Autosomal recessive primary microcephaly (MCPH), a rare neurodevelopmental disorder, is characterized by reduced head circumference of at least 4 standard deviations below the age- and sex- related population specific means; associated with non progressive mental retardation of variable degree but no other neurological deficit. In individuals with MCPH phenotype, the brain weight is markedly reduced and the cerebral cortex is disproportionately small with no major abnormalities in cortical architecture [1, 2]. MCPH is genetically heterogeneous with six loci (MCPH1–MCPH6) mapped to date [3–8] while four of the corresponding genes [Microcephalin (MIM-607117) at MCPH1, CDK5RAP2 (MIM-608201) at MCPH3, ASPM (MIM-605481) at MCPH5 and CENPJ (MIM-609279) at MCPH6] have been identified [9–11].
The MCPH5 is most prevalent in patients with primary microcephaly world wide and at least 30 pathogenic sequence variants in the corresponding gene ASPM (abnormal spindle like microcephaly associated) are known to date without any significant correlation between position of mutation and severity of microcephaly [12–16]. ASPM has preferential expression in neuroepithelium of the lateral ventricles during neurogenic cycle and its various isoforms with different number of IQ motifs have been detected in numerous tissues . Four mutations have been reported in Microcephalin gene at MCPH1 locus, causing MCPH and an allelic form, PCC (Premature chromosome condensation) syndrome (PCC; 606858), characterized by microcephaly, short stature, and misregulated chromosome condensation [9, 18–20]. Microcephalin functions as a proximal factor in the DNA damage checkpoints that control multiple damage sensors and early mediators and also implicated in cell cycle checkpoints, controlling and regulating other important molecules and thus affecting the timing of mitosis; its depletion abolishes the DNA damage response and results in centrosomal abnormalities and chromosomal aberrations [21–23]. Three mutations in the CENPJ (centromere associated protein J) gene at MCPH6 locus are known in Brazilian and Pakistani families with autosomal recessive primary microcephaly [11, 24]. CENPJ encodes a centrosomal protein which is associated with γ-tubulin ring complex and in-vitro evidence suggested its role in inhibition of microtubule nucleation . Therefore, a role for CENPJ in centrosome duplication at the beginning of mitosis has been proposed .
Mutations in Cyclin-dependent protein kinase 5, regulatory subunit associated protein 2, (CDK5RAP2) gene at MCPH3 locus are least involved in causing MCPH phenotype. To date only two homozygous mutations (243T>A, S81X) and (IVS26-15A>G) have been reported in this gene in primary microcephaly patients . The gene CDK5RAP2 has a genomic size of 191290 bps and contains 34 exons, with a transcript length of 6230 bps and 1893 translated residues [11, 27]. The CDK5RAP2 gene has predicted to contain one N-terminal spindle associated domain and two potential chromosome segregation ATPase (SMC, structural maintenance of chromosomes) domains, which are known to play a role in the cohesion and condensation of chromosomes during mitosis . The gene product concentrates at spindles poles during prometaphase and metaphase [11, 29]. Its localization to the spindle poles of mitotic cells suggests involvement of a centrosomal mechanism during mammalian brain development . CDK5RAP2 has a potential role in the inhibition of centrosomal CDK5 during neurogenesis. CDK5 is a promiscuous protein kinase only functioning in the brain with neuron-specific roles in processes including neurogenesis, neural migration and neurodegeneration .
In the present study, we report the identification of a previously described nonsense mutation 243 T>A (S81X) in exon 4 of CDK5RAP2 gene in a Pakistani family of Kashmiri origin.