Identification of Mendelian genes for neurodevelopmental disorders using
exome sequencing to study autosomal recessive (AR) consanguineous pedigrees has
been highly successful. To identify causal variants for syndromic and
non-syndromic intellectual disability (ID), exome sequencing was performed using
DNA samples from 22 consanguineous Pakistani 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 exome variants were tested for
co-segregation with ID using Sanger sequencing. For seven ARID families,
variants were identified in genes not previously associated with ID, including:
EI24, FXR1 and TET3 for which knockout
mouse models have brain defects; and CACNG7 and
TRAPPC10 where cell studies suggest roles in important
neural pathways. For two families, the novel ARID genes 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
ARID genes 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 genes and variants adds to the knowledge base that is
required to further understand human brain function and development.