The cellular distribution of BLM, the Bloom's syndrome protein. M.M. Sanz¹, M. Proytcheva², N.A. Ellis³, W.K. Holloman¹, J. German¹. 1) Departments of Microbiology and Pediatrics, Cornell University Medical College, New York, NY; 2) Department of Pathology, Albert Einstein College of Medicine, New York, NY; 3) Department of Human Genetics, Memorial Hospital for Cancer and Allied Diseases, New York, NY.

   The gene mutated in Bloom's syndrome (BS) encodes a 1,417 aa protein, BLM, which has DNA helicase activity and so is assumed in some way to manipulate DNA. A major question is, What is/are its role(s) in DNA replication, DNA repair, transcription, and, or, recombination, in somatic and, or, germ-line cells? BLM is absent from cells of most persons with BS, which leads to the conclusion that its activity impinges directly or indirectly on the maintenance of genomic stability. In order to obtain leads as to BLM's exact function(s), its distribution has been examined in cells in culture, synchronized and not, using BLM antibodies and immunofluorescence microscopy. In interphase cells, BLM is detected only in the nucleus. In early post-telophase cells it is barely visible. It then increases in amount throughout G1, being both diffuse and also concentrating in multiple (sometimes 20 or more) minute, punctate, brightly fluorescing centers. During S, several (often 10 or more) larger punctate accumulations of BLM become brightly fluorescent, distinctive in appearance from the minute accumulations; these often co-localize with the PML nuclear protein, and in a more transitory way with RPA (the single-strand-binding replication protein A). In addition to the discrete foci, larger but less intensively fluorescing, irregularly shaped aggregates of BLM form in many nuclei. At metaphase, the condensed chromosomes appear to be BLM-free, the BLM then being distributed throughout the entire cell where it fluoresces intensely. Because early G1 cells have so little BLM (above), the conclusion seems inevitable that this is another protein that is eliminated from the cell during late mitosis. These observations will be extended to search for other proteins that co-localize with BLM, and they are to be complemented by immuno- and straightforward biochemical analyses of BLM.