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

2. 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?

3. 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

4. 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 position 2281 which is known as a blmAsh mutation

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

6. 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

7. 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

8. Functions continued

9. 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

10. 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)

11. Sister Chromatid Exchanges
Normal Mutant

12. 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

13. References
Hickson, I. RecQ helicases: caretakers of the genome. Nature Reviews Cancer. 3, (2003).
Luo, G. et al. Cancer predisposition caused by elevated mitotic recombination in Bloom mice. Nature Genetics. 26, (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, (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, (2001).