TAILIEUCHUNG - Chapter 062. Principles of Human Genetics (Part 18)

Phenotypic Heterogeneity Phenotypic heterogeneity occurs when more than one phenotype is caused by allelic mutations (., different mutations in the same gene) (Table 62-4). For example, laminopathies are monogenic multisystem disorders that result from mutations in the LMNA gene, which encodes the nuclear lamins A and C. Twelve autosomal dominant and four autosomal recessive disorders are caused by mutations in the LMNA gene. They include several forms of lipodystrophies, Emery-Dreifuss muscular dystrophy, progeria syndromes, a form of neuronal Charcot-Marie-Tooth disease (type 2B1), and a group of overlapping syndromes. Remarkably, hierarchical cluster analysis has revealed that the phenotypes vary depending on. | Chapter 062. Principles of Human Genetics Part 18 Phenotypic Heterogeneity Phenotypic heterogeneity occurs when more than one phenotype is caused by allelic mutations . different mutations in the same gene Table 62-4 . For example laminopathies are monogenic multisystem disorders that result from mutations in the LMNA gene which encodes the nuclear lamins A and C. Twelve autosomal dominant and four autosomal recessive disorders are caused by mutations in the LMNA gene. They include several forms of lipodystrophies Emery-Dreifuss muscular dystrophy progeria syndromes a form of neuronal Charcot-Marie-Tooth disease type 2B1 and a group of overlapping syndromes. Remarkably hierarchical cluster analysis has revealed that the phenotypes vary depending on the position of the mutation. Similarly identical mutations in the FGFR2 gene can result in very distinct phenotypes Crouzon syndrome craniofacial synostosis or Pfeiffer syndrome acrocephalopolysyndactyly . Locus or Nonallelic Heterogeneity and Phenocopies Nonallelic or locus heterogeneity refers to the situation in which a similar disease phenotype results from mutations at different genetic loci. This often occurs when more than one gene product produces different subunits of an interacting complex or when different genes are involved in the same genetic cascade or physiologic pathway. For example osteogenesis imperfecta can arise from mutations in two different procollagen genes COL1A1 or COL1A2 that are located on different chromosomes Chap. 357 . The effects of inactivating mutations in these two genes are similar because the protein products comprise different subunits of the helical collagen fiber. Similarly muscular dystrophy syndromes can be caused by mutations in various genes consistent with the fact that it can be transmitted in an X-linked Duchenne or Becker autosomal dominant limb-girdle muscular dystrophy type 1 or autosomal recessive limbgirdle muscular dystrophy type 2 manner Chap. 382 . Mutations in

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