas.

Radiographs from a patient with enchondromatosis. Multiple lesions in the long bones of the femur, tibia (A), and humerus (B) are shown. For the patient in panel A, the lower extremity shows a short limb on the more involved side. The enchondromas are pointed out by arrows.

Enchondroma is one of the most common benign bone tumors and is estimated to affect 3% of the population (7, 8). They are mostly present in appendicular bones, uncommon in flat bones, and rarely occur in craniofacial bones (3). Enchondromas occur in solitary lesions or multiple lesions (known as enchondromatosis) in conditions of Ollier disease and Maffucci Syndrome (4). While malignant transformation in solitary enchondroma is rare, the potential for malignant transformation to chondrosarcoma in Ollier disease and Maffucci Syndrome is reported to be as high as 50% (9, 10) (Fig 2). Somatic genetic mutations underlying enchondroma include mutations in isocitrate dehydrogenase 1 and 2 (IDH1/2) and parathyroid hormone 1 receptor (PTHR1) (5, 11–13).

Fig 2: Development of enchondrorna and its progression to chondrosarcoma.

Enchondroma arises from deregulated chondrocyte differentiation in the growth plate. Defects in chondrocyte terminal differentiation leads to formation of cartilage lesions in the growth plate. When enchondroma occur as multiple lesions, they have a high chance to progress into malignant chondrosarcoma upon secondary mutations. Somatic mutations of IDH1/2 are the most common genetic variations in enchondromas and chondrosarcomas.

The aim of this review is to provide an overview of recent findings in genetic mutations associated with enchondromas, potential mechanisms underlying the formation of these tumors, and processes that control growth plate chondrocyte differentiation that are important for enchondroma formation.

Chondrocyte development in growth plate

Endochondral ossification is a process where bone forms from a cartilaginous template. The process starts when undifferentiated mesenchymal cells condense, differentiate to chondrocytes, and form a cartilage primordia that is rich is type II collagen and the proteoglycan aggrecan (1). These chondrocytes do not proliferate rapidly and are called resting chondrocytes. During skeletal development, chondrocyte proliferation and maturation are restricted near the ends of long bones, which is called the growth plate (14). Growth plate chondrocytes differentiate through resting, proliferating, prehypertrophic, hypertrophic, terminal stages, eventually undergo apoptosis or transdifferentiate into osteoblasts (2, 14, 15). The invasion of blood vessels into the hypertrophic cartilage in the center of the long bone is responsible for bringing osteoblasts and osteoclasts to form the bone and remodel the cartilage matrix (1, 16). Growth plate is responsible for the length of the bone. In humans, growth plate closes after puberty. The process of chondrocyte differentiation is tightly regulated by multiple transcription factors, extracellular signaling molecules, mechanical signals, and metabolic processes in a coordinated manner. Deregulated growth plate chondrocyte differentiation leads to pathological conditions such as skeletal dysplasias, or the development of enchondromas (17). Here, we will focus on pathways involved in enchondroma.

PTHLH-Hedgehog signaling in enchondroma

In the initial studies of regulation of growth plate chondrocyte differentiation, it was found Parathyroid Hormone Like Hormone (PTHLH) and Indian Hedgehog (IHH) regulated chondrocyte hypertrophy in a negative feedback loop (Fig 3). PTHLH was expressed at high levels in perichondral cells (18–20). Mouse genetic studies showed that PTHLH signaling suppressed chondrocyte hypertrophy (21–26). IHH was expressed by prehypertrophic and early hypertrophic chondrocytes (19). IHH stimulated the expression of PTHLH by the perichondral cells, which in turn suppressed the expression of IHH (19). IHH also promoted chondrocyte differentiation independently of PTHLH by regulating GLI (Zinc Finger Protein GLI) transcription factors (16, 27). When PTHLH signaling activity was remained at constant level, overexpression of Ihh still increased the length of the columnar zone in the growth plate, suggesting IHH could promote the transition of resting chondrocytes to proliferating chondrocytes independent of PTHLH signaling (27). Ihh mutant mice displayed reduced chondrocyte proliferation and early onset of hypertrophic differentiation, and ablation of Gli3 in Ihh mutant mice restored such changes, showing that IHH could regulate chondrocyte differentiation via inhibiting the repressor activity of GLI3 (28) (Fig 3). PTHLH could also regulate chondrocyte differentiation independent of IHH through protein kinase A and Gli transcription factors. Mau et al. showed that PTHLH inhibited hypertrophic differentiation of chondrocytes in the presence of IHH blockade cyclopamine, but the effects were blunted upon Gli3 deletion, suggesting PTHLH regulated chondrocyte differentiation via GLI3 (29) (Fig 3).

