Homozygosity mapping: familiarity breeds debility.

An efficient strategy for mapping human genes that cause recessive traits has been devised that uses mapped restriction fragment length polymorphisms (RFLPs) and the DNA of affected children from consanguineous marriages. The method involves detection of the disease locus by virtue of the fact that the adjacent region will preferentially be homozygous by descent in such inbred children. A single affected child of a first-cousin marriage is shown to contain the same total information about linkage as a nuclear family with three affected children. Calculations show that it should be practical to map a recessive disease gene by studying DNA from fewer than a dozen unrelated, affected inbred children, given a complete RFLP linkage map. The method should make it possible to map many recessive diseases for which it is impractical or impossible to collect adequate numbers of families with multiple affected offspring. Show more

Friedreich ataxia and ataxia with selective vitamin E deficiency (AVED) share very similar clinical phenotypes. We have mapped the AVED locus to proximal 8q with only three large consanguinous Tunisian families, representing to our knowledge the first use of homozygosity mapping for primary linkage analysis. Subsequently, three additional families showed linkage with the same markers. A maximum lod score of 17.9 was obtained at theta = 0 for the haplotype D8S260-D8S510, consisting of the two closest markers. With only 6 families, the AVED locus is therefore mapped precisely as illustrated by the lod-1 confidence interval of 2.4 cM on either side of D8S260-D8S510. Isolation of a yeast artificial chromosome contig > 800 kilobases (kb) showed that D8S260 and D8S510 are less than 400 kb apart. Show more

Wilson disease (WD) is an autosomal recessively inherited disorder of copper metabolism for which the basic defect is still unknown. Twenty-seven autosomal markers were investigated for linkage in a large inbred kindred with affected individuals in two generations. Also, serum copper and ceruloplasmin were measured on all available members. Close linkage (theta = 0.06) with a logarithm of odds (lod) score of 3.21 was found between the gene for WD and the esterase D locus. Efficient detection of linkage was made possible by the use of a multisibship inbred pedigree. The discovery of a polymorphic marker genetically linked to the WD locus has profound implications both for investigation of the primary gene defect and for clinical services. Show more