RESUMO
Analysis of 784 informative meioses in the CEPH pedigrees revealed a total of 22 recombination events having occurred in the 6-Mb region between D6S265 (70 kb centromeric of HLA-A) and D6S276. These 22 breakpoints were localized with respect to anonymous polymorphic markers, leading to a detailed genetic map of the region telomeric to the human major histocompatibility complex. A nonrandom pattern of recombination was observed throughout this region: the low recombination rate of 0.19% within the 4-Mb interval centromeric to the HLA class I-like candidate gene for hemochromatosis indeed contrasts with the approximate 1% rate observed within the most telomeric two megabases. This reduced rate of recombination may be due to selective constraints depending on environmental factors related to immunity and iron status or to structural variations hampering proper meiotic pairing of homologous sequences. Population data from other human genome segments are now needed to determine whether linkage disequilibrium extending over 4 Mb is unique to this region.
Assuntos
Antígenos HLA/genética , Complexo Principal de Histocompatibilidade/genética , Recombinação Genética/genética , Mapeamento Cromossômico , Feminino , Ligação Genética/genética , Haplótipos/genética , Hemocromatose/genética , Humanos , Desequilíbrio de Ligação , Masculino , Meiose , Repetições de Microssatélites/genética , Linhagem , Polimorfismo Genético/genéticaRESUMO
A candidate gene for hemochromatosis has recently been localized on the short arm of chromosome 6, about 4 megabases telomeric to the major histocompatibility complex. It encodes a protein that exhibits significant similarity to the HLA class I molecules and can be provisionally designated HLA-hc. Genotype analysis of 94 hemochromatosis patients living in France and a similar number of controls confirms that the disease is strongly associated with homozygosity at nucleotide 845 (72% of the patients and none of the controls carry two copies of the 845A variant). The data are consistent with hemochromatosis being a heterogeneous disease: about 79% of the cases in this sample would be caused by a defect in HLA-hc and 21% by an unrelated mechanism. A second variant (187 G) enriched on patient chromosomes that do not carry the 845A mutation might influence the affinity of a ligand for HLA-hc; the exact nature of this ligand remains to be discovered. The 845A variant is the best genetic marker for the disease identified to date, and the detection of 845A homozygosity should now permit diagnosis of a readily curable disease and the prevention of sometimes deadly complications in at least 72% of the patients.
Assuntos
Genes MHC Classe I , Hemocromatose/genética , Antígenos de Histocompatibilidade Classe I/genética , Complexo Principal de Histocompatibilidade , Haplótipos , Humanos , Mutação PuntualRESUMO
Polymorphic (CTC)n and (TAAA)n sequences were identified in exons 1 and 8 of the myelin oligodendrocyte glycoprotein (MOG) gene. The different alleles were detected by a method combining fluorescence labeling of polymerase chain reaction (PCR) products and use of an automated DNA sequencer. Although only two alleles differing by the number of leucine residues encoded by the (CTC)n array were detected at the first locus, seven alleles were identified at the second. The high degree of polymorphism (75%) of the tetranucleotide repeat makes this marker informative for association or linkage studies with diseases such as hemochromatosis or multiple sclerosis.
Assuntos
Éxons , Glicoproteína Associada a Mielina/genética , Polimorfismo Genético , Sequências Repetitivas de Ácido Nucleico , Alelos , Sequência de Bases , Primers do DNA , Feminino , Frequência do Gene , Ligação Genética , Variação Genética , Hemocromatose/genética , Humanos , Masculino , Dados de Sequência Molecular , Esclerose Múltipla/genética , Proteínas da Mielina , Glicoproteína Mielina-Oligodendrócito , Oligodendroglia/metabolismo , Reação em Cadeia da PolimeraseRESUMO
Human myelin oligodendrocyte glycoprotein (MOG), a myelin component of the central nervous system, is a candidate target antigen for autoimmune-mediated demyelination. We have isolated and sequenced part of a cosmid clone that contains the entire human MOG gene. The primary nuclear transcript, extending from the putative start of transcription to the site of poly(A) addition, is 15,561 nucleotides in length. The human MOG gene contains 8 exons, separated by 7 introns; canonical intron/exon boundary sites are observed at each junction. The introns vary in size from 242 to 6484 bp and contain numerous repetitive DNA elements, including 14 Alu sequences within 3 introns. Another Alu element is located in the 3'-untranslated region of the gene. Alu sequences were classified with respect to subfamily assignment. Seven hundred sixty-three nucleotides 5' of the transcription start and 1214 nucleotides 3' of the poly(A) addition sites were also sequenced. The 5'-flanking region revealed the presence of several consensus sequences that could be relevant in the transcription of the MOG gene, in particular binding sites in common with other myelin gene promoters. Two polymorphic intragenic dinucleotide (CA)n and tetranucleotide (TAAA)n repeats were identified and may provide genetic marker tools for association and linkage studies.