Fig 3: PTHLH-Hedgehog regulation of chondrocyte differentiation.

Regulation of chondrocyte differentiation by Indian Hedgehog (IHH) and parathyroid hormone-like hormone (PTHLH). IHH and PTHLH form a feedback loop and regulate chondrocyte differentiation. IHH secreted by prehypertrophic chondrocytes stimulate perichondral cells to secrete PTHLH, which inhibits hypertrophic differentiation and expression of IHH. IHH could promote chondrocyte differentiation by inhibiting the repressor function of Gli3 and the activator function of Gli2 in a manner that is independent of PTHLH. PTHLH could also suppress hypertrophic differentiation independent of IHH by through regulating Gli3 repressor function and promoting protein Idnase A (PKA). PICA signaling leads to activation of histone deacetylase 4 (HDAC4), which inhibits Mef2 function.

The Hedgehog signaling pathway is constitutively active in some cases of enchondromas (30). Independent studies have identified distinct mutations in PTHR1 gene from enchondroma tumors. One of the mutations (R150C) was a hypomorphic mutation, and three other mutations (G121E, A122T, R255H) caused loss-of-function (4, 5, 31). Animals expressing mutant R150C PTHR1 in chondrocytes developed abnormal cellular cartilage islands in the proximity of growth plate cartilage, which were similar to patient enchondromas. Studies in mice showed that the mutant R150C PTHR1 caused constitutive active hedgehog signaling. Overexpression of the Hedgehog regulated transcriptional factor Gli2 led to the formation of similar cartilage lesions (5). PTHR1 mutations of G121E, A122T, and R225H led to either reduced binding affinity for PTH or reduced receptor expression at the cell surface and caused reduced production (31). Recently, Deng et al. showed that activating Hedgehog signaling pathway by deleting patched 1 (Ptch1) gene in mesenchymal stem cells also induced formation of enchondromas in mice. In addition to Hedgehog signaling, Ptch1 deletion also led to activated Wnt (Wingless/integrated) /β-catenin signaling pathway and pharmacological inhibition of Wnt/β-catenin in vivo suppressed tumor formation. Together these data suggested Wnt/β-catenin could be downstream of Hedgehog activation in enchondromas and Wnt/β-catenin signaling could be a potential therapeutic target for enchondromas (32).

IDH mutation and enchondromas

A wide range of mutations in IDH1 (IDH1 R132C, R132G, R132H, etc.) and IDH2 (IDH2 R172M, R172C, R172K, etc,) genes are often found in different tumor types such as glioblastoma, acute myeloid leukemia (AML), chondrosarcomas, and enchondromas. The mutations were initially reported in glioblastomas from whole-genome exon-sequencing analysis, and later in AML (33–35). As patients with enchondromas have an increased chances of developing AML and glioma, researchers investigated the frequency of IDH1/2 mutations in cartilage tumors and found that these mutations in IDH1 and IDH2 also occurred in the majority of enchondromas and chondrosarcomas (11–13).

IDH1/2 encode for isocitrate dehydrogenase 1 and 2, which catalyze the reversible conversion between isocitrate and α-ketoglutarate using nicotinamide adenine dinucleotide phosphate (NADP⁺) as a cofactor (36) (Fig 4). IDH1 is present in the cytoplasm and peroxisome, while IDH2 is present in the mitochondria. IDH1 and IDH2 have distinct functions in cellular metabolism. IDH1 promotes cytoplasmic and nuclear dioxygenases that require α-ketoglutarate as a cofactor (37). It generates non-mitochondrial reduced nicotinamide adenine dinucleotide phosphate (NADPH), which could be used for lipid synthesis and maintaining redox levels (37). IDH1 could also catalyze the carboxylation of α-ketoglutarate to make isocitrate, a metabolite could be further utilized for lipid synthesis (37). IDH2 regulates the relative abundance of isocitrate and α-ketoglutarate in the mitochondria. When mitochondrial α-ketoglutarate level is high, IDH2 converts it to isocitrate, which could feed into the TCA cycle to generate α-ketoglutarate and NADP+ or be converted to citrate for fatty acid biosynthesis (37). IDH2 also generates mitochondrial NADPH that is important for protecting cells from mitochondrial reactive oxygen species (ROS) (37).

