Genetic characterization of hematological malignancies with focus on mantle cell lymphoma

Abstract

Lymphoid cancer affects about 2000 individuals in Sweden every year. Lymphoma is divided into numerous subclasses based on histopathological and phenotypic characterization, however, for some lymphoma entities, the classification criteria are insufficient and clinical diversity is seen in cases of the same diagnosis. Mantle cell lymphoma (MCL) is a highly malignant lymphoma, characterized by t(11;14)(q13;q32), resulting in cyclin D1 over-expression. However, this oncogenic event alone is probably not sufficient for tumor establishment and therefore it is of importance to outline additional genetic alterations involved in the tumorigenesis. Diffuse large B-cell lymphoma (DLBCL) stands for 30-40% of the lymphomas, being the most common non-Hodgkin lymphoma type. It is a very heterogeneous disease, genetically as well as clinically. This thesis is based on genetic studies of different lymphoma entities with a main focus on MCL.

Comparative genomic hybridization (CGH) is an efficient method for screening of the tumor genome in terms of identifying chromosomal gains and losses. Initially, metaphase chromosomes were used as hybridization targets, however, more recent experiments are usually performed as array-CGH, where metaphase chromosomes are replaced by individual clones spotted onto a glass slide. The resolution is thereby remarkably increased and is dependent on the size and genomic distance between the clones.

In Paper I, we studied DLBCL tumors, both de novo and relapses, using metaphase CGH in order to outline the pattern of genetic alterations. Recurrent common early events were losses of 8p and 9p, while loss of 1p, 22q and gain of chromosome 7 appeared late. Loss of 22q was associated with an advanced clinical stage, and 18q was significantly more frequently gained in relapses than in diagnostic tumors. In Paper II, we describe the genetic progression in four subsequent tumor samples; the diagnostic biopsy of MCL and three following relapses, where loss of 8p as well as gains of 7p, X, 3 and 20 appeared late in tumor progression.

A correlation between CGH findings and clinical parameters as well as IGHV gene status in MCL tumors was performed in Paper III. It was revealed that MCL utilizing the IGHV3-21 Ig gene had less genomic alterations with a tendency to favorable prognosis compared to MCL tumors with other IGHV genes. In Paper IV, the MCL tumors were further studied using a 1 Mb array CGH. We detected twice as many alterations, and the main findings were homozygous deletions within 1p32.3 and in 13q32.3, strongly suggesting the localization of tumor suppressor genes in these regions.

In Paper V, material from different hematological neoplasias previously reported to have alterations in 6q (childhood acute lymphoblastic leukemia (ALL), MCL, DLBCL and follicular lymphoma (FL)) were analyzed using a chromosome 6 specific tile path array-CGH in order to outline the exact deletion patterns. 6q21 and 6q23 were frequently deleted in all of the diagnoses included. Furthermore, we could identify overlapping homozygous deletions in DLBCL.