The Dissolvasome in C. elegans Meiosis

Unfaithful chromosome segregation into gametes leads to infertility, embryonic death and syndromes linked to metal retardation. A specialized series of cell divisions during gametogenesis ensures faithful chromosome segregation and genetic diversity. In the first meiotic division parental homologs, and in the second, sister chromatids are partitioned. A physical tether between parental homologs supports their correct orientation, enabling the spindle to pull homologs to opposite poles in the first division. The connections are a result of crossover (CO) recombination. Following induction of DNA double-strand breaks, resection steps generate single-stranded overhangs that invade a sister chromatid of the parental homolog to use it as a repair template for homologous recombination. This culminates in the generation of a DNA double Holliday junction (dHJ) molecule, which can be subjected to resolution to produce CO and non-CO products, depending where a final DNA cut is placed. DNA double strand breaks are always in excess over the final number of COs. Both the lack of COs and unrepaired DNA lesions cause chromosome mis- segregation and genomic instability, making tight control a necessity. How CO control is accomplished is largely unknown. In this project we set out to study the role of the dHJ dissolvasome in formation of COs and non-COs. The dissolvasome provides the major activity for non-CO formation in mitosis. The dHJ dissolvasome contains a topoisomerase, Bloom helicase and RMI1/2, scaffolding components. Patients with mutated Bloom helicase display cancer predisposition syndromes. We isolated several mutants in the C. elegans RMI1 homolog (RMH-1), which allows us to uncover the role of the dHJ dissolvasome in a genetic animal model system with the advantage to combine genetics with high-resolution cytology.

Unfaithful chromosome segregation into gametes leads to infertility, embryonic death and syndromes linked to mental retardation. Meiosis, a specialized series of cell divisions during gametogenesis ensures faithful chromosome segregation and genetic diversity. In the first meiotic division parental homologs, and in the second, sister chromatids are partitioned. A physical tether between parental homologs supports their correct partitioning into germcells. The connections are a result of crossover (CO) recombination. Following induction of DNA double-strand breaks, resection steps generate single-stranded overhangs that invade a sister chromatid of the parental homolog to use it as a repair template for homologous recombination. This culminates in the generation of a DNA double Holliday junction (dHJ) molecule, which can be subjected to resolution to produce CO and non-CO products, depending where a final DNA cut is placed. DNA double strand breaks are always in excess over the final number of COs. Both the lack of COs and unrepaired DNA lesions cause chromosome mis-segregation and genomic instability, making tight control a necessity. How CO control is accomplished is largely unknown. In this project we set out to study the role of the dHJ dissolvasome in formation of COs and non-COs. The dissolvasome provides the major activity for non-CO formation in mitosis. The dHJ dissolvasome contains a topoisomerase, Bloom helicase and RMI1/2, scaffolding components. We isolated several mutants in the C. elegans RMI1 homolog, which allowed us to uncover that the dissolvasome indeed plays an important role in faithful chromosome segregation in meiosis in a genetic animal model system. Furthermore, we generated a detailed picture of the dynamics of RMI1 containing recombination foci.