Fig 4: Function of wildtype and mutant IDH enzymes.

Wild type isocitrate dehydrogenases catalyze reversible conversion between isocitrate and α-ketoglutarate (a-KG). IDH1 is present in the cytosol, and IDH2 and IDH3 localize in the mitochondria. In the majority of enchondromas, somatic mutations of IDH1 or IDH2 are present in chondrocytes. mut-IDH1 and mut-IDH2 lost their original function and gain a neomorphic function that converts α-ketoglutarate to D-2-hydroxyglutarate (D-2HG).

Somatic mutations in IDH1 and IDH2 occur at specific arginine residues in the active sites of the enzymes: IDH1-R132 and IDH2-R140, -R172 (38). They are mostly missense mutations and cause an amino acid substitution. The percentage of different point mutations differs by tumor types. Chondrosarcoma has a broader range of IDH1 mutations than previously identified in other tumor types (6). IDH1^R132C is the most frequent mutation in enchondromas and chondrosarcomas (11–13). Mutant IDH enzymes have impaired activity of producing α-ketoglutarate and NADPH as well as the reverse reductive carboxylation, and acquire a neomorphic activity that reduces α-ketoglutarate to D-2-hydroxyglutarate (D-2HG) in a manner that consumes NADPH (34, 39–41) (Fig 4).

In a genetically modified mouse model, postnatally inducing mutant IDH1^R132Q, a mutation identified in a human chondrosarcoma sample, in chondrocytes was sufficient to initiate enchondroma-like lesion formation in mice (6). The lesions were adjacent to the growth plates and persisted in the animals for at least 6 months after induction of IDH1^R132Q expression without changes in number or size. Immunohistochemistry of type ten collagen was positive in the lesions (6). Animals expressing IDH1^R132Q in chondrocytes during embryonic development showed disrupted growth plate structure and defects in chondrocyte mineralization and cartilage remodeling, supporting that expression of mutant IDH1^R132Q caused defects in chondrocyte terminal differentiation (6). The phenotypes of these animals resembled the ones with Gli2-overexpression, suggesting mutant IDH1 may regulate chondrocyte differentiation via altered Hedgehog signaling activity (6).

The mechanism by which mutant IDH1/2 regulate enchondroma biology are incompletely understood. Studies of other cancers suggest mutant IDH1/2 regulate cells via the putative “oncometabolite” D-2-hydroxyglutarate (D-2HG). D-2HG is a metabolite that normally present in cells at low levels and recycled by 2-HG dehydrogenases that convert them back to α-ketoglutarate (42). Administration of D-2HG or transfection of mutant IDH1/2 was sufficient to increase a wide range of histone methylation (43–45). D-2HG potently inhibited Jumonji-domain-containing family of histone demethylase such as lysine demethylases 2A, 3A, 4A, and 4C (43, 44). IDH1/2 mutations were also associated with DNA hypermethylation in gliomas and AML (44, 46, 47). Introduction of IDH1/2 mutations was sufficient to cause DNA hypermethylation in normal human astrocytes, 293T cells, hematopoietic progenitor cells, and human mesenchymal stem cells (46–48). This was thought to occur through D-2HG competitively inhibiting ten-eleven translocation (TET) family 5-methylcytocine hydroxylase (44, 46–48). D-2HG is believed to exert its effects by competitively inhibiting enzymes that require α-ketoglutarate as a cofactor because the chemical structure of D-2HG is similar to α-ketoglutarate. In almost all the tumors with IDH1/2 mutations (including enchondromas), D-2HG production exceeded its elimination and could accumulate up to millimolar concentrations (40, 49, 50), hence mutant IDH1/2 could potentially regulate enchondroma formation via D-2HG.

