Constitutional hyperrecombinability and its consequences

Lens opacities in Bloom syndrome: case report and review of the literature

Bloom syndrome is an autosomal recessive disorder characterized by proportionate short stature, photosensitivity, immunodeficiency, hypogonadism and a tendency to develop various malignancies. The greatly increased frequency of sister chromatid exchanges (reciprocal exchange of homologous segments between the two sister chromatids of a chromosome) is regarded as pathognomonic for BS. We describe an 18-year old girl who presented with short stature. She was diagnosed with BS based on an extremely increased frequency of sister chromatid exchanges. Ophthalmological examination revealed mild lens opacities bilaterally, which, to our knowledge, has not been previously reported to be associated with BS.

An analysis of univalent segregation in meiotic mutants of Arabidopsis thaliana: a possible role for synaptonemal complex

During first meiotic prophase, homologous chromosomes are normally kept together by both crossovers and synaptonemal complexes (SC). In most eukaryotes, the SC disassembles at diplotene, leaving chromosomes joined by chiasmata. The correct co-orientation of bivalents at metaphase I and the reductional segregation at anaphase I are facilitated by chiasmata and sister-chromatid cohesion. In the absence of meiotic reciprocal recombination, homologs are expected to segregate randomly at anaphase I. Here, we have analyzed the segregation of homologous chromosomes at anaphase I in four meiotic mutants of Arabidopsis thaliana, spo11-1-3, dsy1, mpa1, and asy1, which show a high frequency of univalents at diplotene. The segregation pattern of chromosomes 2, 4, and 5 was different in each mutant. Homologous univalents segregated randomly in spo11-1-3, whereas they did not in dsy1 and mpa1. An intermediate situation was observed in asy1. Also, we have found a parallelism between this behavior and the synaptic pattern displayed by each mutant. Thus, whereas spo11-1-3 and asy1 showed low amounts of SC stretches, dsy1 and mpa1 showed full synapsis. These findings suggest that in Arabidopsis there is a system, depending on the SC formation, that would facilitate regular disjunction of homologous univalents to opposite poles at anaphase I.

Mutation of the murine Bloom's syndrome gene produces global genome destabilization

Bloom's syndrome (BS) is a genetic disorder characterized cellularly by increases in sister chromatid exchanges (SCEs) and numbers of micronuclei. BS is caused by mutation in the BLM DNA helicase gene and involves a greatly enhanced risk of developing the range of malignancies seen in the general population. With a mouse model for the disease, we set out to determine the relationship between genomic instability and neoplasia. We used a novel two-step analysis to investigate a panel of eight cell lines developed from mammary tumors that appeared in Blm conditional knockout mice. First, the panel of cell lines was examined for instability. High numbers of SCEs were uniformly seen in members of the panel, and several lines produced chromosomal instability (CIN) manifested by high numbers of chromosomal structural aberrations (CAs) and chromosome missegregation events. Second, to see if Blm mutation was responsible for the CIN, time-dependent analysis was conducted on a tumor line harboring a functional floxed Blm allele. The floxed allele was deleted in vitro, and mutant as well as control subclones were cultured for 100 passages. By passage 100, six of nine mutant subclones had acquired high CIN. Nine mutant subclones produced 50-fold more CAs than did nine control subclones. Finally, chromosome loss preceded the appearance of CIN, suggesting that this loss provides a potential mechanism for the induction of instability in mutant subclones. Such aneuploidy or CIN is a universal feature of neoplasia but has an uncertain function in oncogenesis. Our results show that Blm gene mutation produces this instability, strengthening a role for CIN in the development of human cancer.