Linkage analysis and candidate gene approach in three disorders

Abstract

Genetically inherited diseases are powerful resources for defining new gene functions. Genetic linkage analysis is the tool used for identifying the locus/loci harboring the gene(s) responsible for a particular disease. The analysis of the genotypes generated with highly dense markers using computer programs for lod score calculations are the basis of genetic linkage analysis. In this study, genome scan in three inherited disorders, Gerodermia Osteodysplastica (GO), Motor Dysfunction, Intellectual Disability and Joint Contractures (MDIDJC) and Medial Cleft Lip (MCL) was performed with microsatellite and/or SNP markers. Two-point and multipoint lod score analyses were performed for locus identification. Candidate gene loci were investigated and gene loci were identified with additional polymorphic microsatellite markers. Geroderma osteodysplastica (GO), a rare autosomal recessive disorder, is characterized by wrinkled skin on the dorsum of the hands and feet, droopy face, hyperextensible joints, skeletal changes such as osteoporosis and an of prematurely aged appearance. The candidate locus was identified in the study family, initially diagnosed as GO. While the thesis was in progress, the gene responsible for GO was identified as GORAB (1q24). In addition, some patients previously diagnosed with GO, Wrinkled Skin Syndrome or De Barsy Syndrome were found to have mutations in PYCR1 (17q25.3), and the disease was named cutis laxa, autosomal recessive, type IIB or ARCL2B. The last disorder has similar features with GO. A previously reported PYCR1 mutation was found in patients studied within the scope of this thesis. Further, the clinical and molecular findings of GO and ARCL2B were reviewed. Since the clinical phenotypes of patients with either GORAB or PYCR1 mutations are similar to GO, it was suggested that the disorder resulting from defects in GORAB should be designated as GO type I (without MR) and that resulting from defects in PYCR1 as GO type II (with MR) rather than ARCL2B. A highly inbred family having twelve members afflicted with autosomal recessive postnatal development of multiple joint contractures, motor disability and mental disability was studied. The syndrome in the study family was assessed as novel and designated motor dysfunction, intellectual disability and joint contractures (MDIDJC). The disease loci was mapped to 8p12, and subsequently a homozygous 2-bp insertion in ERLIN2 (endoplasmic reticulum lipid raft-associated protein 2) was identified. The mutation is predicted to lead to protein truncation. Orofacial defects are very common congenital defects in humans. The kindred we analyzed exhibited a unique form of orofacial defects, median cleft lip (MCL): the four most affected sibs exhibited an inherited form of incomplete median clefting in upper and lower lip together, with involvement of dental dysmorphology. In addition to the affected sibs, the mother and one of the sibs had a subclinical phenotype, whereas two cousins had minimal phenotype. A homozygous intergenic novel deletion larger than 270,000 bp was identified in all affected sibs. Some of the remaining members also carried the mutation. The deletion resided between genes TYW3 and LHX8. LHX8 had been implicated in cleft palate in mouse, and a role in lip and maxilla formation. Thus, we propose that the mutation region might harbor sequences having a regulatory role in the expression of LHX8 gene. It was concluded that a second gene likely contributes to the phenotype. Additionally, several loci that may be associated with the condition were identified.