In the context of chondrocyte differentiation and cartilage neoplasia, mutant IDH1 is believed to regulate chondrocyte and osteoblast differentiation via mechanisms dependent and independent of D-2HG. Jin et al. showed that expressing mutant IDH1^R132C in human mesenchymal stem cells enhanced chondrocyte-related genes and suppressed osteogenic genes through histone modifications (48). The induction of IDH1^R132C in mesenchymal stem cells caused significant increases in active histone modification marker H3K4me3 in the promoter region of chondrocyte-related genes SRY-Box transcription factor 9 (SOX9) and collagen type II alpha 1 chain (COL2A1) and suppressive histone modification marker H3K9me3 in the promoter of the osteoblast-related gene alkaline phosphatase (ALPL) (48). The changes were associated with defects in human mesenchymal stem cells’ ability of forming cartilage pellet and disturbed mineralization after osteogenic induction (48). D-2HG treatment induced similar defects in osteogenic differentiation in human mesenchymal stem cells, suggesting IDH1 mutation might regulate osteogenesis via D-2HG production (51). However, D-2HG treatment did not cause consistent effects on chondrocyte differentiation, suggesting IDH1 mutation might regulate chondrocyte differentiation via mechanisms other than D-2HG production (51). A recent study suggested that mutant IDH1 might regulate enchondromatosis through increased cholesterol biosynthesis. RNA sequencing in primary murine costal chondrocytes revealed that genes in the cholesterol biosynthesis pathway were upregulated upon expression of mutant Idh1 (52). Deleting Sterol Regulatory Element-Binding Protein Cleavage-Activating Protein (SCAP), a positive regulator of the cholesterol synthesis pathway, in IDH1^R132Q-KI mice significantly reduced formation of enchondromas (52). The authors also found that the cell viability and tumor growth of chondrosarcomas were sensitive to lovastatin treatment, a drug that suppressed intracellular cholesterol biosynthesis (52).

Progression to chondrosarcoma

When enchondromas occur as multiple lesions, they have a high chance to progress to malignant central chondrosarcoma. Disrupted signaling pathways that are implied in enchondromas also play roles in chondrosarcomas. Thus, mechanisms by which these signaling pathways regulate chondrosarcomas may be relevant in conditions in enchondromas.

Active Hedgehog signaling is reported in chondrosarcomas. Whole-exome sequencing revealed that 18% of chondrosarcoma tumors harbored mutations in the IHH signaling pathway resulting in IHH activation (53). Suppressing Hedgehog signaling decreased proliferation, cellularity, and tumor size of chondrosarcomas (30). Deleting p53 in Gli2 overexpressing chondrocytes predisposed the mice to develop larger cartilage lesions that resembled malignant chondrosarcomas (54). Deficiency in p53 and overactivation of Gli2 mediated apoptosis in neoplastic chondrocytes via insulin growth factor (IGF) signaling (54). Furthermore, Igf2 deficiency resulted in fewer cartilage lesions in Gli2 overexpressing mice (54). Although preclinical studies showed that active Hedgehog signaling was critical for chondrosarcoma tumor growth, Hedgehog pathway antagonists (IPI-926 and GDC-0449) did not reach the clinical endpoints in two independent clinical trials for chondrosarcomas (55, 56).

Mutations in IDH1/2 are present in about 50% of chondrosarcoma tumors (11, 13). Li et al. found that knocking out mutant IDH1 in chondrosarcoma cells suppressed tumorigenicity in vitro and in vivo (57). The authors showed that deleting mutant IDH1 caused downregulation of multiple integrin genes. Blocking integrin α5β1 and αvβ5 in IDH1 mutant chondrosarcoma cells decreased cells’ migration in vitro. Hence they speculated that mutant IDH1 might regulate the cell – extracellular matrix interaction of chondrosarcomas by altering integrin gene expression (57). Integrin signaling has been shown to be important for chondrocyte differentiation, hence it may also be a potential mechanism regulating enchondroma formation (58). Increased glutamine metabolism is another potential mechanism by which IDH mutations regulate tumorigenicity (59). Glutamine was shown to be the primary carbon source for D-2HG production in chondrosarcoma (60). Chondrosarcomas were reported to rely on glutamine metabolism for viability (59). However, the expression level of glutaminase was significantly higher in high-grade chondrosarcomas when compared to enchondromas and low-grade chondrosarcomas, suggesting glutamine metabolism might play different roles at different stages of tumor development (59).

