The Spectrum of MORC2-Related Disorders: A Potential Link to Cockayne Syndrome

Spectrum of ERCC6-Related Cockayne Syndrome (Type B): From Mild to Severe Forms

(1) Background: Cockayne syndrome (CS) is an ultra-rare multisystem disorder, classically subdivided into three forms and characterized by a clinical spectrum without a clear genotype-phenotype correlation for both the two causative genes ERCC6 (CS type B) and ERCC8 (CS type A). We assessed this, presenting a series of patients with genetically confirmed CSB. (2) Materials and Methods: We retrospectively collected demographic, clinical, genetic, neuroimaging, and serum neurofilament light-chain (sNFL) data about CSB patients; diagnostic and severity scores were also determined. (3) Results: Data of eight ERCC6/CSB patients are presented. Four patients had CS I, three patients CS II, and one patient CS III. Various degrees of ataxia and spasticity were cardinal neurologic features, with variably combined systemic characteristics. Mean age at diagnosis was lower in the type II form, in which classic CS signs were more evident. Interestingly, sNFL determination appeared to reflect clinical classification. Two novel premature stop codon and one novel missense variants were identified. All CS I subjects harbored the p.Arg735Ter variant; the milder CS III subject carried the p.Leu764Ser missense change. (4) Conclusion: Our work confirms clinical variability also in the ERCC6/CSB type, where manifestations may range from severe involvement with prenatal or neonatal onset to normal psychomotor development followed by progressive ataxia. We propose, for the first time in CS, sNFL as a useful peripheral biomarker, with increased levels compared to currently available reference values and with the potential ability to reflect disease severity.

Defining the progeria phenome

Progeroid disorders are a heterogenous group of rare and complex hereditary syndromes presenting with pleiotropic phenotypes associated with normal aging. Due to the large variation in clinical presentation the diseases pose a diagnostic challenge for clinicians which consequently restricts medical research. To accommodate the challenge, we compiled a list of known progeroid syndromes and calculated the mean prevalence of their associated phenotypes, defining what we term the 'progeria phenome'. The data were used to train a support vector machine that is available at https://www.mitodb.com and able to classify progerias based on phenotypes. Furthermore, this allowed us to investigate the correlation of progeroid syndromes and syndromes with various pathogenesis using hierarchical clustering algorithms and disease networks. We detected that ataxia-telangiectasia like disorder 2, spastic paraplegia 49 and Meier-Gorlin syndrome display strong association to progeroid syndromes, thereby implying that the syndromes are previously unrecognized progerias. In conclusion, our study has provided tools to evaluate the likelihood of a syndrome or patient being progeroid. This is a considerable step forward in our understanding of what constitutes a premature aging disorder and how to diagnose them.

A Novel Heterozygous De Novo MORC2 Missense Variant Causes an Early Onset and Severe Neurodevelopmental Disorder

Microrchidia CW-type zinc finger protein 2 (MORC2) is an ATPase-containing nuclear protein which regulates transcription through chromatin remodelling and epigenetic silencing. MORC2 may have a role in the development of neurones, and dominant variants in this gene have recently been linked with disorders including Charcot-Marie-Tooth type 2Z disease, spinal muscular atrophy and, more recently, a neurodevelopmental syndrome consisting of developmental delay, impaired growth, dysmorphic facies, and axonal neuropathy (DIGFAN), presenting with hypotonia, microcephaly, brain atrophy, intellectual disability, hearing loss, faltering growth, and craniofacial dysmorphism. Notably, variants in MORC2 have shown clinical features overlapping with those of Cockayne and Leigh syndromes. Here, we report a case of MORC2-related DIGFAN syndrome in a female infant caused by a novel heterozygous de novo variant. The condition was early onset and severe, further expanding the range of genotypes associated with this disorder. Clinical features included unilateral hearing loss, developmental delay and regression within the first year of life, microcephaly, severe feeding difficulties, and faltering growth, resulting in death at 13 months of age.

Phenotype Presentation and Molecular Diagnostic Yield in Non-5q Spinal Muscular Atrophy

Background and Objectives

Spinal muscular atrophy (SMA) is mainly caused by homozygous SMN1 gene deletions on 5q13. Non-5q SMA patients' series are lacking, and the diagnostic yield of next-generation sequencing (NGS) is largely unknown. The aim of this study was to describe the clinical and genetic landscape of non-5q SMA and evaluate the performance of neuropathy gene panels in these disorders.

Methods

Description of patients with non-5q SMA followed in the different neuromuscular reference centers in France as well as in London, United Kingdom. Patients without a genetic diagnosis had undergone at least a neuropathy or large neuromuscular gene panel.

Results

Seventy-one patients from 65 different families were included, mostly sporadic cases (60.6%). At presentation, 21 patients (29.6%) showed exclusive proximal weakness (P-SMA), 35 (49.3%) showed associated distal weakness (PD-SMA), and 15 (21.1%) a scapuloperoneal phenotype (SP-SMA). Thirty-two patients (45.1%) had a genetic diagnosis: BICD2 (n = 9), DYNC1H1 (n = 7), TRPV4 (n = 4), VCP, HSBP1, AR (n = 2), VRK1, DNAJB2, MORC2, ASAH1, HEXB, and unexpectedly, COL6A3 (n = 1). The genetic diagnostic yield was lowest in P-SMA (6/21, 28.6%) compared with PD-SMA (16/35, 45.7%) and SP-SMA (10/15, 66.7%). An earlier disease onset and a family history of the disease or consanguinity were independent predictors of a positive genetic diagnosis. Neuropathy gene panels were performed in 59 patients with a 32.2% diagnostic yield (19/59). In 13 additional patients, a genetic diagnosis was achieved through individual gene sequencing or an alternative neuromuscular NGS.

Discussion

Non-5q SMA is genetically heterogeneous, and neuropathy gene panels achieve a molecular diagnosis in one-third of the patients. The diagnostic yield can be increased by sequencing of other neuromuscular and neurometabolic genes. Nevertheless, there is an unmet need to cluster these patients to aid in the identification of new genes.