Genetic Alterations of the Retinoblastoma-Related Gene RB2/p130 Identify Different Pathogenetic Mechanisms in and among Burkitt's Lymphoma Subtypes

Abstract

Alterations of cell cycle-associated genes probably contribute to the pathogenesis of Burkitt's Lymphoma (BL), in addition to c-myc translocation. Mutations disrupting the nuclear localization signal of the retinoblastoma-related gene RB2/p130 have been documented recently in BL cell lines and primary tumors. Given the importance of the RB2/p130 gene in controlling cell growth, mutations of this gene may result in uncontrolled cell proliferation. We tested the expression and genomic organization of the RB2/p130 gene in relation to the proliferative features of a series of BL samples collected from the endemic and sporadic regions, regardless of whether the samples were acquired immune deficiency syndrome (AIDS)-related. The expression of the Rb2/p130, p107, and cell proliferation-related proteins (cyclin A and B) was determined by immunohistochemistry. The structures of exons 19 through 22 of the RB2/p130 gene, encoding for the B domain and C terminus, were analyzed by polymerase chain reaction (PCR) analysis and single-strand conformation polymorphism (SSCP) technique. The direct PCR products were sequenced to identify the actual mutations. Our results suggest that BL is composed of a mixture of molecular types with distinct genetic and phenotypic patterns, probably resulting from different pathogenetic mechanisms. In endemic BL, the RB2/p130 gene is mutated in most of the cases, and the protein is restricted to the cytoplasm. In AIDS-related BL, high levels of nuclear expression of the wild-type pRb2/p130, p107, and cell proliferation-related proteins were detected. This finding is in line with the molecular mechanisms observed in virus-linked oncogenesis. Sporadic BLs were mainly characterized by the low nuclear values of the wild-type pRb2/p130 and, conversely, the high values of p107. The increased cell proliferation due to different alterations of cell growth control by Rb-related proteins may be the first step in lymphomagenesis, during which additional genetic changes, including missense mutations of c-myc, may subsequently occur.

Cell cycle regulators p27 and pRb in lymphomas – correlation with histology and proliferative activity

“…This may be explained by the fact that the malignant cells in small lymphocytic lymphoma behave close to normal lymphocytes, which also have low pRb expression. Similarly to our findings, there are recent reports of high levels of pRb in high-grade malignant lymphomas (Geradts et al, 1998;Cinti et al, 2000).…”

“…Considering all these mechanisms in the pRb pathway, our finding of the apparently paradoxical correlation between pRb expression and cell proliferation (Ki-67 expression) seems only rational. Although it would be expected that tumours with high levels of pRb had a reduced proliferative rate, observations similar to ours have been reported recently (Leoncini et al, 1999;Cinti et al, 2000).…”

RB family members as predictive and prognostic factors in human cancer

“…In high-grade NHL, loss of pRB has been reported in 58% of cases, particularly in centroblastic and Burkitt's lymphomas (BLs) (Weide et al, 1994). In particular, in BL, mutations disrupting the nuclear localization signal of the Rb-related gene Rb2/p130 have been documented in cell lines and in primary tumors (Cinti et al, 2000). It seems that BL is composed of a mixture of molecular types with distinct genetic and phenotypic patterns, probably resulting from different pathogenetic mechanisms.…”

Tumor suppressor genes in normal and malignant hematopoiesis

“…The p130 gene, a Rb family member, has a mutation in its nuclear localization site in four of four lymphoblastic cell lines (three T-ALL and one Burkitt-lymphoma cell line) (Cinti et al, 2000a), and in about 50% of primary Burkitt lymphoma samples (Cinti et al, 2000b), resulting in aberrant cytoplasmatic localization of this tumor suppressor protein. Decreased p130 protein expression was found in 25% of non-Hodgkin lymphomas (NHL) which correlated with a better survival (Leoncini et al, 1999).…”