There are conflicting data on whether pharmacologic blockade of D-2HG production could suppress the neoplastic phenotype in established cartilage tumors. One study using two cell lines found that pharmacologic inhibition of D-2HG production inhibited chondrosarcoma cell viability (61), but a second study found that while pharmacologic targeting did lower D-2HG levels, it did not affect chondrosarcoma cell viability (62). In contrast, pharmacologic D-2HG blockade has an anti-cancer effect in AML, and has been now approved for patient use (63). Acute myeloid leukemic cells can derive from progenitor cells in myelodysplastic syndromes (64), and D-2HG blockade could inhibit the initiation of new leukemic cells from progenitors, thus having an anticancer effect by blocking tumor initiation. Taken together these suggest D-2HG plays an important role in tumor initiation, but it may not be as critical in tumor cell maintenance or tumor progression. This concept likely explains the divergent reported data, as the experimental design may alter the results of pre-clinical studies. It is likely that in cartilage tumors, once epigenetic changes occur from D-2HG production, further neoplastic cell maintenance is regulated by other mechanism than D-2HG production itself. In support of this notion, recent work in astrocyte cells and gliomas demonstrated that many chromatin reprogramming changes due to IDH1 mutations were irreversible (65), suggesting that D-2HG blockade would not suppress the malignant behavior of cells once a tumor has formed. Importantly, some patients with chondrosarcoma have been treated with IDH inhibitors. The results to date have been variable, showing at best stabilization of the disease (66), supporting the notion that pharmacologic inhibition is not effective in established cartilage tumors.

Other mutations in enchondroma

In addition to mutations in IDH1/2 genes and genes in the Hedgehog signaling pathway, other mutations have also been reported in enchondroma tumors. Using whole-genome sequencing and target exon sequencing, Totoki et al. reported that 13 / 41 (31.7%) of enchondroma samples harbored somatic missense mutation of COL2A1, a gene encoding alpha 1 chain of type II collagen, an essential component for the cartilage extracellular matrix (67). They also found that 2.4% of the enchondroma tumors had a non-sense somatic mutation in YEATS2, a gene encoding for a subunit of an acetyltransferase complex (67). Somatic missense mutations in ACVR2A, a gene encoding a type II receptor for activin / bone morphogenic protein (BMP) signaling, were found in 7.3% of enchondroma cases (67). In 2019, Saiji et al. identified a germline missense mutation of the tumor suppressor gene serine/threonine kinase 11 (STK11) in a patient with enchondromatosis (68). As previous studies showed that mice lacking Stk11 in chondrocytes failed to terminal differentiate and could develop enchondroma-like lesions, this study suggested STK11 could play similar role in tumorigenicity of enchondromatosis in human (69, 70).

Disrupting regulatory pathways of chondrocyte maturation leads to defects in chondrocyte terminal differentiation and formation of enchondroma-like lesions

In the past few decades research has unraveled our understanding of the regulation of chondrocyte maturation and growth plate development. Disruption of some of these regulatory pathways led to defects of chondrocyte differentiation, and sometimes formation of enchondroma-like lesions in mice. Multiple groups have demonstrated that Mitogen-activated protein kinase / extracellular-signal-regulated kinase (MAPK/ERK) signaling pathway plays important roles in regulating chondrocyte differentiation. In 2014, Chen et al. showed that conditionally deleting Erk2 in Osx-expressing cells in Erk1^−/− mice caused expanded hypertrophic zone during development and formation of enchondroma-like lesions after birth (71). The effects were not present when the authors conditionally deleted Erk2 in Col1a1-expressing cells, suggesting the phenotypes were resulted from defects in terminal differentiation of hypertrophic chondrocytes due to the loss of Erk1 and Erk2 in chondrocytes (71). The authors suggested that Erk1/2 regulated chondrocyte terminal differentiation through the transcription factor early growth response 1/2 (Egr1/2) (71). The tyrosine phosphatase Src homology region 2 domain-containing phosphatase-2 (SHP2), encoded by the Ptpn11 gene, is a ubiquitously expressed non-receptor protein tyrosine phosphatase that is characterized by a Src-homology-2 domain. SHP2 is known to activate ERK1/2 pathway as deletion of Ptpn11 resulted in reduced level of phospho-ERK1/2, but the precise targets remain controversial (72, 73). Bowen et al. showed that deleting Ptpn11 in chondrocytes postnatally disrupted the organization of growth plate and induced formation of enchondroma-like lesions in the mice (74). In 2017, Wang et al. demonstrate that ablating Ptpn11 in Co10a1-expressing hypertrophic chondrocytes impaired transdifferentiation of chondrocytes to osteoblasts via enhanced Sox9 expression (75).

