UC San Diego

Topoisomerase II beta-mediated immunodeficiency is a novel syndromic immunodeficiency characterized by the lack of B lymphocytes, a reduced number of antibodies in the blood, in addition to facial, limb and genital abnormalities. Four patients with this syndrome have been shown to have heterozygous mutations in the gene DNA Topoisomerase II Beta (TOP2B). TOP2B has besen shown to be necessary for long gene transcription by creating transient DNA double-stranded breaks (DSBs) that recruit DSB repair proteins in order for transcription to continue. These DSB repair proteins participate in non-homologous end joining (NHEJ) and homologous recombination (HR) pathways. To investigate the molecular mechanism of patient-derived mutations in TOP2B and the DSB repair pathways, we used the model organism Saccharomyces cerevisiae that contains conserved DSB repair pathways in addition to the highly conserved yeast homolog TOP2. Co-immunoprecipitation analyses demonstrated that TOP2 interacts directly with YKU70, YKU80 and RAD50 and their interaction was unchanged in the presence of top2 mutants. Long gene expression in the presence of top2 mutants showed that long gene expression was significantly decreased. Our results suggest that mutations in TOP2B negatively affect the transcription of long genes, possibly due to the interaction with key components of the NHEJ and HR pathways.