Challenges of diagnostic exome sequencing in an inbred founder population

Exome sequencing was used as a diagnostic tool in a Roma/Gypsy family with three subjects (one deceased) affected by lissencephaly with cerebellar hypoplasia (LCH), a clinically and genetically heterogeneous diagnostic category. Data analysis identified high levels of unreported inbreeding, with multiple rare/novel "deleterious" variants occurring in the homozygous state in the affected individuals. Step‐wise filtering was facilitated by the inclusion of parental samples in the analysis and the availability of ethnically matched control exome data. We identified a novel mutation, p.Asp487Tyr, in the VLDLR gene involved in the Reelin developmental pathway and associated with a rare form of LCH, the Dysequilibrium Syndrome. p.Asp487Tyr is the third reported missense mutation in this gene and the first example of a change affecting directly the functionally crucial β‐propeller domain. An unexpected additional finding was a second unique mutation (p.Asn494His) with high scores of predicted pathogenicity in KCNV2, a gene implicated in a rare eye disorder, retinal cone dystrophy type 3B. This result raised diagnostic and counseling challenges that could be resolved through mutation screening of a large panel of healthy population controls. The strategy and findings of this study may inform the search for new disease mutations in the largest European genetic isolate

Expanding clinical presentations due to variations in THOC2 mRNA nuclear export factor

Multiple TREX mRNA export complex subunits (e.g., THOC1, THOC2, THOC5, THOC6, THOC7) have now been implicated in euro developmental disorders (NDDs), neurodegeneration and cancer. We previously implicated missense and splicing-defective THOC2 variants in NDDs and a broad range of other clinical features. Here we report 10 individuals from nine families with rare missense THOC2 variants including the first case of a recurrent variant (p.Arg77Cys), and an additional individual with an intragenic THOC2 microdeletion (Del-Ex37-38). Ex vivo missense variant testing and patient-derived cell line data from current and published studies show 9 of the 14 missense THOC2 variants result in reduced protein stability. The splicing-defective and deletion variants result in a loss of small regions of the C-terminal THOC2 RNA binding domain (RBD). Interestingly, reduced stability of THOC2 variant proteins has a flow-on effect on the stability of the multi-protein TREX complex; specifically on the other NDD-associated THOC subunits. Our current, expanded cohort refines the core phenotype of THOC2 NDDs to language disorder and/or ID, with a variable severity, and disorders of growth. A subset of affected individuals' has severe-profound ID, persistent hypotonia and respiratory abnormalities. Further investigations to elucidate the pathophysiological basis for this severe phenotype are warranted.Raman Kumar, Elizabeth Palmer, Alison E. Gardner, Renee Carroll, Siddharth Banka ... Jozef Gecz ... et al

Expanding Clinical Presentations Due to Variations in THOC2 mRNA Nuclear Export Factor

Multiple TREX mRNA export complex subunits (e.g., THOC1, THOC2, THOC5, THOC6, THOC7) have now been implicated in neurodevelopmental disorders (NDDs), neurodegeneration and cancer. We previously implicated missense and splicing-defective THOC2 variants in NDDs and a broad range of other clinical features. Here we report 10 individuals from nine families with rare missense THOC2 variants including the first case of a recurrent variant (p.Arg77Cys), and an additional individual with an intragenic THOC2 microdeletion (Del-Ex37-38). Ex vivo missense variant testing and patient-derived cell line data from current and published studies show 9 of the 14 missense THOC2 variants result in