Scientists have discovered a key mechanism involved in the correct separation of chromosomes during the formation of eggs and sperm.

The research shows that BubR1 - a gene recently shown to affect cell division – maintains the cohesion of paired chromosomes (until their time to divide) during the production of reproductive cells. Because BubR1 mutations can result in cells with abnormal numbers of chromosomes, the research has potential implications for human disorders resulting from loss or gain of chromosomes such as Down Syndrome, a disease caused by an extra copy of chromosome 21.

Deletion of the BubR1 gene has been shown to disturb chromosome separation during meiosis - the process by which the reproductive cells, sperm and eggs, are formed - although how this happens is not clear.

Following the discovery of a non-lethal BubR1 mutation in fruit flies Nicolas Malmanche, Claudio E. Sunkel and colleagues decided to try and identify the molecular role of this gene in meiosis. Fruit flies are particularly advantageous in this case as males and females of the species use different molecular mechanisms for the distribution of chromosomes between cells during meiosis, allowing a more detailed analysis of the effects of the BubR1 mutation and consequently also of BubR1 normal role.

Cells normally have two sets of each chromosome (called homologue chromosomes) where one set has come from the father and the other set of the mother. Meiosis, the specialised cell division that produces the sperm and eggs, starts with the duplication of all the chromosomes in the cell – that at this stage stay linked and are called (sister) chromatids - followed by two sets of divisions. During the first division homologues chromosomes are separated with each of the two daughter cells receiving one, while in the second division it is the sister chromatids that are separated with each sex cell receiving one from each pair.

It was by analysing and comparing mutated BubR1 and normal flies throughout these processes that Malmanche, Sunkel and colleagues were able to discover that the BubR1 gene is essential to maintain sister chromatids’ linked throughout meiosis, assuring in this way a correct distribution of the genetic material in the produced sex cells.

The researchers also saw that in BubR1 mutated females, a complex structure called Synaptonemal Complex (SC), which binds homologue chromosomes during the first division of the meiosis and allows recombination (exchange of genetic material between homologous chromosomes, which is essential for generation of diversity) was also disrupted. Accordingly, detailed analysis of this process of recombination in BubR1 mutant cells revealed significant alterations in its frequency and distribution.

Malmanche, Sunkel and colleagues’ discoveries reveal BubR1 gene as crucial for a proper distribution of the genetic material during eggs and sperm formation in fruit flies. But because BubR1 is conserved throughout species and also exist in humans the research have potential implications for the study of human diseases caused by abnormal chromosomal distribution such as Down’s syndrome, which incidence increases with the mother’s age and can affect as much as 4% of the births in women over 45 years old.

Most significantly Down Syndrome individuals are known to have abnormal patterns of recombination and loss of cohesion between sister chromatids exactly like the defects observed in fruit flies with a mutant BubR1gene. As Claudio Sunkel says, “our observations suggest for the first time that inappropriate or reduced function of a gene like BubR1 might be at the heart of age-related chromosome imbalance observed in humans.”

Source: BubR1 is essential to maintain sister chromatid cohesion and Synaptonemal Complex during Drosophila meiosis, Current Biology - (2007)