RNA-based Prognostic Markers in Chronic Lymphocytic Leukemia

Abstract: Chronic lymphocytic leukemia (CLL) is a heterogeneous disease where a significant proportion of patients will develop an aggressive disease. Today, the mutational status of the immunoglobulin heavy variable (IGHV) genes is one of the strongest prognostic markers in CLL, where unmutated IGHV genes correlate with poor outcome. In addition, IGHV3-21 gene usage is associated with poor prognosis independent of mutational status. Recently, several genes were shown to be differently expressed between IGHV mutated and unmutated CLL and were suggested as prognostic markers. The aim of this thesis was to examine the applicability of these RNA-based prognostic markers in CLL. In papers I and II, the prognostic significance of LPL and TCL1A mRNA expression in CLL was investigated in 140 and 144 patients, respectively. High expression was found to be associated with inferior clinical outcome for both markers. However, CLL cases with mutated IGHV3-21 genes displayed low levels of LPL expression, indicating that LPL cannot identify this poor-risk patient group. In contrast, high TCL1A expression was detected in all IGHV3-21 cases. To elucidate the functionality of LPL in CLL, LPL lipase activity was measured in 33 cases. The lipase activity was found to be invariably low, implying an alternative function for LPL in CLL. In paper III, a comprehensive analysis of five RNA-based markers (LPL, TCL1A, ZAP70, CLLU1 and MCL1) was performed in 252 CLL patients. All RNA-based markers except MCL1 predicted clinical outcome, with LPL being the strongest. Moreover, LPL expression independently predicted overall survival when adjusted for established markers. All of the RNA-based markers added additional prognostic information to established markers, e.g. high LPL expression predicted an inferior outcome in patients with mutated IGHV genes or good-risk cytogenetics. For clinical application, over time stability of prognostic markers is crucial. In paper IV, the expression of LPL, TCL1A, ZAP70 and MCL1 was investigated in samples taken at diagnosis and at a follow-up of seven years in 104 CLL patients. LPL was found to be the most stable marker, displaying high correlation between the sequential samples, whereas ZAP70 and MCL1 varied significantly. TCL1A expression increased at follow-up, which may indicate disease progression as TCL1A promotes cell survival. In summary, this thesis highlights the applicability of RNA-based markers in CLL prognostication, both as single markers or in combination with established markers. In particular, LPL was shown to be the strongest RNA-based marker in terms of prognostic strength and stability.