BLOOM HELICASE (and BLOOM SYNDROME)

BLOOM HELICASE(and BLOOM SYNDROME) Amanda DeWitt Tuesday, March 29, 2005

OUTLINE • Overview of Bloom’s Syndrome • Discussion of BLM • Brief description of helicase function in general • Discussion of Bloom helicase function • Interaction of Bloom helicase and RAD51 in homologous recombination • Mouse knockouts • Treatment for Bloom’s Syndrome?

Bloom’s Syndrome • Autosomal recessive disease • Phenotypic traits include: proportional dwarfism, sensitivity to sunlight, type II diabetes, narrow face and prominent ears, male infertility, female subfertility… • Marked by increased predisposition to most types of cancer

BLM • The gene affected in Bloom’s Syndrome is BLM (a tumor-suppressor “caretaker” gene) • Maps to 15q26.1 • Many types of mutations can occur: missense, frameshift, nonsense, splice-site, etc.. • Most common mutation is delATCTGA/insTAGATTC @ position 2281 which is known as a blmAsh mutation

RecQ family Helicases • Highly conserved from bacteria to man Maintain genomic stability Contain a highly conserved helicase domain ~400 amino acids long

Bloom Helicase • In normal cells Bloom helicase interacts with many different proteins to help repair DNA damage • Because Bloom helicase interacts with so many different proteins, any conformational change in the helicase could lead to an ineffective protein

Bloom Helicase continued • Some proposed functions of Bloom Helicase: • Removes “roadblocks” (recombination intermediates) during DNA replication • Re-initiates replication at sites where the replication-fork has been disrupted • Resets a four-way junction to bypass lesions during DNA replication/repair • Aids in telomere maintainance Cellular function is still unknown

Functions continued

BLM and RAD51 as a complex in Homologous Recombination (an example) • Wu, Davies, Levitt, and Hickson found that BLM and RAD51 interact during homologous recombination • They propose that RAD51 acts upstream from BLM to pair homologous sequences and exchange DNA strands to form recombination intermediates • BLM is then needed to remove these intermediates in order to prevent excessive recombination

BLM Mouse Knockouts • Luo, G. et al. created BLM mouse knockouts (Blm -/- ) • Analysis of cells from these knockouts showed higher rates of mitotic recombination (e.g. sister chromatid exchanges), somatic loss of heterozygosity (in other genes) which was paired with a tremendous increase in cancer • These mice (like humans) developed a variety of cancers (lymphomas, sarcomas and carcinomas)

Sister Chromatid Exchanges Normal Mutant

TREATMENT? • There is no known treatment for Bloom Syndrome at the current time • FIY: Dr. Ian Hickson will speak on “Genomic Instability and Cancer: Role of the Bloom’s Syndrome Helicase and its Partner Topoisomerase III” at Duke on May 3, 2005 at 12:30pm

References • Hickson, I. RecQ helicases: caretakers of the genome. Nature Reviews Cancer. 3, 169-177 (2003). • Luo, G. et al. Cancer predisposition caused by elevated mitotic recombination in Bloom mice. Nature Genetics. 26, 424-429 (2000). • McVey, M., LaRocque, J.R., Adams, M.D., & Sekelsky, J. Formation of deletions during double-strand break repair in Drosophilia DmBlm mutants occurs after strand invasion. PNAS. 101, 15694-15699 (2004). • Wu, L., Davies, S.L., Levitt, N.C., & Hickson, I.D. Potential role for the BLM helicase in recombinational repair via a conserved interaction with RAD51. Journal of Biological Chemistry. 276, 19375-19381 (2001).

BLOOM HELICASE (and BLOOM SYNDROME)