Telomere Shortening and Chromosomal Instability in Chronic Lymphocytic Leukemia

Abstract 1761

Chronic lymphocytic leukemia (CLL) is a neoplastic disorder of B-lymphocytes, typically with a high number of peripheral B-lymphocytes and small mature lymphocytes (M Hallek et al, Ann Oncol. 16, Suppl 1:i50-1 (2005). Clinically, some patients have a mild, largely asymptomatic course of the disease and a normal life expectancy, while others suffer from fulminant progression and have a very short survival. An important predictive factor is the presence of typical chromosome aberrations (H Doehner et al, N Engl J Med343, 1910–1916 (2000)). Due to a low proliferative rate of the cells, the gold standard for cytogenetic diagnostics in CLL is fluorescence in situ hybridization (FISH). Therefore, not much is known about the incidence of complex karyotypes, although they are strong predictors of a very poor prognosis in CLL (C Mayr et al, Blood107, 742–751 (2007)).

By stimulating the cells with different interleukins and CpG-oligodeoxynucleotides, we were able to detect complex karyotypes in about 10% of investigated cases of CLL by classical banding analysis. In this study, we characterized 24 patients with CLL and complex karyotype by performing multicolor fluorescence in situ hybridization (mFISH). Hereby, we could identify cryptic aberrations and describe the karyotype in greater detail. In addition to typical aberrations involving 6q, 11q, 13q and 17p and trisomy 12, (iso)dicentric chromosomes and whole-arm translocations of chromosomes Y, 1, 3, 4, 5, 13, 15, 17, 18, 21 and 22 were detected. These chromosome aberrations were mostly generated by breaks in heterochromatic and telomeric regions indicating an increased breakage of these regions. This may indicate that epigenetic alterations and critically short telomeres predispose for the generation of chromosome aberrations in CLL. Telomere shortening and chromosomal instability are believed to play an important role in the development of neoplasia. Recently, it was shown that short telomeres in CLL are associated with a poor survival and increased genetic complexity (G Roos et al, Blood111, 2246–2252 (2008)). So far, published data are only available on the average telomere length in CLL, but not on the telomere length of individual chromosomes. We used a new technique, telomere/centromere-fluorescence in situ hybridization (T/C-FISH), which combines fluorescence R-banding and FISH using a probe against the telomere repeats to measure the telomere length of each chromosome arm. In line with previous results, patients with CLL showed significantly shorter telomeres than those of healthy controls. Comparing the telomere lengths of distinct chromosome arms with specific aberrations, there was no significant association. In addition, we could compare the telomere lengths of cells with aberrations and cells without aberrations within one patient. Aberrant metaphases of the same patient showed significantly shorter telomeres than metaphases with a normal karyotype (p<0.05).

Thus, telomere shortening is not a basic mechanism affecting all hematopoietic cells in CLL patients, e.g. due to aging, but affects only the malignant cells, indicating that telomere attrition is involved in the pathogenesis of CLL with complex karyotypes.