Deletion analysis of chromosome 13q14.3 and characterisation of an alternative splice form of LEU1 in B cell chronic lymphocytic leukemia.

Heterozygous and homozygous deletions of chromosome 13q14.3 are found in 50% of patients with B cell CLL, suggesting the presence of one or more tumour suppressor genes within the deleted region. To identify candidate genes from the region, we constructed a map of 13q14.3 using a combination of genomic and cDNA library screening. The incidence of deletions in CLL patients was 51.5% encompassing a 265 kb region of minimal deletion (RMD) telomeric to markers D13S319. Two CpG islands were identified within the RMD, the telomeric of which is fully methylated whilst the more centromeric is unmethylated. A novel transcript was identified within the RMD that represents an alternative splice version of Leu1. The nine exons of this transcript span a genomic of 436 kb with exon 1 of Leu1 being the common first exon. The remaining exons were shown to be more frequently deleted than Leu1 itself. All splice forms of this transcript were detectable by RT-PCR but Leu1 detected the most abundant message on Northern blotting. Sequence analysis failed to reveal inactivating mutations in patients with heterozygous deletion of 13q14.3, although a polymorphic T to A variant was identified within exon 1 of Leu1 in leukemic and normal controls. As no mutations have been detected for Leu1 or any other transcript so far described, we cannot exclude the existence of control elements within the RMD that may regulate expression of genes lying in this region.