Somatic mutations in VNTR-Locus D1S7 in human colorectal carcinomas are associated with microsatellite instability

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Abstract
To elucidate mutation mechanisms in hypervariable VNTR loci, we have studied somatic mutation events with the minisatellite probe MSI (VNTR locus D1S7) in 224 colorectal carcinomas (CRC). The D1 S7 locus consists of a 9-basepair (bp) repeat unit. The copy number varies from about 100 to 2000, and the germline mutation rate is high. Here we demonstrate a high D1S7 somatic mutation rate in CRC (37/224), higher than indicated earlier by others. We atso demonstrate that the most frequent mutational event by far (n = 34) involves small reductions in VNTR fragment size (median loss 22 repeat units, range 2-1.54), furthermore, in one-half of these cases, this event is biallelic. We wanted to test whether these somatic mutations mirror the same genetic instability as seen by RER (replication error), a phenomenon recently described in tumour DNA from both sporadic and familial cases of CRC. All blood/tumour DNA pairs displaying MS1 mutation (n = 37) as well as 37 randomly selected pairs without MS1 mutation were tested with four tetranucleotide short tandem repeats (STRs, microsatellites). There is a strong association between mutations at the D1S7 locus and the occurrence of new STR alleles (P<0.001). This is the first report of the existence of a minisatellite as a marker for genetic instability/RER in colorectal carcinomas. The findings may also cast light upon the mechanism for somatic mutations in this minisatellite.