The Finland-United States investigation of non-insulin-dependent diabetes mellitus genetics (FUSION) study. I. An autosomal genome scan for genes that predispose to type 2 diabetes.

We performed a genome scan at an average resolution of 8 cM in 719 Finnish sib pairs with type 2 diabetes. Our strongest results are for chromosome 20, where we observe a weighted maximum LOD score (MLS) of 2.15 at map position 69.5 cM from pter and secondary weighted LOD-score peaks of 2.04 at 56.5 cM and 1.99 at 17.5 cM. Our next largest MLS is for chromosome 11 (MLS = 1.75 at 84.0 cM), followed by chromosomes 2 (MLS = 0.87 at 5.5 cM), 10 (MLS = 0.77 at 75.0 cM), and 6 (MLS = 0.61 at 112.5 cM), all under an additive model. When we condition on chromosome 2 at 8.5 cM, the MLS for chromosome 20 increases to 5.50 at 69.0 cM (P=.0014). An ordered-subsets analysis based on families with high or low diabetes-related quantitative traits yielded results that support the possible existence of disease-predisposing genes on chromosomes 6 and 10. Genomewide linkage-disequilibrium analysis using microsatellite marker data revealed strong evidence of association for D22S423 (P=.00007). Further analyses are being carried out to confirm and to refine the location of these putative diabetes-predisposing genes.

Complete maternal isodisomy of chromosome 8 in an individual with an early-onset ileal carcinoid tumor.

Uniparental disomy (UPD) is a condition in which diploid individuals possess a chromosome pair from a single parent. In some instances, UPD causes an abnormal phenotype due to imprinting effects, reduction to homozygosity at recessive disease loci, or trisomy mosaicism. Here we report the first account of an individual with apparently nonmosaic complete maternal isodisomy of chromosome 8. This individual was identified during routine genotyping in a genomewide search for type 2 diabetes susceptibility genes, although he does not have diabetes. He is of normal appearance, stature, and intelligence, but there is an unusual history of early onset ileal carcinoid. The discovery of other maternal UPD 8 cases will be necessary to define whether this condition causes a distinct phenotype.

The mammalian FEN-1 locus: structure and conserved sequence features.

Flap endonuclease 1 (FEN-1) is an enzyme that is very important for DNA replication in all eukaryotes because it cleaves the 5' DNA flaps that arise between Okazaki fragments. In addition, FEN-1 is important for base excision repair and for nonhomologous DNA end joining in all eukaryotes from yeast to human. Here we report the structure and sequence of the murine genomic FEN-1 locus, and we compare it to the human FEN-1 locus. The transcriptional initiation zone of FEN-1 is within a CpG island, and the coding region of FEN-1 is a single exon in both the murine and human genomes. There are striking regions of nucleotide sequence homology within the 5' or 3'UTR or immediately upstream of the 5'UTR. These regions range from 30 to 230 bp. The functions of these conserved sequence blocks could be in transcriptional regulation, or they may represent a gene that overlaps in its initiation zone with FEN-1, but is oriented in the opposite transcriptional direction.

The nonhomologous DNA end joining pathway is important for chromosome stability in primary fibroblasts.

There are two types of chromosome instability, structural and numerical, and these are important in cancer. Many structural abnormalities are likely to involve double-strand DNA (dsDNA) breaks. Nonhomologous DNA end joining (NHEJ) and homologous recombination are the major pathways for repairing dsDNA breaks. NHEJ is the primary pathway for repairing dsDNA breaks throughout the G0, G1 and early S phases of the cell cycle [1]. Ku86 and DNA ligase IV are two major proteins in the NHEJ pathway. We examined primary dermal fibroblasts from mice (wild type, Ku86(+/-), Ku86(-/-), and DNA ligase IV(+/-)) for chromosome breaks. Fibroblasts from Ku86(+/-) or DNA ligase IV(+/-) mice have elevated frequencies of chromosome breaks compared with those from wild-type mice. Fibroblasts from Ku86(-/-) mice have even higher levels of chromosome breaks. Primary pre-B cells from the same animals did not show significant accumulation of chromosome breaks. Rather the pre-B cells showed increased cell death. These studies demonstrate that chromosome breaks arise frequently and that NHEJ is required to repair this constant spontaneous damage.

A large sample of finnish diabetic sib-pairs reveals no evidence for a non-insulin-dependent diabetes mellitus susceptibility locus at 2qter.

In the first reported positive result from a genome scan for non-insulin-dependent diabetes mellitus (NIDDM), Hanis et al. found significant evidence of linkage for NIDDM on chromosome 2q37 and named the putative disease locus NIDDM1 (Hanis et al. 1996. Nat. Genet. 13:161-166). Their total sample was comprised of 440 Mexican-American affected sib-pairs from 246 sibships. The strongest evidence for linkage was at marker D2S125 and best estimates of lambdas (risk to siblings of probands/population prevalence) using this marker were 1.37 under an additive model and 1.36 under a multiplicative model. We examined this chromosomal region using linkage analysis in a Finnish sample comprised of 709 affected sib-pairs from 472 sibships. We excluded this region in our sample (multipoint logarithm of odds score /= 1.37. We discuss possible reasons why linkage to 2q37 was not found and conclude that this region is unlikely to be playing a major role in NIDDM susceptibility in the Finnish Caucasian population.

Methods for precise sizing, automated binning of alleles, and reduction of error rates in large-scale genotyping using fluorescently labeled dinucleotide markers. FUSION (Finland-U.S. Investigation of NIDDM Genetics) Study Group.

Large-scale genotyping is required to generate dense identity-by-descent maps to map genes for human complex disease. In some studies the number of genotypes needed can approach or even exceed 1 million. Generally, linkage and linkage disequilibrium analyses depend on clear allele identification and subsequent allele frequency estimation. Accurate grouping or categorization of each allele in the sample (allele calling or binning) is therefore an absolute requirement. Hence, a genotyping system that can reliably achieve this is necessary. In the case of affected sib-pair analysis without parents, the need for accurate allele calling is even more critical. We describe methods that permit precise sizing of alleles across multiple gels using the fluorescence-based, Applied Biosystems (ABI) genotyping technology and discuss ways to reduce genotyping error rates. Using database utilities, we show how to minimize intergel allele size variation, to combine data effectively from different models of ABI sequencing machines, and automatically bin alleles. The final data can then be converted into a format ready for analysis by statistical genetic packages such as MENDEL.

Substrate nucleotide-determined non-templated addition of adenine by Taq DNA polymerase: implications for PCR-based genotyping and cloning.

The Applied Biosystems PRISM fluorescence-based genotyping system as well as the Invitrogen TA Cloning vector system are influenced by the tendency of Taq DNA polymerase to add an adenine nucleotide to the 3' end of PCR products after extension. Incomplete addition of adenine to a majority of PCR product strands creates problems in allele-calling during genotyping and potentially diminishes the cloning efficiency of such products. Experiments reported here show that certain terminal nucleotides can either inhibit or enhance adenine addition by Taq and that PCR primer design can be used to modulate this activity. The methods we propose can substantially improve allele-calling for problematic microsatellite markers when using GENOTYPER software.