The tumor suppressor retinoblastoma protein pRB is important for proper chondrocyte development. pRB, p107, and p130 are members of the pocket protein family (76). They regulated proliferation via interactions with E2F family of transcription factors (76). Conditionally deleting Rb in p107^−/− mesenchymal stem cells caused defects in long bone development (77). Embryos of these Rb^CKO;p107^−/− animals had expanded and disorganized proliferating and hypertrophic zones at E17.5 (77). These animals developed enchondroma-like lesions at 6 months (77). The enchondroma phenotypes of these animals were mediated through E2f3a or E2f3b (77). Along the same line, it was reported that combinatorial deletion of p107 and p130 resulted in deregulated chondrocyte proliferation and defects in hypertrophic chondrocyte differentiation (78, 79).

Other signaling pathways and genes have also been shown to regulate chondrocyte differentiation and disruption of these pathways could cause enchondroma formation. In 2015, Zhou et al. showed that fibroblast growth factor receptor 3 (Fgfr3) deficiency in chondrocytes led to dysregulation of growth plate chondrocytes and formation of enchondromas and osteochondromas (80). These cartilage lesions displayed impaired MAPK signaling activation and enhanced Ihh and Pthlp expression (80). Smoothened inhibitor treatment significantly reduced chondroma-formation in the Fgfr3-deficiency mice, further suggesting that FGFR3 regulated chondrocyte maturation and chondroma formation via overactivation of Hedgehog signaling (80). The tumor suppressor phosphatase and tensin homolog (PTEN) also has a regulatory role of chondrocyte maturation. Targeted deletion of Pten in chondrocytes caused severe dyschondroplasia and formation of cartilage lesions, however, these neoplasms disappeared at 6 months of age (81). The authors showed that Pten deficiency was associated with increased ER stress and activation of Hif1α signaling (81).

Conclusion

Several signaling pathways that dysregulate chondrocyte differentiation play a role in enchondromas. Some of these dysregulated pathways are associated with known genetic mutations in human tumors. Distinct from metaphyseal chondrodysplasia, only a proportion of chondrocytes are affected in the condition of enchondroma. The reasons for this are not known. Possibilities include that other pathways are important for the development of enchondroma, hence only a subset of cells in which these signaling cascades are active eventually become enchondroma. Enchondroma may be derived from only selected cell types in the growth plate. Growth plate and skeletal stems reside in the growth plate (82–86).It is possible that dysregulation in only a specific subpopulation of skeletal stem cells gives rise to enchondroma. The somatic mutations might need to be restricted to a small proportion of cells, otherwise it might be so severe of a phenotype that animals could not survive.

Findings from the study of enchondromas may have generalized applicability in the context of cartilage and growth plate. The recent study of cholesterol biosynthesis in enchondromas suggest normal and well-controlled cholesterol level is critical for chondrocyte differentiation and homeostasis (52). This echoes with studies of the role of cholesterol in achondroplasia (87), osteoarthritis (88), an long bone growth (89).

Potential therapies for enchondromas may be developed from targeting the mutant IDH1/2 enzymes as well as downstream pathways that are deregulated by these mutations. Techniques such as metabolomics and single-cell RNA sequencing could provide candidates for pharmaceutical targeting. Future work in this field could focus on further elucidating pathways that are deregulated in enchondromas, identifying cell of origin in enchondromas, and understanding what regulates malignant progression in enchondromatosis.