Detection of Activation-Induced Cytidine Deaminase RNA In CLL Cells Correlates with Shorter Patient Survival and High Numbers of CD38⁺ Cells

Abstract 2415

Activation-induced cytidine deaminase (AID) is normally expressed in germinal center B cells and causes immunoglobulin somatic hypermutation and isotype class switching. High AID expression in B-cell chronic lymphocytic leukemia (CLL) associates with unmutated IGHV and cytogenetic aberrations that correlate with unfavorable prognosis. Mistargeted AID mutational activity may promote genetic changes leading to aggressive disease.

To directly examine patient outcome and AID expression, peripheral blood cells from 126 CLL patients were tested for AID mRNA by nested PCR and then these results compared to time to first treatment (TFT) and overall survival (OS). In our cohort, patients have been followed for up to 393 months. AID⁺ CLL patients had a significantly shorter TFT (75 months; n = 53) compared to AID⁻ CLL patients (243 months; n = 51)(P < 0.0025). AID⁺ patients also had a significantly shorter OS (134 months; n= 55) than AID⁻ CLL patients (245 months; n =54)(P < 0.0001). AID⁺ patients also died at a significantly younger median age than AID⁻ patients (74 versus 83 years, respectively)(P < 0.0001), yet both groups were diagnosed at about same median age (58 versus 61 years, P < 0.5879). These data imply that AID⁺ CLL patients have an inherently more aggressive and/or less treatment responsive disease.

IGHV mutation status in this cohort was determined and compared to AID expression. Unmutated IGHV (U-CLL) was confirmed to significantly correlate with AID mRNA expression: 31/46 U-CLL versus 21/58 mutated IGHV CLL (M-CLL) cells express detectable AID (P < 0.0002). One hypothesis to explain the apparent paradox of finding elevated levels of AID expression in cells lacking IGHV mutations is that the initial leukemogenic event is independent of AID activity, but subsequent AID activation in the leukemic clone results in more aggressive disease. Although IGHV mutation status correlates with AID expression, a fair number of AID⁻ U-CLL (33%) and AID⁺ M-CLL (36%) exist. AID⁺ expression in M-CLL significantly correlates with worse OS (183 versus 286 month median survival, P < 0.0275), whereas AID⁺ expression did not significantly discriminate OS in U-CLL patient (102 versus 151 month median survival, P < 0.2691).

Eight common CLL cytogenetic aberrations at 13q14.3, 17p13.1, 11q22.3, 12-CEN, 13q34, 11q13, 14q32, 6q23.3 were determined by FISH in a subset of these patients. We confirmed that AID⁺ CLL tended to have more cytogenetic aberrations (n = 21, average aberrations = 1.6) than AID⁻ CLL (n = 21, average aberrations = 1.0). Because 13q14.3 aberrations are not associated with unfavorable clinical courses, such cases were excluded from a subsequent analysis. In this assessment, AID expression correlated with the other genetic aberrations (AID⁺ = 1.0 average aberrations versus AID⁻ = 0.2 aberrations), suggesting that 13q14.3 deletion may be an early genetic change that occurs independently of AID expression, while subsequent genetic changes are AID dependent.

Finally, high CD38 levels correlate with poor CLL patient outcome. CD38 is induced upon activation of mature B cells and CD38⁺ CLL cells are enriched in cells engaged in the cell cycle. Furthermore, CD38 expression is highest among B cells in germinal centers, where AID is normally expressed. Thus, CD38 levels on CLL B cells were measured by flow cytometry on a subset of patients and compared to AID expression. AID expression (14/19) significantly correlated with high CD38 (≥ 30%) levels as compared to expression (5/20) in those with low CD38 levels (P < 0.0038). This suggests that AID expression is found in CLL patients with a higher proportion of activated, cycling cells.

Thus, AID expression correlates with unmutated IGHV genotype, more cytogenetic aberrations, high numbers of CD38⁺ cells, and more aggressive disease as manifested by shorter TFT and OS.