MOLECULAR CHARACTERIZATION OF 2 DELETION EVENTS INVOLVING ALU-SEQUENCES, ONE NOVEL BASE SUBSTITUTION AND 2 TENTATIVE HOTSPOT MUTATIONS IN THE HYPOXANTHINE PHOSPHORIBOSYLTRANSFERASE (HPRT) GENE IN 5 PATIENTS WITH LESCH-NYHAN-SYNDROME

Mutations identified in the hypoxanthine phosphoribosyltransferase (HP RT) gene of patients with Lesch-Nyhan (LN) syndrome are dominated by s imple base substitutions. Few hotspot positions have been identified, and only three large genomic rearrangements have been characterized at the molecular level. We have identified one novel mutation, two tenta tive hot spot mutations, and two deletions by direct sequencing of HPR T cDNA or genomic DNA from fibroblasts or T-lymphocytes from LN patien ts in five unrelated families. One is a missense mutation caused by a 610C-->T transition of the first base of HPRT exon 9. This mutation ha s not been described previously in an LN patient. A nonsense mutation caused by a 508C-->T transition at a CpG site in HPRT exon 7 in the se cond patient and his younger brother is the fifth mutation of this kin d among LN patients. Another tentative hotspot mutation in the third p atient, a frame shift caused by a G nucleotide insertion in a monotono us repeat of six Gs in HPRT exon 3, has been reported previously in th ree other LN patients. The fourth patient had a tandem deletion: a 57- bp deletion in an internally repeated Alu-sequence of intron 1 was sep arated by 14 bp from a 627-bp deletion that included HPRT exon 2 and w as flanked by a 4-bp repeat. This complex mutation is probably caused by a combination of homologous recombination and replication slippage. Another large genomic deletion of 2969 bp in the fifth patient extend ed from one Alu-sequence in the promoter region to another Alu-sequenc e of intron 1, deleting the whole of HPRT exon 1, The breakpoints were located within two 39 bp homologous sequences, one of which overlappe d with a well-conserved 26-bp Alu-core sequence previously suggested a s promoting recombination. These results contribute to the establishme nt of a molecular spectrum of LN mutations, support previous data indi cating possible mutational hotspots, and provide evidence for the invo lvement of Alu-mediated recombination in HPRT deletion mutagenesis.