PROXIMAL 6Q, A REGION SHOWING ALLELE LOSS IN PRIMARY BREAST-CANCER

To define regions of deletion of chromosome 6q in breast cancer, we scored 18 (CA)(n) microsatellites for allelic imbalance (AI) in 42 paired blood/tumour samples. Heterozygosity frequencies of the markers in the sample population ranged from 31% to 92% (mean 68%). Two regions of the chromosome arm showed Al values greater than the background range of 10-22% (mean 17%) of informative cases that was observed with five markers spanning 6q21-q25.2. Firstly, seven markers gave Al values that averaged 35% in a region flanked by D6S313 (AI = 10%) at 6q13 and D6S283 (AI = 17%) at 6q16.3-21. The second region showed marginally increased Al at 6q25.2-q27 and included D6S193, previously shown to be close to a tumour-suppressor gene involved in ovarian carcinoma. Since AI of 6q in breast cancer was shown previously to be due predominantly to loss of heterozygosity, our results suggest the presence of at least two tumour-suppressor genes on 6q that are involved in breast cancer. The proximal region has not been recognised in breast cancer before and is involved in a higher frequency of tumours than the distal region.