SEQUENCE-ANALYSIS OF DELETION MUTATIONS AT THE HPRT LOCUS OF HUMAN T-LYMPHOCYTES - ASSOCIATION OF A PALINDROMIC STRUCTURE WITH A BREAKPOINTCLUSTER IN EXON-2

To study the structure and mechanism of deletion mutation in human som atic cells in vivo, we have identified and sequenced the breakpoints o f 16 independent deletions at the hypoxanthine-guanine phosphoribosyl transferase (HPRT) locus in human T-lymphocytes, Seven deletions were found in exon 2, three in each of exons 3 and 6, and one in each of in tron 3, exon 8 and exon 9, Most of the deletions seemed to result from non-homologous recombination, possibly by a slippage-misalignment mec hanism between short repeat sequences, Putative secondary DNA structur es, possibly acting as intermediates in the deletion formation, were i dentified in several mutants, Six of the seven exon 2 deletions had a breakpoint within a 12 bp region (in the 5' end of exon 2) which conta ins a 9 nucleotide palindrome (AACCAGGTT) and is preceded by a TGA dir ect repeat tract, One of the mutants had two deletions in tandem, sepa rated by the palindrome. Another mutant, in which 23 bp containing the palindromic sequence was deleted, had an additional base (C) inserted between the breakpoints forming a direct repeat (gACGAC) in the delet ion junction, Taken together with previously reported deletion mutatio ns at the HPRT locus, these results suggest that the deletion cluster in the 5' part of HPRT exon 2 in T-cells in vivo is promoted by the 9 nucleotide palindrome sequence and the TGA repeat tract. The former ma y act as a stabilizer in a putative intermediate structure, and the la tter may induce slippage and misalignment during replication.