Genetic characterization of non-Hodgkin lymphomas using in situ hybridization

Sammanfattning: Fluorescence in situ hybridization (FISH) has been shown to be a valuable andimportant technique in cytogenetics, as a complement to traditional bandinganalysis. This thesis focus on the characterization of chromosomalrearrangements in two hematological neoplasias, myelodysplastic syndrome(MDS) and non-Hodgkin lymphoma (NHL) using FISH.The increased cytogenetic resolution obtained by FISH, made it possible toidentify a true-whole arm translocation consisting of 17q and 18q, in a case ofMDS. The derivative chromosome had a centromere with sequences from bothchromosomes 17 and 18. Thus, a breakpoint on either of the p-arms could beexcluded, and it could be concluded that the rearrangment resulted inmonosomy of 17p and 18p without affecting expressed genes located in thecentromeric region.Chromosome painting, FISH with chromosome specific libraries, on short termcultures of NHL showed that chromosomal rearrangements were often morecomplex than the banding analysis predicted. Painting analysis couldcontribute with new information, not only of translocations, but also of truedeletions and amplification. This could significantly clearify the true karyotypeof the malignant cell.A recurring rearrangement in NHL is additional material on the tip of the shortarm of chromosome 1. Using chromosome specific libraries it was possible toshow that, in three out of nine such cases, the extra chromosomal materialoriginated from the long arm of chromosome 2, resulting in a der(1)t(1;2). Thethree cases were all follicular NHL, although non of them had the t(14;18)found in 85-90% of follicular lymphomas. In addition, one of the cases, had theder(1)t(1;2) as the only rearrangement in one of two malignant clones. All threecases had the der(1)t(1;2) in addition to two normal chromosome 2 homologs,leading to a duplication of 2q31-qter. These findings exemplifies that newcytogenetic subgroups can be identified when banding analysis iscomplemented with chromosome painting.Furthermore, we found a case of NHL with a der(22)t(2;22) as a sole aberration.This results in a cytogenetically identical duplication of 2q31-qter, as in thecases with der(1)t(1;2). These findings suggest that cancer relevant genes maybe located at the distal 2q.LAZ3/BCL6 gene has been implicated in tumorigenesis of NHL. Therefore theexpression pattern of the LAZ3/BCL6 gene was studied, using RNA in situhybridization. It was shown that LAZ3/BCL6 is expressed in follicular center(FC) B-cells of both non malignant reactive lymph nodes and FC-derived non-Hodgkin lymphomas. Furthermore, LAZ3/BCL6 is expressed in a number ofdifferent tissues such as skeletal muscle, spinal cord, trachea, and peripheralblood leukocytes. The data clearly show that the expression of the LAZ3/BCL6is not confined to the malignant cells, and thus its role in the malignanttransformation must be reconsidered.Key words: Myelodysplastic syndrome, non-Hodgkin lymphoma, fluorescencein situ hybridization, LAZ3/BCL6 expression, chromosome rearrangementsISBN 91-628-1994-1 Stockholm, 1996