Loss or Mutation of TP53 Is Highly Associated with Complex Aberrant Karyotype and Chromosomal Translocations in Chronic Lymphocytic Leukemia.

In chronic lymphocytic leukemia (CLL) cytogenetic aberrations such as del(17p) and del(11q) predict inferior outcome. In addition, complex aberrant karyotypes as well as chromosomal translocations as defined by metaphase cytogenetics were suggested as poor prognostic markers for overall survival. We screened 194 consecutive CLL patients for del(17p)/TP53-deletion by fluorescence in situ hybridization (FISH) and for TP53-mutations by denaturing high performance liquid chromatography (DHPLC) and subsequent direct sequencing of aberrant fragments. In addition 160 of these CLL patients were analyzed by classical metaphase cytogenetics to determine the incidence of TP53-aberration in different cytogenetic subgroups. Interphase FISH on 194 samples detected TP53-deletions in 9.3% (n=18) of cases. In parallel, exons 3–9 of the TP53 gene were screened by DHPLC and an aberrant pattern was detected in 9.8% (n=19) of cases. TP53-mutations were confirmed and further characterized by direct sequencing in 16 of the 19 cases. The residual 3 samples had an aberrant pattern in DHPLC for the amplicon of exons 8–9 which pointed to a small population of TP53-aberrant cells which was beyond the detection limit of sequencing. 16 of 18 (89%) cases with TP53-deletion were accompanied by a TP53-mutation affecting the residual allele. 3 samples with TP53-mutations had no deletion of one TP53 allele. Therefore, the overall incidence of TP53-aberrations was 10.8 % (21/194) with a significant association of TP53-deletion and TP53-mutation (p<0.0001). Metaphase cytogenetics was performed on 160 CLL samples. A complex aberrant karyotype defined by ≥ 3 aberrations was identified in 14% of samples (22/160). The incidence of TP53-aberrations in this cytogenetic subgroup was 50% (11/22) and therefore significantly higher than in other cytogenetic subgroups (p<0.0001). Among 160 samples with cytogenetic analysis 49 (31%) exhibited translocations. We divided these translocations into subgroups with karyotypes carrying balanced translocations only (n=18), carrying unbalanced translocations only (n=20) as well as karyotypes with both balanced and unbalanced translocations (n=11). Within the entire group of translocations the incidence of TP53-aberration was 27% (13/49). The incidence of TP53-aberrations was 5.5% (1/18) in the group with only balanced translocations, 40% (8/20) in the group with only unbalanced translocations and 36% (4/11) where balanced and unbalanced translocation occurred in combination. When the latter two groups with unbalanced translocations were combined TP53-aberration occurred in 39% (12/31) of cases. Altogether the association of TP53-aberration with translocations was strong (p<0.0001) especially with unbalanced translocations (p<0.0001) whereas no coherency with balanced translocations could be demonstrated (p>0.05). Furthermore, translocations were detected in 91% (20/22) and unbalanced translocations in 82% (18/22) of complex karyotypes. The association of translocations, in particular unbalanced translocation with complex aberrant karyotype was significant (for both p<0.0001).

In conclusion:

1. Loss of TP53 and TP53 mutations occur with a frequency of 9.3% and 9.8%, respectively and are significantly associated.

2. A highly significant association of TP53-aberrations with complex aberrant karyotypes and unbalanced translocations was observed.

We hypothesize that TP53-aberrations might contribute to genetic instability leading to accumulation of cytogenetic aberrations especially unbalanced translocations.