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Antigenic variation in Trypanosoma brucei has selected for the evolution of a massive archive of silent Variant Surface Glycoprotein (VSG) genes, which are activated by recombination into specialized expression sites. Such VSG switching can occur at rates substantially higher than background mutation and is dependent on homologous recombination, a core DNA repair reaction. A key regulator of homologous recombination is BRCA2, a protein that binds RAD51, the enzyme responsible for DNA strand exchange. Here, we show that T. brucei BRCA2 has undergone a recent, striking expansion in the number of BRC repeats, a sequence element that mediates interaction with RAD51. T. brucei BRCA2 mutants are shown to be significantly impaired in antigenic variation and display genome instability. By generating BRCA2 variants with reduced BRC repeat numbers, we show that the BRC expansion is crucial in determining the efficiency of T. brucei homologous recombination and RAD51 localization. Remarkably, however, this appears not to be a major determinant of the activation of at least some VSG genes.

Neurofilament accumulations are characteristic of a number of neurological conditions including amyotrophic lateral sclerosis, giant axonal neuropathies and several chemically-induced neuropathies. Although the mechanism(s) leading to neurofilament accumulation are unknown, it is possible that similar processes occur both in disease and in chemically-induced neuropathies. Understanding the mechanism(s) of chemically-induced neurofilament accumulation, which is more amenable to experimental manipulation, may give insight into the neurological diseases they mimic. We have compared the effects of two chemically-dissimilar neurotoxins, 2,5-hexanedione and acrylamide, on neurofilaments in the human neuroblastoma cell line, SH-SY5Y. Both undifferentiated and differentiated SH-SY5Y cells were exposed to 2,5-hexanedione or acrylamide and changes in cytoskeletal organization examined by immunofluorescence and electron microscopy. Although distinct morphological differences have previously been characterized in the neuropathies induced by 2,5-hexanedione and acrylamide in vivo, we have found that both compounds had similar direct effects on neurofilaments in SH-SY5Y cells, inducing formation of perikaryal inclusion bodies. In addition, differentiated SH-SY5Y cells were more sensitive to both 2,5-hexanedione and acrylamide compared with undifferentiated cells. These similar effects of 2,5-hexanedione and acrylamide lend further support that a common mechanism(s) may lead to neurofilament accumulation in these neuropathies. SH-SY5Y cells provide a useful model to investigate further the biochemical basis of neurofilament accumulation.