Abstracts

Rationale: More than 60 genes are known to cause developmental and epileptic encephalopathies (DEEs), with mutations often appearing de novo in affected probands. Recently, 8.3% of probands were found to have a parent mosaic for the DEE-associated mutation, increasing the risk of recurrence in subsequent offspring. Conversely, detection of mosaicism in affected probands decreases parental reproductive risks. Mosaicism at low levels can be difficult to detect due to technological limitations; mosaicism at high levels is readily detectable but may be difficult to differentiate from heterozygous mutations. We sought to ascertain the frequency of mosaicism in probands with DEE and to describe the genotypic and phenotypic spectrum of these patients. Methods: A cohort of 114 trios, consisting of probands with DEE with a known mutation and their biological parents, were screened for mosaicism using single-molecule molecular inversion probes (smMIPs) and next-generation sequencing (NGS). Additional cases were ascertained by research testing (smMIPs or whole exome sequencing) and by diagnostic testing (direct sequencing and NGS panels). We performed detailed phenotypic analysis on each patient based on a validated seizure questionnaire and medical records. Results: Three/114 (2.6%) patients were mosaic for their heterozygous mutation. An additional 7 patients were identified through our ongoing research studies and diagnostic testing. Three patients were mosaic for their SCN1A mutation, and one patient was mosaic in each of ALG13, CDKL5, GNAO1, KIAA2022, PCDH19, SCN2A and STXBP1. The mean minor allele frequency across all tissue types was 18.05% (range 1.9-32%). Seven patients had missense mutations and 3 had truncation mutations. Sanger sequencing was performed in 6 patients; mosaicism was suspected in 3 but not detected in the remaining 3 (minor allele frequency range 17-26% and 25-32% respectively). All patients’ phenotypes were within the phenotypic spectrum of their genetic diagnosis, with 7/10 being just as severe as patients with non-mosaic heterozygous mutations of the same gene. Three patients displayed milder features than expected for their genetic diagnoses: later age at seizure onset (2), absence of characteristic seizure types (2), later onset of developmental delay (1), infrequent seizures at onset (1) and better cognitive outcome (1). However, one of these patients also displayed features that were more severe than expected for their diagnosis of Dravet syndrome with tonic seizures and 250-800 tonic-clonic seizures per year. Conclusions: Patients with a mosaic genetic cause for their DEE are not always less severe than their heterozygous peers. Finding a mosaic mutation in a patient with DEE has significant implications for parents with reduced reproductive risks, informing genetic counselling for these families. Funding: National Institute of Neurological Disorders and Stroke, National Health and Medical Research Council AUS, Health Research Council NZ, Cure Kids NZ, LGS Foundation, AES, Dravet Syndrome Foundation, Murdoch Children’s Research Institute and University of Melbourne.