Linkage disequilibrium mapping of the cornea plana congenita gene CNA2.

We recently assigned a gene for autosomal recessive cornea plana congenita (CNA2; MIM No. 217300) by linkage analysis to the approximately 3-cM interval between markers D12S82 and D12S327. Here, we extended these studies by exploiting the haplotype and linkage disequilibrium information that can be derived from the genetically isolated Finnish population and its subpopulations. By testing 32 independent families with 10 polymorphic markers in the CNA2 interval, strong allelic association between CNA2 and a set of markers with a peak at marker D12S351 was detected. Based on linkage disequilibrium analysis, the critical region for CNA2 could be narrowed to only 0.04-0.3 cM from marker D12S351, thus defining a critical interval 0.08-0.60 cM in length. These results provide a basis for highly focused positional cloning of CNA2.

Cornea plana congenita gene assigned to the long arm of chromosome 12 by linkage analysis.

We report the mapping of the locus for autosomal recessive cornea plana congenita (CNA2; MIM 217300) by linkage analysis to the approximately 10-cM interval between markers D12S82 and D12S327. The recessively inherited disorder studied here is more severe than dominant forms. Its main manifestations are reduced curvature and hazy limbus of the cornea, opacities in the corneal stroma, and marked corneal arcus at early age. Our results provide a starting point for the positional cloning of CNA2 and the elucidation of the pathogenesis of the disease.

The gene for a recessively inherited human childhood progressive epilepsy with mental retardation maps to the distal short arm of chromosome 8.

A recently delineated childhood epilepsy has hitherto been observed only in a small geographic region in northern Finland, where, with the exception of one, both parents of all of the 11 sibships with affected individuals descend from one or two founding couples. The disease is characterized by generalized tonic-clonic seizures with onset at 5-10 years and progressive, severe mental retardation with onset 2-5 years after the first seizures. In this study the gene locus is assigned to the telomeric region of chromosome 8p by linkage. Analyses of recombinations place the locus in the 7-centimorgan interval between AFM185xb2 and D8S262 in which three markers, D8S504, D8S264, and AFM077yg5, show no recombinations with the phenotype. Haplotypes comprising alleles at the above five loci support the hypothesis of a single founding mutation for all affected chromosomes except the one belonging to the unrelated parent, who has a very different haplotype, suggesting another mutation or a very old ancestry of a single mutation. This study raises to three the number of heritable epilepsies whose gene loci have been mapped and provides a starting point for the cloning of the gene. It also suggests the possibility that the disease might not be limited to the northern Finnish population.

Cohen syndrome gene assigned to the long arm of chromosome 8 by linkage analysis.

Cohen syndrome is an autosomal recessive disorder characterized by mental and motor retardation, short stature, microcephaly, several dysmorphic features, major ocular symptoms and granulocytopenia. Major research challenges are the confusing nosology and the pleiotropy of the gene. We report the mapping of a locus (CHS1) by linkage analysis in as few as four two-generation pedigrees with uniform clinical features. CHS1 was assigned to an interval of approximately 10 cM between D8S270 and D8S521. Our results provide a tool to a more accurate definition of Cohen syndrome(s) and a starting point for the positional cloning of CHS1.