SMAD5, A TUMOR-SUPPRESSOR CANDIDATE AT 5Q31.1, IS HEMIZYGOUSLY LOST AND NOT MUTATED IN THE RETAINED ALLELE IN HUMAN LEUKEMIA-CELL LINE HL60

Deletions of the long arm of chromosome 5 with common overlapping segm ent 5q31.1 are among the most frequent cytogenetic aberrations in myel odysplastic syndromes and acute myeloid leukemias (MDS/AML). We have c onstructed a YAC-based physical map of the 5q31.1 critical locus and l ocalized the transcriptional transactivator Smad5 adjacent to loci sho wing consistent loss of heterozygosity in these disorders. Smad5 plays a key role along the bone morphogenetic protein-4 (BMP-4) inhibitory signalling pathway inducing embryonic hematopoiesis. Smad5 homologs Sm ad2 and DPC4 have recently been linked to human cancer. FISH analysis of AML-M2 cell line HL60 and of four MDS/AML patients revealed consist ent hemizygous loss of the SmadS locus. In HL60 cells, a translocation event within 5q31.1 associated with loss of adjacent material leads t o disruption of the critical locus with partial retention of the 5q31. 1 genomic sequences on a marker chromosome. RT-PCR sequencing analysis of the HL60 Smad5 remaining allele ruled out the functional inactivat ion of the gene analogous to that occurring in the Smad5 homologs DPC4 and Smad2 in cases of pancreatic and colorectal cancers. Mutational a nalysis of Smad5 in MDS/AML cases is in progress.