Identification of genes and mutations involved in primary microcephaly and inherited limb disorders in Pakistani families

Abstract

Numerous genetic conditions have been described clinicall y but the molecular etiology for most of them is sti ll unknown. With th e advancement in the fie ld of molecular biology powerful techniques have been developed to locali ze these conditions in the human genome and subsequent identification of causative genes. Functional analysis of causative genes leads to the discovery and understand ing of novel genetic processes and pathways underlying disease conditions including normal developmental pathways. Li nkage analysis studi es in Mendelian disorders to identify the causative genes and mutations are possible using large pedigrees with multiple affected individuals. Analysis of alleles using microsatellite markers and genome wide SNPs lead to the discovery of novel genes and loci for specific disorders. The main aim of this thesis was to analyze families with autosomal recessive primary microcephaly and families with inherited limb disorders parti cularly polydactyl, syndactyly and brachydactyly to identify the causative mutations or chromosomal loci. Autosomal recessive primary microcephaly (MCPH) is a neurogenic disorder characterized by reduced head circumference (::;4 SD) and variable degree of mental retardation without any other neurological man ifestations. The normal brain architecture is preserved despite the fact that brain size is reduced to three fo lds. In the first part of this study, genetic analysis of eleven primary microcephaly families was carried out. Linkage analysis using highl y polymorphic microsatellite markers confirmed linkage in six fam ilies to ASPM (MCPH5), two CENPJ (MCPH6), one MCPH2 locus and haplotype analysis in two families demonstrated compound heterozygosity for ASPM Sequencing of ASPM in six potentially linked families (MCP3, MCP6, MCP7, MCP9, MCP 11 and MCP 17) revealed six homozygous mutations in the affected subj ects (A1160fsI181X, Y2245fs2258X, R3233X, Y3164X, S3 186X, and R3244X respectively) and two possible compound heterozygous families (MCP35 and MCP 18) demonstrated compound heterozygous mutations (W 1326X/R31 07X and RIO 19X/Q2632X, respectively). Compound heterozygous patients (W 1326X & R31 07X) also have add itional clinical symptoms of seizures. Two families linked to MCPH6 locus (MCP2 1, MCP22) demonstrate Ibp deletion mutation c.17 _ 18de1C (T6fsX3) in exon 2 of CENP J lead ing to premature termination of protein. This mutation was previously reported in two Northern Pakistani fam ilies.