A new locus for autosomal recessive non-syndromal sensorineural hearing impairment (DFNB27) on chromosome 2q23-q31.

Non-syndromic sensorineural deafness is an extremely genetically heterogeneous condition. We have used autozygosity mapping in a large consanguineous United Arab Emirate family to identify a novel locus for autosomal recessive non-syndromic sensorineural deafness, DFNB27, on chromosome 2q23-q31, with a maximum two-point lod score of 5.18 at theta = 0 for marker D2S2257. The DFNB27 locus extends over a 17 cM region between D2S2157 and D2S2273, and may overlap the DFNA16 locus for dominantly inherited, fluctuating, progressive non-syndromal hearing loss. However, genotype data suggests that the locus is likely to be refined to between D2S326 and D2S2273 and thus distinct from the DFNA16 locus.

Transmission of haplotypes of microsatellite markers rather than single marker alleles in the mapping of a putative type 1 diabetes susceptibility gene (IDDM6).

Allelic association methods based on increased transmission of marker alleles will have to be employed for the mapping of complex disease susceptibility genes. However, because the extent of association of single marker alleles with disease is a function of the relative frequency of the allele on disease-associated chromosomes versus non disease-predisposing chromosomes, the most associated marker allele in a region will not necessarily be closest to the disease locus. To overcome this problem we describe a haplotype-based approach developed for mapping of the putative type 1 diabetes susceptibility gene IDDM6. Ten microsatellite markers spanning a 550 kb segment of chromosome 18q21 in the putative IDDM6 region were genotyped in 1708 type 1 diabetic Caucasian families from seven countries. The most likely ancestral diabetogenic chromosome was reconstructed in a stepwise fashion by analysing linkage disequilibrium between a previously defined haplotype of three adjacent markers and the next marker along the chromosome. A plot of transmission from heterozygous parents to affected offspring of single marker alleles present on the ancestral chromosome versus the physical distance between them, was compared with a plot of transmission of haplotypes of groups of three adjacent markers. Analysing transmission of haplotypes largely negated apparent decreases in transmission of single marker alleles. Peak support for association of the D18S487 region with IDDM6 is P = 0.0002 (corrected P = 0.01). The results also demonstrate the utility of polymorphic microsatellite markers to trace and delineate extended and presumably ancient haplotypes in the analysis of common disease and in the search for identical-by-descent chromosome regions that carry an aetiological variant.

Fine mapping of the diabetes-susceptibility locus, IDDM4, on chromosome 11q13.

Genomewide linkage studies of type 1 diabetes (or insulin-dependent diabetes mellitus [IDDM]) indicate that several unlinked susceptibility loci can explain the clustering of the disease in families. One such locus has been mapped to chromosome 11q13 (IDDM4). In the present report we have analyzed 707 affected sib pairs, obtaining a peak multipoint maximum LOD score (MLS) of 2.7 (lambda(s)=1.09) with linkage (MLS>=0.7) extending over a 15-cM region. The problem is, therefore, to fine map the locus to permit structural analysis of positional candidate genes. In a two-stage approach, we first scanned the 15-cM linked region for increased or decreased transmission, from heterozygous parents to affected siblings in 340 families, of the three most common alleles of each of 12 microsatellite loci. One of the 36 alleles showed decreased transmission (50% expected, 45.1% observed [P=.02, corrected P=.72]) at marker D11S1917. Analysis of an additional 1,702 families provided further support for negative transmission (48%) of D11S1917 allele 3 to affected offspring and positive transmission (55%) to unaffected siblings (test of heterogeneity P=3x10-4, corrected P=. 01]). A second polymorphic marker, H0570polyA, was isolated from a cosmid clone containing D11S1917, and genotyping of 2,042 families revealed strong linkage disequilibrium between the two markers (15 kb apart), with a specific haplotype, D11S1917*03-H0570polyA*02, showing decreased transmission (46.4%) to affected offspring and increased transmission (56.6%) to unaffected siblings (test of heterogeneity P=1.5x10-6, corrected P=4.3x10-4). These results not only provide sufficient justification for analysis of the gene content of the D11S1917 region for positional candidates but also show that, in the mapping of genes for common multifactorial diseases, analysis of both affected and unaffected siblings is of value and that both predisposing and nonpredisposing alleles should be anticipated.