Genetic Linkage And Mutation Analyses Of Symphalangism And Brachydactyly A1 In Chinese Families

One of the most important features of a gene is its accurate replication when transmitted from one generation to the next. However, during the replication process, errors occasionally occur and lead to the generation of mutations. Some mutations are gross changes, including chromosomal anomalies (changes of the number or structure of chromosomes; for example, translocations, inversions, aneuploidy, deletions, duplications) and deletions, insertions, and duplications of large DNA fragments. Some mutations are small changes within or around genes, including point mutations, small deletions, and insertions. Mutations may affect the function and expression of critical genes that are important for the normal human development and physiology, which will cause various human diseases. Recent development of human genetic technologies has allowed the identification of disease genes and mutations based on their chromosomal locations (positional cloning). The current study used the human genetic approach to characterize three Chinese families with autosomal dominant diseases affecting the bone system. This thsis consists of two major sections:First, to identify a novel disease-causing mutation in GDF5 in two Chinese families affected with proximal symphalangism (SYM1). SYM1 is an autosomal dominant disorder characterized by ankylosis of the proximal interphalangeal joints and fusion of carpal and tarsal bones. We identified and characterized two five-generation Chinese families with SYM1. The two families share some similarities (e.g. osseous fusion of interphalangeal joints of the 2-4 fingers) with SYM1 families with mutations in the NOG gene or the family with mutation R438L recently reported in the GDF5 gene (encoding a bone morphogenetic protein family member). However, they show some unique features including the absence of cuboid bone, the lack of shortness of the first and fifth metacarpal bones, and manifestation of flat feet. Genome-wide linkage analysis of the two families linked the disease gene to marker D20S112 with a combined LOD score of 4.32. Mutational analysis revealed a novel E491K mutation in the GDF5 gene in both families. The mutation occurs at a highly conserved residue in the TGF-βdomain of GDF5 and represents the second GDF5 gene mutation identified for SYM1 to date. The E491K mutation co-segregated with the affected individuals in the two families, and did not exist in unaffected family members or 200 normal controls. These results indicate that defects in GDF5 can cause SYM1 in the Chinese population, and expand the spectrum of clinical phenotypes associated with mutant GDF5.Second, to identify a novel mutation in IHH that caused brachydactyly type A1 (BDA1). BDA1 is caused by mutations in the Indian hedgehog gene, IHH on chromosome 2q35-36. In this study, a large five-generation Chinese family with BDA1 was identified and characterized. All affected family members demonstrated significant intrafamilial homogeneous phenotype and some unique clinical features different from those associated with the reported BDA1 mutations in IHH. Linkage analysis showed that the causative mutation in the family was linked to marker D2S126 close to IHH with a LOD score of 4.74 at a recombination fraction of 0. DNA sequence analysis revealed a heterozygous C to T transition at nucleotide 461 of IHH, resulting in a novel T154I substitution. The T154I mutation co-segregated with all affected individuals in the family, and was not present in normal family members or 200 normal controls. These results identify a novel IHH mutation associated with BDA1 and expand the spectrum of clinical phenotype associated with IHH mutations.