The Huntington's disease (HD) gene has been mapped in 4~16.3 but has eluded identification. We have used haplotype analysis of linkage disequilibrium to spotlight a small segment of 4~16.3 as the likely location of the defect. A new gene, IT15, isolated using cloned trapped exons from the target area contains a polymorphic trinucleotide repeat that is expanded and unstable on HD chromosomes. A (CAG), repeat longer than the normal range was observed on HD chromosomes from all 75 disease families examined, comprising a variety of ethnic backgrounds and 4~16.3 haplotypes. The GAG), repeat appears to be located within the coding sequence of a predicted-346 kd protein that is widely expressed but unrelated to any known gene. Thus, the HD mutation involves an unstable DNA segment, similar to those described in fragile X syndrome, spino-bulbar muscular atrophy, and myotonic dystrophy, acting in the context of a novel 4~16.3 gene to produce a dominant phenotype.

Morphometric studies of the tail of the caudate nucleus, the site where the pathology is first seen, were performed on 16 brain specimens collected from individuals at risk for inheriting Huntington's disease (HD). Medical records and information obtained from immediate family members indicated that all had died without symptoms of HD. Six individuals had 37 or more CAG repeats and were designated HD gene carriers, whereas 10 were determined to be non‐carriers. Cell counts of the tail of the caudate nucleus revealed an increased density of oligodendrocytes among the presymptomatic HD gene carriers (mean cells/field: carriers = 40.0, noncarrier = 21.3; age, sex, repeated measure adjusted F[126] = 11.7, p = 0.0008). No statistically significant differences were found between HD carriers and noncarriers in the density of neurons (carriers = 16.9, noncarriers = 15.5), astrocytes (carriers = 27.8, noncarriers = 21.3) or microglial cells (carriers = 7.9, noncarriers = 5.6). Ubiquitin immunostaining performed in 3 gene carriers revealed intranuclear inclusions in all 3 cases, including 1, with 37 repeats, who died 3 decades before the expected age for onset of the clinical syndrome. Normal densities of other cell types and careful macroscopic examination suggest that the increase in oligodendroglial density is not a consequence of atrophy and may instead reflect a developmental effect of the HD gene. Ann Neurol 2001;49:29–34

Studies have shown that hemophilia B (Christmas disease; factor IX deficiency) results from many different mutations in the factor IX gene, of which >95% are single nucleotide substitutions. This study has identified a previously unreported form of hemophilia B in a patient who was a somatic mosaic for a guanine-to-cysine transversion at nucleotide 31,170 in the factor IX gene. This point mutation changes the codon for residue 350 in the catalytic domain of factor IX from a cysteine to a serine. We used differential termination of primer extension to confirm and measure the degree of mosaicism. Our study shows that a varying proportion of cells from hepatic, renal, smooth muscle, and hematopoietic populations possessed normal as well as mutant factor IX sequences. These results indicate that the mutation in this patient occurred either as an uncorrected half-chromatid mutation in the female gamete or as a replication or postreplication error in the initial mitotic divisions of the zygote preceding implantation. In addition, this kindred also contains two females in successive generations who have moderately severe factor IX deficiency. The molecular pathogenesis of this latter phenomenon has been studied and seems to relate to the unaccompanied expression of the mutant factor IX gene consequent upon a second, as yet undefined, genetic event that has prevented inactivation ofsequences including the mutant factor IX gene on the X chromosome inherited from the affected male.Hemophilia B (Christmas disease) is a chromosome X-linked recessive inherited bleeding disorder that has an incidence of approximately 1 in 30,000 males (1). The disorder is a result ofeither a deficient or defective factor IX molecule, a vitamin K-dependent serine protease that participates in the intrinsic pathway of hemostasis (2). The variability of the clinical and laboratory manifestations of hemophilia B suggests that the disease is the result of many different mutations within the factor IX gene, and studies have shown that >95% of these mutations are single nucleotide substitutions (3-5).The genetic pathology responsible for hemophilia B has been the focus of intense investigation since the cloning and characterization of the factor IX gene in 1982 (6-8). Study of the patterns ofthese mutations suggests that certain recurring mild mutations appear to have arisen from a common ancestor (9), whereas other severe phenotypes result from repeated mutations often at sequences involving CpG dinucleotides (5). The origin of these latter mutations has until recently been assumed to involve replication or postreplication DNA repair errors occurring during meiosis. There is growing evidence, however, to suggest that some (perhaps many) of these previously unreported mutations arise as a result of errors in replication during mitosis in early embryogenesis (10).The studies described here were prompted by two unusual observations in a family with hemophilia B. Elucidation of the causative mutation in the one affected male in the family revea...

We present a family in which a fragile X mosaic male, who carries both premutation and full mutation alleles in his peripheral blood leukocytes, has a daughter with both premutation and partially methylated full mutation alleles and a significant developmental disability. To our knowledge, this is the first report of such an occurrence and it challenges current thinking about the expansion and transmission of unstable FMR1 alleles from men to their daughters. It is currently accepted that neither males with premutations nor full mutations are at risk for having daughters with full mutations and fragile X syndrome. The sperm cells of full mutation males are thought to carry only premutation alleles. These alleles, when transmitted through a male, regardless of his cognitive status, are thought to be unable to expand to full mutations in the next generation. In effect, the expansion from premutation to full mutation has only been observed through female meioses. The sperm cells in the father in this family have been shown to contain only alleles in the premutation range. Since his daughter has both premutation and full mutation alleles the expansion to full mutation in this case must have occurred postzygotically.