Bloom’s Syndrome and Bloom helicase

1. Bloom’s Syndrome and Bloom helicase Alexandra Otto March 16, 2004

2. Bloom Syndrome • Syndrome was first described by New York dermatologist David Bloom in 1954 • Extremely rare ~ 220 cases worldwide • Death before age 30 • Mean age of cancer diagnosis ~ 24 • BS is associated with a predisposition to cancers of all types • Autosomal recessive disorder • Arises from a mutation in the gene BLM

3. Proportional dwarfism Sun-induced erythema Type-II diabetes Narrow face and prominent ears Male infertility and female sub-fertility Frequent infections Clinical Features of BS http://www.skinsite.com/erythema

4. How was BLM identified? • Prevalence of BS among the Ashkenazi Jewish population (carrier rate of 1%) • Used positional cloning (like Rb) • Maps to chromosome 15q26.1 ) http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM

5. Helicases are enzymes that separate the complementary strands of nucleic-acid duplexes essential for all aspects of DNA metabolism BLM encodes a helicase http://www.blc.arizona.edu/marty/411/Modules/Lectures/Figures/helicase.GIF

6. The RecQ helicase family • BLM helicase is a member of the RecQ family • RecQ family gets its name from the recQ gene in E. coli. • Family members share a homologous region with E. coli • Conserved region is flanked by stretches of amino acids called the N-terminal region and the C-terminal region

7. http://www.nature.com/nrc/journal/v3/n3/images/nrc1012-f1.jpghttp://www.nature.com/nrc/journal/v3/n3/images/nrc1012-f1.jpg

8. RecQ helicases • Unicellular organisms express 1 RecQ enzyme whereas humans express 5 • Defects in 3 of these human RecQ helicases (BLM, WRN, and RECQ4) give rise to clinical disorders associated with cancer predisposition • Bloom’s syndrome, Werner’s syndrome, and Rothmund-Thomson syndrome

9. Role of Bloom helicase • Required for the maintenance of genomic integrity • Duplex unwinding • ‘Caretaker’ tumor-suppressor • Caretakers influence genomic stability without directly regulating tumorigenesis • Repair of double-strand breaks

10. http://nar.oupjournals.org/cgi/content/full/31/21/6272

11. Role of BLM helicase • bloom helicase normally plays a role in the repair of DSB by the homologous recombination pathway • In Bloom’s syndrome cells, repair may occur through the error-prone NHEJ pathway • increased genomic instability and predisposition to malignancy. • BLM helicase has not been placed at an exact step in the HR pathway

12. Possible roles of BLM • Ability to process recombination intermediates during DNA replication - G-quadruplexes, hairpins • Bloom helicase could reset the replication fork by branch migration http://www.nature.com/nrc/journal/v3/n3/images/nrc1012-f1.jpg

13. BLM helicase as a roadblock remover . http://www.biochemj.org/bj/374/0577/bj3740577.htm

14. Branch migration http://www.biochemj.org/bj/374/0577/bj3740577.htm

15. Interaction with crucial proteins • BLM has not been definitively placed at a certain step in the homologous recombination pathway, but is known to interact with a number of crucial proteins http://www.nature.com/nrc/journal/v3/n3/images/nrc1012-f1.jpg

16. Features of BLM helicase mutants • Abnormal DNA replication • Elevated level of homologous recombination In Bloom’s syndrome cells → accumulation of abnormal replication intermediates → increase in the frequency of reciprocal exchanges → ~ 10 fold increase in sister-chromatid exchanges

17. Mouse Model Knockout mice - death by extreme anemia at 13.5 days - immortalized cell line showed a high frequency of sister chromatid exchange - characteristic short stature is seen in early stages of embryo development Viable BLM-/- Mouse - elevated rate of mitotic recombination - high frequency of sister-chromatid exchanges and somatic loss of heterozygosity - high cancer incidence (lymphomas, carcinomas, sarcomas) http://www.weizmann.ac.il/home/ligivol/publications/PNAS%201999.pdf

18. Cancer Predisposition What features of hyper-recombination underlie the cancer predisposition? • Recombination events are not carried out with perfect fidelity • Events are not carried out to completion This leads to: • Chromosomal duplication or breakage • Genomic instability and therefore cancer

19. BLM helicase and cancer Concluding points • BLM helicase is a caretaker tumor suppressor • Proposed to act in HR pathway • Homologous recombination exists to repair double strand breaks and damaged replication forks • Sister chromatid exchanges arise during HR from the crossing over of chromatid arms • BS cells have high frequency of SCE • This hyper-recombination results from defective replication • Without BLM helicase, replication cannot proceed smoothly • Genomic instability → CANCER predisposition • Cancer of all types because all cells need to repair damages in replication machinary

20. Works Cited http://www.ncbi.nlm.nih.gov http://www.nature.com/nrc/journal/v3/n3/images/nrc1012-f1.jpg http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12691817 http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12427531 http://www.biochemj.org/bj/374/0577/bj3740577.htm http://www.mssm.edu/jewish_genetics/genetic_diseases.shtml http://www.nature.com/cgi-taf/DynaPage.taf?file=/onc/journal/v21/n58/full/1205959a.htm http://tmm.trends.com http://www.biomedcentral.com/1471-2121/4/15