The field of CMT and related diseases has developed rapidly in recent years, with the discovery of a growing number of causal genes and an expanding phenotypic spectrum. MORC2-related axonal CMT disease was firstly described in 2016 as later onset CMT2 or early onset spinal muscular atrophy-like phenotype [2]. Recent studies on MORC2 mutations show a remarkable heterogeneity in clinical features and disease severity, ranging from axonal motor and sensory neuropathy to a complex multisystem disorder.
In this study, we identified 4 index patients in unrelated Chinese families with MORC2 mutations, accounting for 1.4% (4/284) in our cohort. A recently published study in North China has reported a MORC2 mutation frequency rate of 1.7% (2/115) [10]. Combining the data on the prevalence rate as identified in previous studies in Korea 2.6% (4/152) [9], Japan 2.7% (13/487) [15] and the present study, we find that the mean frequency distribution rate of MORC2 mutations in the East Asian population is 2.2% (23/1038). Our study confirmed that MORC2 is a CMT2 disease-causing gene of relatively high mutation rate. As an interesting aspect, the MORC2 mutations have shown a high rate of de novo events. In our four families, de novo mutations were confirmed in three families by testing the parents.
Review of previous literature, patients carrying MORC2 mutations showed a wide and complex phenotypic spectrum: (i) late onset or early onset axonal CMT [2, 8, 10, 13, 15];(ii) congenital or early onset SMA-like syndromes [2, 9, 13, 16]; (iii) axonal neuropathy plus CNS symptoms (pyramidal signs, seizures, leukomalacia, mental retardation, spinal cord atrophy, tremor, hearing loss, et al) [7, 9, 15]; (iiii) axonal neuropathy plus multisystemic disorders (cerebellar atrophy, diaphragmatic paralysis, nocturnal hypoventilation, scoliosis, dysmorphic face, et al)[12, 14]. These findings support a new and more comprehensive recognition of MORC2-related diseases as a multisystemic spectrum.
Our cohort study provided new evidence supporting this notion. Patients belonging to Family 1 showed an adult onset of chronic axonal motor and sensory neuropathy associated with asymptomatic hyperCKemia. The most frequent initial symptom was hand weakness, and the common clinical feature was distal limbs weakness and sensory loss. Previous reported clinical characteristics, proximal limbs involvement, prominent sensory disturbances and asymmetric impairment [2, 10] also presented in some patients within this family. Another unusual clinical presentation in all affected individuals manifested isolated asymptomatic hyperCKemia. Elevated CK concentration has been reported in axonal CMT with MPZ and NEFL mutations[20–23], but the cause of hyperCKemia remains unclear. Possible mechanisms could include impaired muscle membrane integrity caused by denervation deriving from axonal damage [20], or an altered or slowed muscle fiber type differentiation caused by inadequate nerve function [23]. In our family, the presence of spontaneous muscle activity at EMG (proband 1-IV9, patient 1-V2, patient 1-V3) and neuropathic changes in muscle biopsy (proband 1-IV9) confirmed acute denervation together with chronic neurogenic changes. Other possible causes of myopathy were ruled out, however, the possibility of a coincidental association between an idiopathic hyperCKaemia and MORC2 mutation cannot be excluded.
The patient in Family 2 presented with a delay in the acquisition of motor and mental milestones, then developed into a SMA-like disease with cerebellar hypoplasia and mental retardation. All reported patients (one Spanish and two Koreans) with the p.S87L mutation exhibited similar SMA-like phenotype [2, 9]. Schottmann et al. [12] and Zanni et al. [14] described respectively a Germany patient and a Italy patient with de novo p.T424R mutation associated with SMA-like neuropathy, cerebellar atrophy, diaphragmatic paralysis or nocturnal hypoventilation. Our report confirmed that patients carrying the p.S87L or p.T424R mutation shared a similar phenotype of SMA-like disease with complex syndrome. Sancho et al. observed the p.S87L mutation of MORC2 led to an increase in axonal swellings in neurons, which represented abnormal accumulation of axonal cargos and cytoskeletal proteins, directly affected the axonal transport systems through microtubules and motor proteins [24]. As a hallmark of axonal injury, which underlie the pathogenesis of the neuropathy and contribute to disease severity and progression.
