ELSEVIER Complementation of Bloom Cellular Phenotype by Human Chromosome 15 Bloom Syndrome (BS) is a rare autosomal recessive disor- der characterized by proportional dwarfing, immunodefi- ciency, predisposition to various malignancies, and by the pathognomonic feature of an increased level of spontane- ous sister chromatid exchanges (SCEs). Shiraishi and Li [1] recently reported lack of complementation of the elevated SCE phenotype after fusion of BS cells with B-cell lymphoma cell lines bearing structural abnormalities on 14q32, suggest- ing that the BS gene might be located in this region. McDaniel and Schultz [2] had previously obtained the correc- tion of the SCE phenotype by microcell-mediated chromo- some transfer (MMCT) of a human chromosome 15. This re- sult was rather unexpected, because the previous candidate genes were located on chromosomes 12, 17, 18, 19, and 21. Since xenogenic hybrids often develop chromosome rear- rangements, we performed MMCT with a human chromo- some 15 of different origin, to test the evidence for the cor- rection of the BS cellular defect. The MMCT experiment was carried out as previously described [3] using as recipient an SV40-transformed fibroblast cell line (GMO8505B) obtained from a BS patient (NIGMS Human Genetic Mutant Cell Repository) and the MC 200-7 mouse-human monochromo- somic hybrid [4], containing a neo-tagged human chromo- some 15, as donor cells. Three G418-resistant clones were identified, but only one (BS/H15) could be expanded in cul- ture. A supranumerary chromosome 15 was detected by DA- DAPI banding and the SCE level was significantly lowered in clone BS/H15 as compared to BS control cells (p < 0.01) (Table 1). The present experiments therefore confirm, with a different chromosome source, that the elevated SCE pheno- type of BS cells is complemented by human chromosome 15, supporting the chromosome 15 localization of the BS gene. Heterogeneity of BS has never been reported in spite of the wide use of complementation experiments by cell fusion. Therefore the lack of complementation of the SCE pheno- type of BS by lymphoma cells with chromosomal breaks at 14q32 [1] can hardly be explained with a second BS locus on chromosome 14. This assumption is supported by the reported failure to correct the SCE phenotype following trans- fer of a normal chromosome 14 to BS cells [2]. The inability of B-cell lymphoma cells to complement the BS phenotype by whole cell fusion [1] suggests that the BS gone is altered in these cells, in agreement with the frequent abnormalities of chromosome 15 observed in B-cell lymphoma [5]. Con- sidering the predisposition to neoplasia in BS patients, it Table 1 SCE frequency for the BS cell line and the chromosome 15 BS/H15 hybrid clone Metaphases Chromosome No SCEs per Cell line counted number chromosome BS 9 81.0 0.94 BS/H15 9 80.9 0.32 a a p < 0.01. is conceivable that the BS gene may be involved by deletion or mutation in the development or progression of B-cell lym- phomas and other common sporadic neoplasias. A. SENSI A. BONFATTI R. GRUPPIONI F. GUALANDI P. RIMESSI C. TRABANELLI G. BARBANTI-BRODANO E.J. STANBRIDGE Institute of Medical Genetics, and Institute of Microbiology, University of Ferrara, Ferrara, Italy; Department of Microbiology and Molecular Genetics, University of California, Irvine, California NOTE ADDED IN PROOF: While this article was in press, a paper by German et al. (Proc Natl Acad Sci 91:6669-6673, 1994) assigned the Bloom locus to chromosome 15q 26.1 by linkage analysis. REFERENCES 1. Shiraishi Y, Li M (1993): Uncorrected SCE levels of Bloom Syn- drome cells by cell hybridization with malignant cells with 14q32 structural abnormalities. Cancer Genet Cytogenet 69:45-50. 2. Mc Daniel L, Schultz R (1992): Elevated sister chromatid exchange phenotype of Bloom Syndrome cells is complemented by hu- man chromosome 15. Proc Natl Acad Sci USA 89:7968-7972. 3. Saxon PJ, Srivatsan ES, Leipzig GV, Sameshima JH, Stanbridge EJ (1985): Selective transfer of individual human chromosomes to recipient cells. Mol Cell Biol 5:140-146. 4. Saxon PJ, Schultz RA, Stanbridge EJ, Friedberg EC (1989): Hu- man chromosome 15 confers partial complementation of pheno- types to Xeroderma Pigmentosum Group F cells. Am J Med Ge- net 44:474-485. 5. Mitelman F (1991): Catalog of Chromosome Aberrations in Can- cer, 4th Ed. Wiley Liss, New York pp. 1170-1172. Cancer Genet Cytogenet 80:167 (1995) © Elsevier Science Inc., 1995 655 Avenue of the Americas, New York, NY 10010 0165-4608/95/$9.50 SSDI 0165-4608(94)00169-C