RFLP mapping of thymidine kinase mutants places Dl7S4 proximal to the human TK1 locus

There is uncertainty in the literature concerning the relative map order of D17S4 and the thymidine kinase (TK1) locus on human chromosome 17. Meiotic linkage analysis has placed D17S4 both distal (1) and proximal (2) to TK1 on 17q. In this report, loss of heterozygosity in a set of thymidine kinase mutants of a human lymphoblastoid cell line indicates that D17S4 is proximal to TK1. X-ray induced mutants (3) which had lost heterozygosity at a Sad RFLP downstream of the TK1 locus (4) were examined for allele loss at other markers on chromosome 17. The informative markers examined were D17S21, a locus proximal to TK1 (5), D17S24 and D17S77, loci distal to TK1 (1), and D17S4. Allele loss was detected by standard RFLP analysis. The parent cell line used is cytogenetically stable in culture, and karyotypic analysis showed two normal chromosomes 17 (6). All of the markers examined in the mutants behaved as would be predicted from their reported linkage. Table 1 presents the results for these markers in the wild-type parent line and in several derivative mutant cell lines. The data include 7 mutants which had lost the proximal marker D17S21 and D17S4, and retained the known distal marker D17S77. In addition, there were 7 mutants which had lost the distal marker D17S77 but not D17S4. This pattern of concerted allele loss is consistent with the ordering of these loci: cen . -D17S21-D17S4-TK1-D17S77D17S24—q-ter. These data are supported by another mutant (number 16 in Table 1) from a related cell line (7) which demonstrates allele loss for TK1, D17S77, and D17S24, but not D17S4.