Genetic Mapping of Genes Underlying Human Skin Disorders

Abstract

Skin is a highly specialized organ system of human body. It is the exterior most part of our body and acts as a boundary between the organs inside the body and environment, facing the external stresses. Genodermatosis is a term used for a congenital skin disease for which the genetic cause is determined. Genodermatoses possessing skin phenotypes without systemic manifestations are excellent models for genetic research because disorders of skin and its appendages make more than one third part of all monogenic diseases. Autozygosity mapping is a powerful statistical technique used for the identification of genes causing recessive disorders in human consanguineous families. In a country like Pakistan, consanguinity or inbreeding is a very common incident due to highly conserved ethnic groups, separated by customs, cultural and geographical barriers. Such population, with high consanguinity in families, experiences rare recessive disorders, which are more likely to be caused by identical-by-descent (IBD) mutations. In the present investigation 11 families (A-K) segregating autosomal recessive disorders of skin or its appendages were ascertained. Ten of these families were identified in the remote areas of Pakistan and one was provided by collaborators from Islamic Republic of Iran. In family A, affected individuals showed characteristic clinical features of cutis laxa syndrome having generalized skin laxity, progeroid appearance, facial anomalies with hypertelorism, sagging cheeks, broaden nasal bridge and hiatal hernia. Human genome scan using 534 microsatellite markers revealed a common homozygous region of 14.6 Mb, flanked by markers D9S1862 and D9S167, on chromosome 9q13-21.32 in the affected individuals of the family. Maximum multipoint LOD of > 3.0 was obtained at recombination fraction zero with several markers on chromosome 9q13-q21.32. Sequencing of the eight selected putative candidate genes (PRKACG, TJP2, ALDH1A1, RFK, GCNT1, GNAQ, PSAT1, CHCHD9) did not reveal any functional sequence variant and therefore their involvement in causing cutis laxa syndrome phenotype at the present novel locus is not supported. In family B, affected individuals showed clinical features of autosomal recessive hypotrichosis (LAH1). The disease equally affected both males and females with sparse Genetic Mapping of Genes Underlying Human Skin Disorders XVII Abstract hair on scalp, sparse to absent eyebrows and eyelashes, and sparse hair on rest of the body. Genotyping data showed linkage of the family to DSG4 gene on chromosome 18q12. Sequence analysis of DSG4 gene revealed a previously reported large intragenic homozygous deletion mutation (Ex5_8del) in all the affected individuals. In family C, affected individuals showed clinical features of autosomal recessive hypotrichosis (LAH2). The patients showed thin fragile hair on scalp, sparse to absent eyebrows and eyelashes, and sparse hair on rest of the body. Genotyping data showed linkage of the family to LIPH gene on chromosome 3q27. Sequence analysis of LIPH gene revealed a novel homozygous two base pairs deletion mutation (c.659_660delTA, p.Ile220ArgfsX25) in all the affected individuals. Three families (D, E, F) showed clinical features of autosomal recessive hypotrichosis (LAH3). All the affected individuals presented an equal intensity of the disease with sparse thin fragile hair on scalp, sparse to absent eyebrows and eyelashes, and arrested hair follicle growth. Genotyping data showed linkage of the families to P2RY5 gene on chromosome 13q14.11-q23.21. Sequence analysis of the P2RY5 gene revealed three novel homozygous mutations including an insertion (c.160insA, p.Thr54AsnfsX5) in family D, and missense (c.436G>A, p.Gly146Arg) in family E and (c.830C>T, p.Lue277Pro) in family F. Two families (G, H) showed clinical features of atrichia with papular lesions. Patients in these families showed complete hair loss of scalp, absent eyebrows and eyelashes, loss of axillary and pubic hair. Genotyping results showed linkage of the two families to human hairless gene (HR) located on chromosome 8p21.3. In family G, sequence analysis revealed a novel sequence variant involving three base pairs homozygous deletion (c.1839-1841delATG, p.Cys614del) in