Cytological studies of meiotic recombination in human males.

We combined immunostaining and fluorescence in situ hybridization (FISH) methodology to directly examine meiotic exchanges in over 2,000 pachytene stage spermatocytes from 25 individuals. Our results indicate that, on average, there are about 50 exchanges per cell and that, with the exception of the acrocentric chromosomes, all chromosome arms harbor at least one exchange. We also identified significant among-individual variation in the mean number of exchanges, with an approximate 20% difference between individuals with "low" and those with "high" exchange frequencies.

Maternal folate polymorphisms and the etiology of human nondisjunction.

Attempts to identify genetic contributors to human meiotic nondisjunction have met with little, if any, success. Thus, recent reports linking Down syndrome to maternal polymorphisms at either of two folate metabolism enzymes, methylenetetrahydrofolate reductase (MTHFR) and methionine synthase reductase (MTRR), have generated considerable interest. In the present report, we asked whether variation at MTHFR (677C-->T) or MTRR (66A-->G) might be associated with human trisomies other than trisomy 21. We analyzed maternal polymorphisms at MTHFR and MTRR in 93 cases of sex-chromosome trisomy, 44 cases of trisomy 18, and 158 cases of autosomal trisomies 2, 7, 10, 13, 14, 15, 16, 18, or 22, and compared the distributions of genotypes to those of control populations. We observed a significant increase in the MTHFR polymorphism in mothers of trisomy 18 conceptuses but were unable to identify any other significant associations. Overall, our observations suggest that, at least for the sex chromosomes and for a combined set of autosomal trisomies, polymorphisms in the folate pathway are not a significant contributor to human meiotic nondisjunction.

Is disomic homozygosity at the APECED locus the cause of increased autoimmunity in Down's syndrome?

AIMS: To examine the age of onset of insulin dependent diabetes mellitus (IDDM) in children with Down's syndrome compared with non-trisomic individuals, and to assess whether differences might be related to disomic homozygosity at the autoimmune polyglandular disease type 1 (APECED) gene locus. METHODS: Children with Down's syndrome and IDDM were identified through the Down's syndrome association newsletter and from paediatricians. DNA was extracted from mouthbrush preparations provided by the parents and patients using standard techniques. Mapping techniques were then used to identify areas of reduction to homozygosity, including a marker that overlaps the locus for APECED. The frequency of disomic homozygosity for all markers (n = 18) was compared with a control group of 99 patients with Down's syndrome and their parents. The families also answered a questionnaire concerning diabetes and related autoimmune conditions in the family. Details were compared with the British Paediatric Surveillance Group 1988 diabetes study. RESULTS: Children with Down's syndrome and IDDM were diagnosed significantly earlier than the general population (6.7 v 8.0 years) with a far higher proportion diagnosed in the first 2 years of life (22% v 7%). There was no evidence of increased disomic homozygosity in the region of the APECED locus in Down's syndrome patients with IDDM compared with simple Down's syndrome. CONCLUSIONS: The natural history of IDDM in Down's syndrome is different from that of the general population. Although children with Down's syndrome have features similar to cases of APECED, disomic homozygosity in this region does not explain the predilection for autoimmune disease.

FISH studies of the sperm of fathers of paternally derived cases of trisomy 21: no evidence for an increase in aneuploidy.

Paternal nondisjunction accounts for approximately 5% of cases of trisomy 21. To test the hypothesis that, in some such cases, the fathers might be predisposed to meiotic nondisjunction, we utilized fluorescence in situ hybridization (FISH) to screen for aneuploidy in sperm. We analyzed sperm samples from ten males with a trisomy 21 offspring of paternal origin. Among these individuals, the overall frequency of disomy 21 was 0.15%, comparable to estimates of disomy 21 in the general male population. Furthermore, none of the ten fathers of trisomy 21 individuals had significantly elevated levels of disomic sperm. Thus, our results provide no evidence that the occurrence of a trisomy 21 conceptus of paternal origin imparts an increased risk of trisomy in subsequent pregnancies.

Effects of histone acetylation on chromatin topology in vivo.

Recently a model for eukaryotic transcriptional activation has been proposed in which histone hyperacetylation causes release of nucleosomal supercoils, and this unconstrained tension in turn stimulates transcription (V. G. Norton, B. S. Imai, P. Yau, and E. M. Bradbury, Cell 57:449-457, 1989; V. G. Norton, K. W. Marvin, P. Yau, and E. M. Bradbury, J. Biol. Chem. 265:19848-19852, 1990). These studies analyzed the effect of histone hyperacetylation on the change in topological linking number which occurs during nucleosome assembly in vitro. We have tested this model by determining the effect of histone hyperacetylation on the linking number change which occurs during assembly in vivo. We find that butyrate treatment of cells infected with simian virus 40 results in hyperacetylation of the histones of the extracted viral minichromosome as expected. However, the change in constrained supercoils of the minichromosome DNA is minimal, a result which is inconsistent with the proposed model. These results indicate that the proposed mechanism of transcriptional activation is unlikely to take place in the cell.