A CCG expansion in ABCD3 causes oculopharyngodistal myopathy in individuals of European ancestry

Individuals affected by inherited neuromuscular diseases often present with a specific pattern of muscle weakness, which can guide clinicians in genetic investigations and variant interpretation. Nonetheless, more than 50% of cases do not receive a genetic diagnosis. Oculopharyngodistal myopathy (OPDM) is an inherited myopathy manifesting with a particular combination of ptosis, dysphagia and distal weakness. Pathologically it is characterised by rimmed vacuoles and intranuclear inclusions on muscle biopsy. In recent years GCC • CCG repeat expansion in four different genes have been identified in individuals affected by OPDM in Asian populations. None of these have been identified in affected individuals of non-Asian ancestry. In this study we describe the identification of CCG expansions in ABCD3 in affected individuals across eight unrelated OPDM families of European ancestry. In two large Australian OPDM families, using a combination of linkage studies, short-read WGS and targeted ONT sequencing, we identified CCG expansions in the 5’UTR of ABCD3 . Independently, the ABCD3 CCG expansion was identified through the 100,000 Genomics England Genome Project in three individuals from two unrelated UK families diagnosed with OPDM. Targeted ONT sequencing confirmed the presence of mono-allelic CCG repeat expansions ranging from 118 to 694 repeats in all tested cases ( n =19). The expansions were on average 1.9 times longer in affected females than affected males, and children of affected males were ∼2.3 times more likely to have the disease than those of affected females, suggesting inheritance of an expanded allele from an affected mother may have reduced penetrance. ABCD3 transcripts appeared upregulated in skeletal muscle and cells derived from affected OPDM individuals, suggesting a potential role of over-expression of CCG repeat containing ABCD3 transcript in progressive skeletal muscle degeneration. The study provides further evidence of the role of non-coding repeat expansions in unsolved neuromuscular diseases and strengthens the association between the GCC • CCG repeat motif and a specific pattern of muscle weakness with prominent cranial involvement across different populations. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This project was supported by an NHMRC Ideas Grant (APP2002640) to G.R, N.G.L, M.R.D., P.J.Lamont and M.C-S and Medical Research Council (MR/T001712/1), Fondazione Cariplo (grant n. 2019-1836), the Inherited Neuropathy Consortium, Fondazione Regionale per la Ricerca Biomedica (Regione Lombardia, project ID 1751723) and Italian Ministry of Health (Ricerca Corrente 2021-2022) to A.C. C.K.S. and L.D. are supported by an Australian Government Research Training Program (RTP) Scholarship, I.W.D. is supported by an MRFF Investigator Grant (MRF1173594) and G.R. is supported by an NHMRC EL2 Investigator Grant (APP2007769). M.B. is supported by an NHMRC L1 Investigator Grant (APP1195236), This work was also supported by the Australian State of Victoria Government Operational Infrastructure Support Program, and the NHMRC Independent Research Institute Infrastructure Support Scheme (IRIISS). This work was supported by resources provided by the Pawsey Supercomputing Research Centre with funding from the Australian Government and the Government of Western Australia. Library preparation and RNA-sequencing was conducted in the Genomics WA Laboratory in Perth, Australia. This facility is supported by BioPlatforms Australia, State Government Western Australia, Australian Cancer Research Foundation, Cancer Research Trust, Harry Perkins Institute of Medical Research, Telethon Kids Institute and the University of Western Australia. We gratefully acknowledge the Australian Cancer Research Foundation and the Centre for Advanced Cancer Genomics for making available Illumina Sequencers for the use of Genomics WA. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study was approved by the Human Research Ethics Committee of the University of Western Australia (RA/4/20/1008) the Human Research Ethics Committee of the Royal Children Hospital (HREC 28097) and Northeast-Newcastle and North Tyneside 1 Research Ethics Committee (22/NE/0080) I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors