INTERSTITIAL DELETION CONSTITUTES THE MAJOR MECHANISM FOR LOSS OF HETEROZYGOSITY ON CHROMOSOME 20Q IN POLYCYTHEMIA-VERA

An acquired deletion of the long arm of chromosome 20 is a recurrent a bnormality in myeloproliferative disorders, particularly polycythemia vera and myelodysplastic syndromes, The association of 20q deletions w ith myeloid ''stem cell'' disorders suggests that the deletions mark t he site of one or more genes, loss or inactivation of which plays a ro le in the regulation of normal hematopoietic progenitors, We have rece ntly performed a detailed molecular analysis of 20q deletions in perip heral blood (PB) granulocytes and defined a commonly deleted region of 16 to 21 centimorgan (cM), To further reduce the size of the common d eleted region we have searched for small deletions or mitotic recombin ation events, neither of which would be detected by conventional cytog enetics. We have studied 48 patients with polycythemia vera and four p atients with idiopathic myelofibrosis, In each case, cytogenetic analy sis had either failed or had shown no abnormalities of chromosome 20, Seventeen microsatellite markers that span the common deleted region w ere used to search for loss of heterozygosity in granulocyte DNA. No i nstance of microsatellite instability was observed in a total of 880 c omparisons of granulocyte and T-cell DNA. Granulocyte DNA from four pa tients exhibited allele loss on 20q, In each case the allele loss was caused by an interstitial deletion because heterozygosity at distal ma rkers was retained and because quantitative Southern blotting demonstr ated hemizygosity. Loss of heterozygosity in PB granulocytes would be masked by the presence of significant numbers of normal granulocytes n ot derived from the malignant clone, Therefore, the human androgen rec eptor assay (HUMARA) was used to determine granulocyte clonality, In 2 1 of 27 informative female patients the majority of the granulocytes w ere clonally derived, In 5 patients the granulocytes appeared polyclon al and in 1 patient unilateral X inactivation was observed in both gra nulocytes and T cells, These results show that, in the vast majority o f patients presented here, the failure to detect loss of heterozygosit y cannot be attributed to the presence of normal polyclonal granulocyt es. Our results therefore show that allele loss on chromosome 20q in p olycythemia vera does not commonly involve mitotic recombination or ch romosome loss and that microsatellite instability is a rare event in t his disorder. (C) 1996 by The American Society of Hematology.