NEW ALLELES AT MICROSATELLITE LOCI IN CEPH FAMILIES MAINLY ARISE FROMSOMATIC MUTATIONS IN THE LYMPHOBLASTOID CELL-LINES

In the analysis of 40 CEPH families, under the EUROGEM project, with a total of 29 microsatellites (26 CA-repeats, a TCTA-repeat within the vWFII-3 gene, a TTA-repeat within the PLA-2 gene, and an AAAT-repeat i ntragenic to the NF1 gene) from human chromosomes 12, 17, and 21, we h ave detected 21 cases of abnormal segregation of alleles in 16 pedigre es for a total of 14 markers (48%). In 11 cases, the abnormal transmis sions were of somatic origin, 10 of which (91%) occurred in the lympho blastoid cell lines. In 9 other cases, it was not possible to determin e if the origin of the new alleles was somatic or germline, and in one case hemizygosity in several family members was observed, so its orig in was germline. The 20 new mutations detected in the 22,852 meioses a nalysed represent a mutation frequency of 8.7 x 10(-4) per locus per a llele. The germline mutation rate could be as high as 3.9 x 10(-4) per locus per gamete (from 0 to 3.9 x 10(-4)), but the rate of somatic mu tations detected in the study was much higher (4.8 x 10(-4) to 8.7 x 1 0(-4) per locus per allele). Individual mutation rates ranged from 0 t o 3.8 x 10(-3). Among the markers analysed, all three that were tri- o r tetranucleotide repeats showed one or two new alleles, compared to o nly 10 of the 26 (38%) CA-repeats showing mutations. Three CEPH famili es (102, 45 and 1333) each had several mutational events, and one indi vidual (10210) had somatic mutations for two microsatellites from diff erent chromosomes. The mutation rate at microsatellite loci within fam ilies, using DNA directly obtained from cells from the individual, is less than 1 x 10(-4) (true germline mutation rate), which should not a ffect the use of these markers in diagnosis and linkage. However, thes e results and previous data suggest that for DNA obtained from cell li nes, mutations are much more frequent (1 x 10(-2)-1 X 10(-3)). (C) 199 4 Wiley-Liss, Inc.