Whole exome sequencing identifies a novel mutation in ASPM and ultra-rare mutation in CDK5RAP2 causing Primary microcephaly in consanguineous Pakistani families.

Ehtisham Ul Haq Makhdoom,Haseeb Anwar,Shahid Mahmood Baig,Ghulam Hussain

doi:10.12669/pjms.38.1.4464

Abstract

ABSTRACTBackground & Objectives:Primary Microcephaly (MCPH) is a rare neurogenetic disease, manifesting congenitally reduced head circumference and non-progressive intellectual disability (ID). To date, twenty-eight genes with biallelic mutations have been reported for this disorder. The study aimed for molecular genetic characterization of Pakistani families segregating MCPH.Methods:We studied two unrelated consanguineous families (family A and B) presenting >2 patients with diagnostic symptoms of MCPH, born to asymptomatic parents. We employed whole-exome sequencing (WES) of probands to find putative causal mutations. The candidate variants were further confirmed and analyzed for co-segregation by Sanger sequencing of all available members of each family. This study was conducted at Government College University, Faisalabad, Pakistan, and Cologne Center for Genomics (CCG), University of Cologne, Germany; during 2017-2020.Results:We identified a novel homozygous variant c.10097_10098delGA, p.(Gly3366Glufs*19) in exon 26 of ASPM gene in family A which presents with moderate intellectual disability, speech impairment, visual abnormalities, seizures, and ptyalism. Family B was found to segregate nonsense, homozygous variant c.448C>T p.(Arg150*) in CDK5RAP2. The patients also exhibited mild to severe seizures without ptyalism that has not been previously reported in patients with mutations in the CDK5RAP2 gene.Conclusion:We report a novel mutation in ASPM and ultra-rare mutation in the CDK5RAP2 gene, both causing primary microcephaly. The study expands the mutational spectrum of the ASPM gene to 212, and also adds to the clinical spectrum of CDK5RAP2 mutations. It also demonstrated the utility of WES in the investigation and genetic diagnosis of genetically heterogeneous disorders like MCPH. These findings would aid in diagnostic and preventive strategies including carrier screening, cascade testing, and genetic counselling.

Highlights

Autosomal recessive primary microcephaly (MCPH, MIM251200) refers to congenitally reduced occipitofrontal circumference/ head circumference (HC) by >3 standard deviation (SD) as compared to mean for same age and gender along with sloping forehead and non-progressive cognitive impairment
26 genes have been reported for this disorder that include, Microcephalin, WDR62, CDK5RAP2, CASC5, ASPM, CENPJ, STIL, CEP135, CEP152, ZNF335, PHC1, CDK6, CENPE, SASS6, MFSD2A, ANKLE2, CIT, WDFY3, COPB2, KIF14 NCAPD2, NCAPD3, NCAPH, NUP37, RRP7A and recently, MAP114 Products of these genes function in diverse cellular processes critical for optimal neurogenesis such as DNA repair, cell cycle regulation, centrosome maturation, position, duplication, spindle orientation, and positioning.[5]
Clinical features: Family A originated from District Swat of Pakistan and is comprised of nine siblings born to consanguineous parents (Fig.1A)

Summary

Introduction

Autosomal recessive primary microcephaly (MCPH, MIM251200) refers to congenitally reduced occipitofrontal circumference/ head circumference (HC) by >3 standard deviation (SD) as compared to mean for same age and gender along with sloping forehead and non-progressive cognitive impairment. It is the manifestation of insufficient growth of the foetal brain, mainly. The study expands the mutational spectrum of the ASPM gene to 212, and adds to the clinical spectrum of CDK5RAP2 mutations It demonstrated the utility of WES in the investigation and genetic diagnosis of genetically heterogeneous disorders like MCPH. These findings would aid in diagnostic and preventive strategies including carrier screening, cascade testing, and genetic counselling

Methods

Results

Discussion

Conclusion