BLOOM HELICASEBLOOM HELICASE(and BLOOM SYNDROME)(and BLOOM SYNDROME)

Amanda DeWittAmanda DeWittTuesday, March 29, 2005Tuesday, March 29, 2005

OUTLINEOUTLINE

Overview of Bloom’s SyndromeOverview of Bloom’s Syndrome Discussion of Discussion of BLMBLM Brief description of helicase function in generalBrief description of helicase function in general Discussion of Bloom helicase functionDiscussion of Bloom helicase function Interaction of Bloom helicase and RAD51 in Interaction of Bloom helicase and RAD51 in

homologous recombinationhomologous recombination Mouse knockoutsMouse knockouts Treatment for Bloom’s Syndrome?Treatment for Bloom’s Syndrome?

Bloom’s SyndromeBloom’s Syndrome

Autosomal recessive disease Autosomal recessive disease

Phenotypic traits include: proportional Phenotypic traits include: proportional dwarfism, sensitivity to sunlight, type II dwarfism, sensitivity to sunlight, type II diabetes, narrow face and prominent ears, diabetes, narrow face and prominent ears, male infertility, female subfertility…male infertility, female subfertility…

Marked by increased predisposition to Marked by increased predisposition to most types of cancer most types of cancer

BLMBLM

The gene affected in Bloom’s Syndrome is The gene affected in Bloom’s Syndrome is BLM BLM (a tumor-suppressor “caretaker” gene)(a tumor-suppressor “caretaker” gene)

Maps to 15q26.1Maps to 15q26.1 Many types of mutations can occur: Many types of mutations can occur:

missense, frameshift, nonsense, splice-missense, frameshift, nonsense, splice-site, etc..site, etc..

Most common mutation is Most common mutation is delATCTGA/insTAGATTC @ position 2281 delATCTGA/insTAGATTC @ position 2281 which is known as a blmAsh mutation which is known as a blmAsh mutation

RecQ family HelicasesRecQ family Helicases

Highly conserved from Highly conserved from bacteria to manbacteria to man

Maintain genomic stabilityMaintain genomic stability

Contain a highly conservedContain a highly conservedhelicase domain ~400helicase domain ~400amino acids long amino acids long

Bloom HelicaseBloom Helicase

In normal cells Bloom helicase interacts with In normal cells Bloom helicase interacts with many different proteins to help repair DNA many different proteins to help repair DNA damagedamage

Because Bloom helicase interacts with so many Because Bloom helicase interacts with so many different proteins, any conformational change in different proteins, any conformational change in the helicase could lead to an ineffective proteinthe helicase could lead to an ineffective protein

Bloom Helicase continuedBloom Helicase continued

Some proposed functions of Bloom Some proposed functions of Bloom Helicase:Helicase: Removes “roadblocks” (recombination Removes “roadblocks” (recombination

intermediates) during DNA replicationintermediates) during DNA replication Re-initiates replication at sites where the Re-initiates replication at sites where the

replication-fork has been disruptedreplication-fork has been disrupted Resets a four-way junction to bypass lesions Resets a four-way junction to bypass lesions

during DNA replication/repairduring DNA replication/repair Aids in telomere maintainanceAids in telomere maintainance

Cellular function is still unknown Cellular function is still unknown

Functions continuedFunctions continued

BLM and RAD51 as a complex in BLM and RAD51 as a complex in Homologous Recombination (an example)Homologous Recombination (an example)

Wu, Davies, Levitt, and Hickson found that BLM Wu, Davies, Levitt, and Hickson found that BLM and RAD51 interact during homologous and RAD51 interact during homologous recombination recombination

They propose that RAD51 acts upstream from They propose that RAD51 acts upstream from BLM to pair homologous sequences and BLM to pair homologous sequences and exchange DNA strands to form recombination exchange DNA strands to form recombination intermediatesintermediates

BLM is then needed to remove these BLM is then needed to remove these intermediates in order to prevent excessive intermediates in order to prevent excessive recombinationrecombination

BLM Mouse KnockoutsBLM Mouse Knockouts

Luo, G. Luo, G. et alet al. created BLM mouse knockouts . created BLM mouse knockouts ((Blm Blm -/--/- ) )

Analysis of cells from these knockouts showed Analysis of cells from these knockouts showed higher rates of mitotic recombination (e.g. sister higher rates of mitotic recombination (e.g. sister chromatid exchanges), somatic loss of chromatid exchanges), somatic loss of heterozygosity (in other genes) which was heterozygosity (in other genes) which was paired with a tremendous increase in cancerpaired with a tremendous increase in cancer

These mice (like humans) developed a variety of These mice (like humans) developed a variety of cancers (lymphomas, sarcomas and cancers (lymphomas, sarcomas and carcinomas)carcinomas)

Sister Chromatid ExchangesSister Chromatid Exchanges

Normal MutantNormal Mutant

TREATMENT?TREATMENT?

There is no known treatment for Bloom There is no known treatment for Bloom Syndrome at the current timeSyndrome at the current time

FIY: Dr. Ian Hickson will speak on FIY: Dr. Ian Hickson will speak on “Genomic Instability and Cancer: Role of “Genomic Instability and Cancer: Role of the Bloom’s Syndrome Helicase and its the Bloom’s Syndrome Helicase and its Partner Topoisomerase III” at Duke on Partner Topoisomerase III” at Duke on May 3, 2005 at 12:30pmMay 3, 2005 at 12:30pm

ReferencesReferences

Hickson, I. RecQ helicases: caretakers of the genome. Hickson, I. RecQ helicases: caretakers of the genome. Nature Reviews Nature Reviews CancerCancer. . 33, 169-177 (2003)., 169-177 (2003).

Luo, G. Luo, G. et alet al. Cancer predisposition caused by elevated mitotic . Cancer predisposition caused by elevated mitotic recombination in Bloom mice. recombination in Bloom mice. Nature GeneticsNature Genetics. . 2626, 424-429 (2000)., 424-429 (2000).

McVey, M., LaRocque, J.R., Adams, M.D., & Sekelsky, J. Formation of McVey, M., LaRocque, J.R., Adams, M.D., & Sekelsky, J. Formation of deletions during double-strand break repair in deletions during double-strand break repair in DrosophiliaDrosophilia DmBlm DmBlm mutants occurs after strand invasion. mutants occurs after strand invasion. PNASPNAS. . 101101, 15694-15699 (2004)., 15694-15699 (2004).

Wu, L., Davies, S.L., Levitt, N.C., & Hickson, I.D. Potential role for the 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 BLM helicase in recombinational repair via a conserved interaction with RAD51. RAD51. Journal of Biological ChemistryJournal of Biological Chemistry. . 276276, 19375-19381 (2001)., 19375-19381 (2001).