Muhammad Wajid

Low prevalence of Connexin 26 (GJB2) variants in Pakistani families with autosomal recessive non‐syndromic hearing impairment

The Pakistani population has become an important resource for research on autosomal recessive non‐syndromic hearing impairment (ARNSHI) due to the availability of large extended and highly consanguineous pedigrees. Here is presented the first report on the prevalence of gap junction beta‐2 (GJB2) variants in Pakistan. One hundred and ninety‐six unrelated Pakistani families with ARNSHI were recruited for a study on the genetics of NSHI. DNA sequencing of the GJB2 coding region was done on two affected individuals per family. Evolutionary conservation and predicted effect on the protein product were studied in order to hypothesize whether or not a variant was potentially deleterious. Homozygous putatively functional GJB2 variants were identified in 6.1% of families. None of the putatively functional GJB2 variants were observed in the compound heterozygous state. The six putatively causative variants noted were 231G > A(W77X), 71G > A(W24X), 167delT, 95G > A(R32H), 358–360delGAG(delE120), and 269T > C(L90P), with 231G > A(W77X) and 71G > A(W24X) being the most common. In addition, five benign polymorphisms, 380G > A(R127H), 457G > A(V153I), 493C > T(R165W), 79G > A(V27I), and 341 A > G(E114G), were identified within this population. In a few individuals, benign polymorphisms were observed to occur on the same haplotype, namely [457G > A(V153I); 493C > T(R165W)] and [79G > A(V27I); 341 A > G(E114G)]. The spectrum of GJB2 sequence variants in Pakistan may reflect shared origins of hearing impairment alleles within the Indian subcontinent. The high degree of consanguinity within Pakistan may have maintained the GJB2 prevalence at a much lower rate than within India and other populations.

WNT10A missense mutation associated with a complete Odonto-Onycho-Dermal Dysplasia syndrome

Wnt signalling is one of a few pathways that are crucial for controlling genetic programs during embryonic development as well as in adult tissues. WNT10A is expressed in the skin and epidermis and it has shown to be critical for the development of ectodermal appendages. A nonsense mutation in WNT10A was recently identified in odonto-onycho-dermal dysplasia (OODD; MIM 257980), a rare syndrome characterised by severe hypodontia, nail dystrophy, smooth tongue, dry skin, keratoderma and hyperhydrosis of palms and soles. We identified a large consanguineous Pakistani pedigree comprising six individuals affected by a complete OODD syndrome. Autozygosity mapping using SNP array analysis showed that the affected individuals are homozygous for the WNT10A gene region. Subsequent mutation screening showed a homozygous c.392C>T transition in exon 3 of WNT10A, which predicts a p.A131V substitution in a conserved α-helix domain. We report here on the first inherited missense mutation in WNT10A with associated ectodermal features.

A mutation in the hair matrix and cuticle keratin KRTHB5 gene causes ectodermal dysplasia of hair and nail type

Background: Ectodermal dysplasias are developmental disorders affecting tissues of ectodermal origin. To date, four different types of ectodermal dysplasia involving only hair and nails have been described. In an effort to understand the molecular bases of this form of ectodermal dysplasia, large Pakistani consanguineous kindred with multiple affected individuals has been ascertained from a remote region in Pakistan. Objective: To identify the gene underlying the phenotype. Methods: Microsatellite markers were genotyped in candidate regions and two point and multipoint parametric linkage analysis carried out. Results: The disease locus was mapped to a 16.6 centimorgan region on chromosome 12q12–q14.1 (Zmax = 8.2), which harbours six type II hair keratin genes. DNA sequence analysis revealed a homozygous missense mutation in the hair matrix and cuticle keratin KRTHB5, leading to histidine substitution of a conserved arginine residue (R78H) located in the head domain. Conclusions: This report provides the first direct evidence relating to the molecular pathogenesis of pure hair–nail ectodermal dysplasias.

A novel autosomal recessive nonsyndromic hearing impairment locus (DFNB42) maps to chromosome 3q13.31-q22.3.

A consanguineous family with autosomal recessive nonsyndromic hearing impairment (NSHI) was ascertained in Pakistan and displayed significant evidence of linkage to 3q13.31‐q22.3. The novel locus (DFNB42) segregating in this kindred, maps to a 21.6 cM region according to a genetic map constructed using data from both the deCode and Marshfield genetic maps. This region of homozygosity is flanked by markers D3S1278 and D3S2453. A maximum multipoint LOD score of 3.72 was obtained at marker D3S4523. DFNB42 represents the third autosomal recessive NSHI locus to map to chromosome 3.