The twin sisters in family 3 suffered from axonal motor neuropathy with high variability in disease severity and duration. One twin sister presented disease onset in early infancy with delay in the acquisition of motor milestones, never walked normally and never ran or jumped. A rapid progression was further noticed in foot deformity and distal muscular atrophy. Another twin sister described the first signs with foot deformity and slightly abnormal gait until 6 years of age, without any difficulty with walking, run and jump. It is speculated that the phenotypic differentiation of twins may be caused by epigenetic modification, such as DNA methylation, histone modification, and microRNAs-mediated regulation. Mechanisms of epigenetics regulate gene expression and display some level of phenotypic discordance.
The patient of family 4 showed a typical axonal motor and sensory neuropathy, developed early onset (first decade) and rapid progression. A reported Japanese patient with the same mutation showed almost the same phenotypes as follows: the age at onset, the most frequent initial symptom as cramps in the lower limbs, distal lower limb weakness and sensory loss during the initial examination[15].
Genotype–phenotype correlation is still unclear in MORC2-mutated patients. To explore the correlation between genotype and phenotype, we summarized all reported patients with MORC2-mutated phenotype (Fig. 4).
MORC2 is a 1032-amino acid (AA) protein predicted to contain several functional domains. The N-terminal catalytically active ATPase module is composed of Gyrase B, Hsp90, histidine kinase, and MutL (GHKL) and S5-fold domains (AA residues 1-469), which has been mechanistically linked to gene transcription and DNA repair by remodeling chromatin[3–4].The CW-type zinc finger domain(AA residues 490–544) participates in chromatin regulation through the recognition of epigenetic signals [4].The C-terminal chromo-like domain(AA residues 795–850) is commonly found in eukaryotic chromatin proteins and can recognize methylated peptides in histones and nonhistone proteins [25].The coiled-coil domains is suggested to be an important structural determinant for protein assembly and molecular recognition[25]. To date, four MORC2 mutations (p.S87L, p.R132L, p.E236G and p.R252W) are known to be located in the ATPase domain, one MORC2 mutation (p.R319H) is located in the first coiled-coil domain, other five known MORC2 mutations (p.Q400R, p.C407Y, p.T424R, p.A431V and p.D466N) and the novel mutation p. D466G in this study are located in the ribosomal protein S5 domain. All identified mutations associated with MORC2-related neuropathies are localized in the N-terminal ATPase module, which might be critical to MORC2 protein function.
It is interesting to note that the different mutations causing distinct changes in biochemical properties (Table 3) [6], which may be helpful to understand MORC2 mutations cause the complex range of clinical symptoms. The p.R252W mutation is supposed to be the most common mutation, and with a highly variable clinical features including late onset or early onset axonal CMT, axonal neuropathy plus CNS symptoms, and congenital or early onset SMA-like syndromes [2, 7, 8, 9, 15]. Recent study showed R252W mutation hyperactivated HUSH-mediated epigenetic silencing in neuronal cells, which weakened the regulatory ATPase–CW interaction [5]. Another known mutational hotspot is p.D466N, which is associated with axonal CMT and SMA-like syndromes [10, 13]. In our study, we identified a novel mutation p. D466G presented symptom of axonal CMT with hyperCKemia, which provided new evidence. This mutation caused destabilize ATPase similar to R252W. The other three mutations p.S87L, p.R319H and p.T424R presented with SMA-like syndromes or plus complex disorders [2, 9, 12, 14]. Some researches showed S87L caused kinetic stabilization of MORC2 dimers, whereas T424R increased the rate of dimer assembly and disassembly [5]. These two biochemical effects were more distinct and affected patients presenting with severe phenotype of early onset. The p.Q400R and p.C407Y mutations were both shown as typical axonal CMT[10, 15], otherwise p.R132L, p.E236G, p.A431V manifested as axonal neuropathy plus CNS symptoms [7, 9, 15].These mutations causing changes in biochemical is unclear, so more work will be needed to explore how MORC2 mutations cause the complex range of clinical symptoms.
In conclusion, we described four Chinese axonal CMT families caused by MORC2 mutations, accompanied by hyperCKemia, cerebellar hypoplasia and mental retardation. Our findings expand the ethnic, phenotypic, and genetic diversity of MORC2-related neuropathies. It should be considered in the diagnostic workup of unresolved cases of axonal neuropathy, SMA-like syndromes, and with other complex associated phenotype. MORC2 mutations are transmitted as an autosomal dominant trait, with a high rate of dominant de novo, further studies will elucidate the mechanism underlying the diversity of MORC2-related diseases.