Novel candidate genes and variants underlying autosomal recessive neurodevelopmental disorders with intellectual disability

2018
Identification of Mendelian genes for neurodevelopmental disordersusing exome sequencingto study autosomal recessive (AR) consanguineouspedigrees has been highly successful. To identify causal variants for syndromic and non-syndromic intellectual disability (ID), exome sequencingwas performed using DNA samples from 22 consanguineousPakistani families with ARID, of which 21 have additional phenotypes including microcephaly. To aid in variant identification, homozygosity mapping and linkage analysis were performed. DNA samples from affected family member(s) from every pedigree underwent exome sequencing. Identified rare damaging exomevariants were tested for co-segregation with ID using Sanger sequencing. For seven ARIDfamilies, variants were identified in genes not previously associated with ID, including: EI24, FXR1 and TET3 for which knockout mousemodels have brain defects; and CACNG7 and TRAPPC10 where cell studies suggest roles in important neural pathways. For two families, the novel ARIDgenes CARNMT1 and GARNL3 lie within previously reported ID microdeletion regions. We also observed homozygous variants in two ID candidate genes, GRAMD1B and TBRG1, for which each has been previously reported in a single family. An additional 14 families have homozygous variants in established ID genes, of which 11 variants are novel. All ARIDgenes have increased expression in specific structures of the developing and adult human brain and 91% of the genes are differentially expressed in utero or during early childhood. The identification of novel ARID candidate genesand variants adds to the knowledge base that is required to further understand human brain function and development.
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