CHRONIC LYMPHOCYTIC-LEUKEMIA CELLS WITH ALLELIC DELETIONS AT 13Q14 COMMONLY HAVE ONE INTACT RB1 GENE - EVIDENCE FOR A ROLE OF AN ADJACENT LOCUS

We have previously shown that 30% of patients with B-cell chronic lymp hocytic leukemia (B-CLL) have hemizygous deletions of the retinoblasto ma (RB1) gene at 13q14. RB1 gene deletions may thus participate in mal ignant transformation of B-CLL, but it is also possible that a neighbo ring gene on 13q is the relevant one. To answer this question the re R B1 allele of eight clones with hemizygous deletions was studied by rev erse transcription-polymerase chain reaction (RT-PCR), single-strand c onformation polymorphism (SSCP) analysis, and immunofluorescense techn iques. Cells from 10 patients without RB1 gene deletions were also stu died by these methods. Lack of RB1 mRNA and RB protein expression was seen in leukemia cells from one of the patients. All other cases were found to be normal with regard to immunofluorescense, RT-PCR, and SSCP analysis, indicating at least one functional RB1 allele and supportin g the importance of another gene in the 13q14 deletions. We then perfo rmed extended Southern blot analyses of the 13q region, using probes f or 10 different loci. In 14 of 31 CLL clones (45%), deletions of a reg ion telomeric to the RB1 gene (D13S25) were ohserved. In 4 of the case s the deletions were homozygous. Hemizygous deletions of the RB1 gene were observed in 11 of these patients and in none of the patients with out D13S25 deletions. These data thus indicate that a gene(s) telomeri c to RB1 is involved in the malignant transformation of CLL clones and that deletions of this region are a common event in this disease.