Identification of Genes Involved in Human Hereditary Skin Disorders

Abstract

A fully functional gene is one of the most indispensable elements for perfect operational human machinery. Any anomaly in epigenetic mechanism or disturbance in gene expression through mutations can cause functional, developmental and structural abnormalities that are transmitted inheritably in human. Disease causing mutations involved in such genetic disorders are screened by molecular genetics and recent advancements in this field have facilitated investigators to understand the complex mechanisms regulating skin and its associated appendages. The present dissertation, a part of my PhD studies, describes clinical and molecular analysis of 18 consanguineous kindred (A-R) presenting various types of human skin disorders. Affected individuals in family A demonstrated clinical features of autosomal recessive hair loss including sparse hair on the scalp, sparse eyebrows and eyelashes, and papules formation on different parts of body. Human genome scan using 500 highly polymorphic microsatellite markers established linkage in the family to a novel 3.67 Mb region on chromosome 8p22. The maximum multipoint LOD score of 2.41 was achieved with five microsatellite markers mapped at chromosome 8p22. Three other families (B, C, D), exhibited clinical features of congenital atrichia with papular lesions (APL), mapped to the nearby hairless gene on chromosome 8p21.3. Mutational screening in patients of these three families failed to reveal any functional variant. Affected individuals in five families (E-I) showed clinical features of autosomal recessive hypotrichosis/woolly hair. Majority of the affected members exhibited sparse scalp hair, sparse eyebrows and eyelashes, and sparse thin hair on other parts of the body. Genetic mapping established linkage in two families (E, F) to the gene LIPH at chromosome 3q27 and in three families (G, H, I) to LPAR6 on chromosome 13q14.11-q23.21. Sequence analysis of the gene LIPH revealed a novel deletion mutation (c.682delT; p.Leu228TrpfsX31) in exon 5 in affected individuals of the family E. Sequencing of the gene LPAR6 led to the identification of three recurrent missense mutations (p.Glu189Lys, p.Asp63Val, p.Ile188Phe) in family G, H and I. In the tenth family (J), affected members presented clinical features of autosomal recessive hypotrichosis including sparse thin light brown hair on the scalp, sparse eyebrows and sparse to absent eyelashes, and sparse hair growth on the chest, arms, legs and other parts of the body. Genetic mapping established linkage in the family to a 12.69 cM disease interval, flanked by markers D10S1665 and D10S2337, on chromosome 10q11.23-22.3. Three genes (ZMYND17, KAT6B/MYST4, P4HA1), located in the candidate linkage interval, were sequenced in affected members of the family but failed to detect any functional sequence variant. Autosomal dominant form of hypotrichosis/woolly hair was observed only in two families (K, L) in the present investigation. Affected individuals in family K presented patchy hair loss leading to sparse thin hair on the scalp and normal growth of hair at beard, mustache, eyebrows and eyelashes. Affected individuals in the other family (L) presented tightly curled and twisted scalp hair entangling with each other. Eyebrows, eyelashes, mustache and beard hair were found normal in the affected individuals. Genotyping analysis established linkage in both the families to the genes KRT71-KRT74 mapped on chromosome 12q12-q14.1. Sequence analysis of the gene KRT74 detected a novel heterozygous missense mutation (c.1444G>A; p.Asp482Asn) in all the five affected members of family K. Sequencing of both the genes KRT71 and KRT74 in affected individuals of the family L however failed to detect sequence variants. The present study located in a remote area of the country a family (M) in which affected members showed features of hypotrichosis with juvenile macular dystrophy (HJMD). Genetic mapping showed linkage of the family to the gene CDH3 at chromosome 16q22.1. Subsequently, sequence analysis of the gene revealed a novel splice acceptor site mutation (c.IVS10-1 G→A) in all affected members of family. Two families (N, O) showed segregation of X-linked recessive isolated hypodontia. Affected incisors and to some extent canines and premolars with a variable expression were observed in patients of these two families. Genotyping showed linkage of the families to the gene EDA at chromosome Xq12-13.1. Mutational screening of the gene EDA revealed a missense mutation (c.1091T>C; p.Met364Thr) in family N but failed to detect potential sequence variants in family O. Autosomal recessive hypohidrotic ectodermal dysplasia (ARHED) was observed in affected individuals in two families (P, Q). All the affected members showed fine thin hair on the scalp and sparse to absent eyebrows and eyelashes, hypohidrosis, saddled-shaped nose, protruding prominent lips, only two teeth with conical shape and normal nails. Mapping analysis exhibited linkage of the families to the gene EDAR on chromosome 2q11-q13. Subsequently, sequencing of the gene revealed two novel mutations: a missense (c.1163T>C; p.Ile388Thr) in family P and insertion (c.1014insA; p.Val339SerfsX6) in family Q. In family R, the patients demonstrated phenotypes of isolated congenital bilateral clubbing of all finger- and toenails. The nails were shiny, thick, long and broad. Genotyping established linkage of the family to the gene HPGD at chromosome 4q32.3-q34.2. Sequence analysis of the gene identified a recurrent mutation (c.577T>C; p.Ser193Pro) in all affected members of the family. The data presented in the dissertation has been published in the following articles. 1. Naqvi SK, Wasif N, Javaid H, Ahmad W (2011). Two novel mutations in the gene EDAR causing autosomal recessive hypohidrotic ectodermal dysplasia. Orthodontics and Craniofacial Research 14: 156-159 2. Wasif N, Kamran-Ul-Hassan Naqvi S, Basit S, Ali N, Ansar M, Ahmad W (2011). Novel mutations in the keratin-74 (KRT74) gene underlie autosomal dominant woolly hair/hypotrichosis in Pakistani families. Human Genetics 129: 419-424 (Joint First Authors) 3. Khan S, Habib R, Mir H, Umm-E-Kalsoom, Naz G, Ayub M, Shafique S, Yasmin T, Ali N, Basit S, Wasif N, Kamran-Ul-Hassan Naqvi S, Ali G, Wali A, Ansar M, Ahmad W (2011) Mutations in the LPAR6 and LIPH genes underlie autosomal recessive hypotrichosis/woolly hair in 17 consanguineous families from Pakistan. Clinical and Experimental Dermatology 36: 652-654 4. Kamran-Ul-Hassan Naqvi S, Azeem Z, Ali G, Ahmad W (2010) A novel splice- acceptor site mutation in CDH3 gene in a consanguineous family exhibiting hypotrichosis with juvenile macular dystrophy. Archives and Dermatological Research 302: 701-703 5. Azeem Z, Kamran-Ul-Hassan Naqvi S, Ansar M, Wali A, Naveed AK, Ali G, Hassan MJ, Tariq M, Basit S, Ahmad W (2009) Recurrent mutations in functionally-related EDA and EDAR genes underlie X-linked isolated hypodontia and autosomal recessive hypohidrotic ectodermal dysplasia. Archives and Dermatological Research 301: 625-629 (Joint First Authors) 6. Kamran-Ul-Hassan Naqvi S, Raza SI, Naveed AK, John P, Ahmad W (2008) A novel deletion mutation in the phospholipase H (LIPH) gene in a consanguineous Pakistani family with autosomal recessive hypotrichosis (LAH2). British Journal of Dermatology 160: 194-196 7. Azeem Z, Jelani M, Naz G, Tariq M, Wasif N, Kamran-Ul-Hassan Naqvi S, Ayub M, Yasinzai M, Amin-ud-din M, Wali A, Ali G, Chishti MS, Ahmad W (2008) Novel mutations in G protein-coupled receptor gene (P2RY5) in families with autosomal recessive hypotrichosis (LAH3). Human Genetics 123: 515-519