exon 6 of the HR gene in all the affected individuals. In family H, sequence analysis of HR gene failed to detect any functional sequence variant. In family I, affected individuals exhibited characteristic features of alopecia with mental retardation syndrome. Complete loss of scalp hair and, absent eyebrows and eyelashes were observed in the affected individuals. Both the affected individuals were mentally retarded and attending school for special education. Genotyping data showed linkage of the family to APMR1 locus on chromosome 3q26.33-q27.3. The region of homozygousity spanning 11.39 cM (6 Mb) flanked by markers D3S1232 and D3S3686 contains several Genetic Mapping of Genes Underlying Human Skin Disorders XVIII Abstract genes. To search for potential sequence variants, coding exons and splice junction sites of the three genes (LIPH, AADACL1, AHSG) were sequenced in two affected individuals of the family. Sequence analysis of the three genes failed to identify any functional sequence variant, which could be responsible for the disease phenotype. In family J, the patients exhibited hypotrichosis with juvenile macular dystrophy phenotype. The scalp hairs were fine and curly that could easily be pulled out. Eyebrows and eyelashes, pelvic and pubic hair showed normal growth. Fundus examination of the patients disclosed Electrophysiological marked tests degeneration including of the macular electroretinography pigment showed epithelium. severe retinal dysfunction. Sequence analysis of CDH3 gene at chromosome 16q22.1 revealed a novel homozygous splice-acceptor site mutation (IVS10-1G>T) in affected individuals of the family. In family K, affected individuals showed clinical features of ectodermal dysplasia cutaneous syndactyly (EDCS) syndrome. The patients exhibited hypotrichosis with sparse hair on scalp, and sparse to absent eyebrows and eyelashes. Conical and cylindrical shaped teeth with ill-defined surface morphology. Hypoplastic nails, mild keratoderma over the palms and soles and bilateral cutaneous syndactyly of 3-4 digit of hand and 2-3 toe-digit was observed in all patients. Human genome scan mapped the disease locus to chromosome 1q23.1-q23.3. This region corresponds to 5.63 Mb according to the sequenced based physical map (Build 36.2) of the human genome and flanked by markers D1S1653 and D1S1677. A maximum two-point LOD score of 5.05 was obtained with marker D1S484. Sequence analysis revealed a homozygous missense mutation (c.635C>G; p.Pro212Arg) in the recently reported PVRL4 gene causing ectodermal- dysplasia-syndactyly syndrome (EDSS). The data presented here has been published in the following articles. 1 Jelani M, Wasif N, Ali G, Chishti M, Ahmad W (2008) A novel deletion mutation in LIPH gene causes autosomal recessive hypotrichosis (LAH2). Clinical Genetics 74: 184-188 2 Azeem Z, Jelani M, Naz G, Tariq M, Wasif N, Naqvi SK, Ayub M, Yasinzai M, Amin-Ud-Din M, Wali A, Ali G, Chishti MS, Ahmad W (2008) Novel mutations in G Genetic Mapping of Genes Underlying Human Skin Disorders protein-coupled receptor gene (P2RY5) in families with autosomal recessive hypotrichosis (LAH3). Human Genetics 123: 515-915 3 Jelani M, Salman Chishti M, Ahmad W (2009) A novel splice-site mutation in the CDH3 gene in hypotrichosis with juvenile macular dystrophy. Clinical and Experimental Dermatology 34: 68-73 4 Balighi K, Lajevardi V, Moeineddin F, Jelani M, Tamizifar B, Nikoo A, Javed Q, Ahmad W, Parvaneh N (2009) A novel deletion mutation in the human hairless (HR) gene in an Iranian family with atrichia and papular lesions. Clinical and Experimental Dermatology 34: e498-500 5 Tariq M, Ayub M, Jelani M, Basit S, Naz G, Wasif N, Raza SI, Naveed AK, ullah Khan S, Azeem Z, Yasinzai M, Wali A, Ali G, Chishti MS, Ahmad W (2009) Mutations in the P2RY5 gene underlie autosomal recessive hypotrichosis in 13 Pakistani families. British Journal of Dermatology 160: 1006-1010 6 Jelani M, Tariq M, Jan IA, Ullah H, Naeem M, Ahmad W (2011) Congenital cutis laxa syndrome maps to a novel locus on chromosome 9q13-q21.32. Journal of Dermatological Science 61:134-136 7 Jelani M, Chishti MS, Ahmad W (2011) Mutation in PVRL4 gene encoding Nectin-4 underlies Ectodermal-dysplasia-cutaneous-syndactyly (EDCS) syndrome. (Accepted to Journal of Human Genetics)