DFNB39, a recessive form of sensorineural hearing impairment, maps to chromosome 7q11.22–q21.12

This article describes the identification of a novel locus (DFNB39) responsible for an autosomal recessive form of hearing loss segregating in a Pakistani consanguineous family. The hearing impaired members of this family present with profound prelingual sensorineural hearing impairment and use sign language for communications. Linkage was established to microsatellite markers located on chromosome 7q with a maximum multipoint lod score of 3.8. The region of homozygosity spans a 19 cM region that is bounded by markers D7S3046 and D7S644.

Autosomal recessive pure hair and nail ectodermal dysplasia linked to chromosome 12p11.1-q14.3 without KRTHB5 gene mutation

Hair-nail ectodermal dysplasia (HNED; OMIM 602032) constitutes a rare subgroup of ectodermal dysplasias characterised by onychodystrophy, hypotrichosis and brittle hair. We identified a large consanguineous Pakistani family with four siblings affected by a congenital autosomal recessive form of the disease. Based on previous genetic findings in HNED we performed linkage analysis in the family using chromosome 12 markers. A genetic linkage analysis revealed a lod score of 2.92 ( = 0.0) at locus D12S368, indicating the disease gene to be located on chromosome 12. Candidate genes on chromosome 12, including the KRTHB5 gene and four additional keratin II genes, were sequenced in affected family members. Sequence analysis of the coding regions of keratin KRTHB5 gene, previously associated with a distinct clinical form of hair-nail dysplasia, revealed normal coding regions. Our study confirms linkage of a variant clinical form of hair-nail ectodermal dysplasia to chromosome 12 without any mutation in the coding sequences of the KRTHB5 gene. The results suggest this family to have either a non-coding mutation in the KRTHB5 gene, or a mutation in a yet unknown gene within the linked region on chromosome 12.

Synpolydactyly and HOXD13 polyalanine repeat: addition of 2 alanine residues is without clinical consequences

BackgroundType II syndactyly or synpolydactyly (SPD) is clinically very heterogeneous, and genetically three distinct SPD conditions are known and have been designated as SPD1, SPD2 and SPD3, respectively. SPD1 type is associated with expansion mutations in HOXD13, resulting in an addition of ≥ 7 alanine residues to the polyalanine repeat. It has been suggested that expansions ≤ 6 alanine residues go without medical attention, as no such expansion has ever been reported with the SPD1 phenotype.MethodsWe describe a large Pakistani and an Indian family with SPD. We perform detailed clinical and molecular analyses to identify the genetic basis of this malformation.ResultsWe have identified four distinct clinical categories for the SPD1 phenotype observed in the affected subjects in both families. Next, we show that a milder foot phenotype, previously described as a separate entity, is in fact a part of the SPD1 phenotypic spectrum. Then, we demonstrate that the phenotype in both families segregates with an identical expansion mutation of 21 bp in HOXD13. Finally, we show that the HOXD13 polyalanine repeat is polymorphic, and the expansion of 2 alanine residues, evident in unaffected subjects of both families, is without clinical consequences.ConclusionIt is the first molecular evidence supporting the hypothesis that expansion of ≤ 6 alanine residues in the HOXD13 polyalanine repeat is not associated with the SPD1 phenotype.

Mapping of a novel autosomal recessive nonsyndromic deafness locus (DFNB46) to chromosome 18p11.32-p11.31

Hereditary nonsyndromic deafness (NSD) is extremely heterogeneous. Autosomal recessive (AR) forms account for ∼75% of genetic cases. To date, over 40 ARNSD loci have been mapped. A novel locus (DFNB46) for ARNSD was mapped to chromosome 18p11.32‐p11.31 in a five‐generation Pakistani family. A 10 cM genome‐wide scan and fine mapping was carried out using microsatellite markers. A maximum multipoint LOD score of 3.8 was obtained at two markers, D18S481 and D18S1370. The three‐unit support interval is flanked by markers D18S59 and D18S391, corresponds to a 17.6 cM region according to the deCode genetic map and spans 5.8 Mb on the sequence‐based physical map.