Confirmation of Rb gene defects in B‐CLL clones and evidence for variable predominance of the Rb defective cells within the CLL clone

Summary. In 70 B‐CLL patients, deletion or translocation, at or near the retinoblastoma (Rb) site, was detected in 20 by cytogenetic analysis. Purified B cell clones from 13 of these B‐CLL patients were isolated and studied for Rb gene status, Rb mRNA and the Rb protein product. Southern blot analysis of the Rb site detected internal deletions (N= 1) or a single allele loss (N= 2) in five patients. Northern blots detected reduced Rb mRNA in four patients. Immunoblot of whole cell lysate revealed reduced levels of unphosphorylated Rb protein in six CLL patients. No CLL B cell clone contained phosphorylated Rb species. These molecular studies have confirmed the cytogenetic alteration of 13q12–14 sites in B‐CLL cells. In addition, cytogenetic and molecular biologic analysis suggest heterogeneity in the B cell clone for Rb gene abnormality. B‐CLL patients with abnormalities in both cytogenetic and Rb DNA/RNA analysis will have a dominance of B cells with an Rb abnormality (N= 5). In patients whose Rb defective CLL cells constitute only a minor subpopulation of the total B cell clone, only cytogenetic defects would likely be detected (N= 7).