Emily Newstrom

Introduction. Bloom (BLM) Helicase is a multifunctional protein that, when mutated, is primarily implicated in Bloom Syndrome, an autosomal recessive disorder characterized by genomic instability. Bloom Syndrome was first recognized as a clinical presentation with features of short stature, developmental abnormalities, immunodeficiency, UV sensitivity, and a predisposition for cancer. Cancer propensity has the greatest negative effect, often leading to not only early onset cancers, but also multiple cancers and early death^1,2. It is theorized that this predisposition is due to DNA instability creating a higher risk for accumulation of mutations, which was largely attributed to BLM’s role in regulating double strand break excision and homologous recombination during DNA repair^3,4,5,6. Recently, an additional mechanism has been presented that involves BLM protecting DNA stability during replication by unwinding guanine-rich quadruplexes (G4 complexes) that can form within DNA and block/collapse replication forks^2,7. This mechanism lends new insight into BLM’s contribution to genomic stability, a full understanding of which can create opportunities for treatment development. Methods. Single-molecule fluorescence resonance energy transfer (smFRET) was used to visualize the step-by-step interaction of BLM helicase with G4 complexes. Green and red dyes were injected into DNA segments before and after G4 motifs. High or low FRET emission was based on the degree of energy transference between the two regions⁷. Results. G4 motifs adjacent to ssDNA displayed the lowest FRET values, meaning they were successfully unwound by BLM such that transference was limited. FRET remained high after interaction with G4 next to dsDNA, however, showing that BLM is only functional before G4 complexes fully form and integrate with dsDNA. FRET values also alternated throughout the BLM-G4 interaction, suggesting a step-wise mechanism involving constant unfolding and refolding of G4 by BLM until it is completely unwound. In this way, BLM can operate in an ATP-independent environment, confirmed by similar FRET values resulting from interactions with and without ATP present⁷. Conclusions. Studies of BLM Helicase reveal complex and widespread functions in its protection of the human genome. Early studies outlined its action in DNA repair and newer research has presented a separate replication mechanism. Because there are currently no treatments for Bloom Syndrome, continued study is necessary both to expand on understanding of these mechanisms and explore potential unknown functionalities. As a fuller picture of BLM Helicase is established, perhaps techniques for restoring function can be developed to retain genomic stability and reduce the considerable cancer risk in these patients.

1. Cunniff C, Bassetti JA, Ellis NA. Bloom’s Syndrome: Clinical Spectrum, Molecular Pathogenesis, and Cancer Predisposition. MSY. 2017;8(1):4-23. doi:10.1159/000452082
2. van Wietmarschen N, Merzouk S, Halsema N, Spierings DCJ, Guryev V, Lansdorp PM. BLM helicase suppresses recombination at G-quadruplex motifs in transcribed genes. Nat Commun. 2018;9. doi:10.1038/s41467-017-02760-1
3. Nimonkar AV, Özsoy AZ, Genschel J, Modrich P, Kowalczykowski SC. Human exonuclease 1 and BLM helicase interact to resect DNA and initiate DNA repair. PNAS. 2008;105(44):16906-16911. doi:10.1073/pnas.0809380105
4. Nimonkar AV, Genschel J, Kinoshita E, et al. BLM–DNA2–RPA–MRN and EXO1–BLM–RPA–MRN constitute two DNA end resection machineries for human DNA break repair. Genes Dev. 2011;25(4):350-362. doi:10.1101/gad.2003811
5. Gravel S, Chapman JR, Magill C, Jackson SP. DNA helicases Sgs1 and BLM promote DNA double-strand break resection. Genes Dev. 2008;22(20):2767-2772. doi:10.1101/gad.503108
6. Wang Y, Li S, Smith K, Waldman BC, Waldman AS. Intrachromosomal recombination between highly diverged DNA sequences is enabled in human cells deficient in Bloom helicase. DNA Repair. 2016;41:73-84. doi:10.1016/j.dnarep.2016.03.005
7. Chatterjee S, Zagelbaum J, Savitsky P, et al. Mechanistic insight into the interaction of BLM helicase with intra-strand G-quadruplex structures. Nature Communications. 2014;5:5556. doi:10.1038/ncomms6556