Monoclonal B Cell Lymphocytosis as a Marker of Inherited Predisposition to Chronic Lymphocytic Leukemia.

Background: Segregation analyses suggest that CLL has one of the highest inherited risks among all types of cancer. However, no common germline mutations that confer an inherited predisposition to CLL have been found. Identification of a CLL predisposition gene should lead to improved prevention and treatment strategies. Gene identification studies to date have been limited by few affected patients per family, late age of disease onset, and probable genetic heterogeneity. Identification of a phenotypic marker for carriers of a CLL predisposition gene would dramatically improve the likelihood of finding a causative gene. Monoclonal B-cells with typical CLL immunophenotype (monoclonal B-cell lymphocytosis, MBL) can be identified in normal relatives of patients with overt CLL. We hypothesize that MBL identify unaffected carriers of a CLL predisposition gene.

Methods: We used 6-color flow cytometric methods to analyze peripheral blood samples for MBL. We defined MBL as a population of CD5⁺, CD20^lo cells that were kappa/lambda restricted and comprised at least 2% of the CD19⁺ peripheral B cell compartment. We identified extended families segregating 2 or more members with CLL from patients receiving care at Duke and the Durham VA. We collected peripheral blood from both affected and unaffected family members for complete blood count, flow cytometry for MBL detection, and DNA purification. Genotyping studies were performed using PCR to amplify highly polymorphic short tandem repeat sequences (STR) at candidate loci.

Results: We ascertained 16 extended families with CLL and obtained samples from 138 family members. Twelve of the 113 (11%) unaffected participating family members had MBL. In one family, evaluation of 32 members revealed 3 with overt CLL and 5 with MBL. Two distinct inheritance patterns emerge upon review of the families ascertained and analyzed to date. Five families suggested a dominant mode of inheritance of the combined CLL/MBL phenotype: multiple family members with MBL, vertical transmission, and an approximate 50% affection rate for among persons over age 40. In these 5 families, 11 of 64 (17%) unaffected family members had MBL while in the other 11 families no unaffected family members had MBL (0 of 48). In some of the latter 11 families, analysis of obligate carriers of a CLL predisposition gene showed no MBL. We pooled the 5 families with MBL for screening of candidate loci associated with an inherited predisposition to CLL from previously published linkage studies. Using markers at these candidate loci, we excluded linkage to 5q22.1, 6p22, 9q21.33, 10q24.1, 11q13.1, 13q22.2, and 14q32.2.

Conclusions: A combined MBL/CLL phenotype appears to be inherited as a dominant trait in some cases of familial CLL. This supports the hypothesis of genetic heterogeneity in the development of CLL. A familial analysis of both CLL and MBL greatly increases the statistical power for genetic analysis, and may allow for families to be pooled for genetic analysis based on the MBL phenotype. Gene identification studies are ongoing.