Genetic Aberrations In Leukemias And Lymphomas

On average, leukemias and lymphomas contain fewer genetic aberrations than carcinomas. Very often, their karyotype is near-diploid with few chromosomes altered. Those aberrations that are present, however, are characteristic for the particular type of leukemia or lymphoma. Although chromosomal losses and gains are also found, many hematological cancers are characterized by typical translocations (Table 10.2).

In some hematological cancers, these translocations are so characteristic that they allow to classify the disease accordingly. So, >90% of chronic myeloid leukemias contain a characteristic translocation between the long arms of chromosomes 9 and 22, t(9;22) (q34;q11). On the other hand, apparently similar diseases may result from disparate translocations. So, the same t(9;22) translocation is found in «25% of acute lymphoblastic leukemias as well. However, in this group of diseases, a variety of other translocations are found (Figure 10.2).

Unlike deletions such as those of 6q or 13q found in some leukemias, translocations do not simply destroy genes and their functions, but alter them. Two mechanisms can be distinguished (Figure 10.3, cf. 4.3).

(1) The coding region of the gene at the translocation site remains intact, but its regulation is altered, since the translocation removes some of its own regulatory regions and/or brings the gene under the influence of novel regulatory elements. In most cases, this leads to ectopic or deregulated expression of a gene product acting as an oncogenic protein. Burkitt lymphoma with its translocations activating the MYC gene is a case in point (^10.3). Another frequent translocation t(14;18) (q32;q21) leads to deregulation and overexpression of the anti-apoptotic regulator protein BCL2 (^7.2) in follicular lymphoma. Here, the BCL2 locus is brought under the control of a heavy-chain immunoglobulin enhancer (cf. 10.3).

Table 10.2. Some characteristic translocations in leukemias and lymphomas

Translocation

Genes involved

Consequence

Cancer type*

t(8;14) (q24;q32)

MYC, Ig loci

deregulation of MYC

Burkitt lymphoma

t(2;8) (p12;q24)

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