Genomic analysis of patients in a South Indian Community with autosomal dominant cortical tremor, myoclonus and epilepsy suggests a founder repeat expansion mutation in the SAMD12 gene

Autosomal Dominant Cortical Tremor, Myoclonus and Epilepsy is a non-progressive disorder characterized by distal tremors. Autosomal Dominant Cortical Tremor, Myoclonus and Epilepsy has been reported globally with different genetic predispositions of autosomal dominant inheritance with a high degree of penetrance. In south India, Autosomal Dominant Cortical Tremor, Myoclonus and Epilepsy has been reported in a large cohort of 48 families, in which the genetic defect was not identified. This report pertains to the whole-genome analysis of four individuals followed by repeat-primed PCR for 102 patients from a familial cohort of 325 individuals. All the patients underwent extensive clinical evaluation including neuropsychological examinations. The whole-genome sequencing was done for two affected and two unaffected individuals, belonging to two different families. The whole-genome sequencing analysis revealed the repeat expansion of TTTTA and TTTCA in intron 4 of the SAMD12 gene located on chromosome 8 in the patients affected with Autosomal Dominant Cortical Tremor, Myoclonus and Epilepsy, whereas the unaffected family members were negative for the similar expansion. Further, the repeat-primed PCR analysis of 102 patients showed the expansion of the TTTCA repeats in the intron 4 of SAMD12 gene. All patients registered for this study belong to a single community called "Nadar" whose nativity is confined to the southern districts of India, with reported unique genetic characteristics. This is the largest and most comprehensive single report on clinically and genetically characterized Autosomal Dominant Cortical Tremor, Myoclonus and Epilepsy patients belonging to a unique ethnic group worldwide.