Early Events in the Clonal Evolution in Patients with Chronic Lymphocytic Leukemia Carrying TP53 Mutations and 17p Deletion

Abstract

The leukemia cells of patients with chronic lymphocytic leukemia (CLL) may experience clonal evolution of TP53 mutations and/or deletions in the short arm of chromosome 17 (del(17p)). Such changes are associated with poor prognosis and resistance to standard chemotherapy. We evaluated for TP53 mutations and p53 protein expression before and after irradiation in leukemia cells with del(17p) of 14 patients at various times prior to therapy. The proportion of leukemia cells harboring del(17p) ranged from 3% to 99.5%, as assessed by fluorescence in situ hybridization (FISH). Four cases had less than 5% CLL cells with del(17p) at the initial sample date. Ten out of 14 patients had leukemia cells that used unmutated IGHV (71%), and 9 (64%) or 6 (43%) had CLL cells with high-level expression of ZAP-70 or CD38, respectively. The median time from diagnosis to sample collection was 2.6 years and the median follow up was 3.8 years. We found 10 of 14 (71%) had proportions of CLL cells with del(17p) ranging from 3% to 55%, but no detectable TP53 mutations in exons 5 through 8, where cancer-related mutations in TP53 are most common. These cases also had functional TP53, as assessed by leukemia-cell induction of p53 and p53-target genes in response to ionizing radiation. Four cases of 14 (28.6%) had TP53 mutation within one of the exons studied and constitutive p53 protein expression both before and after irradiation. In particular, 2 patients with del(17p) detected in 83.5% and 93% of the CLL cells, had leukemia-cell somatic mutations in exon 7 of TP53 at codon 244 and codon 242, respectively. A third patient with 99.5% of the CLL cells with del(17p) had a mutation in exon 5 codon 174. We identified heterozygosity in exon 5 at codon 179 associated with constitutive expression of dysfunctional p53 protein in a fourth patient in which only 14% of the CLL cells had del(17p). The mutation observed at codon 179 of exon 5 resulted in an amino acid change from histidine to leucine. Collectively, even though located in the DNA-binding domain of p53, none of the mutations identified were common to hot spot p53 mutations identified in other types of cancer. However similar mutations (histidine to arginine or histidine to tyrosine) at codon 179 were observed in 3 CLL cases recently published (Zenz T. et al. prepublished Blood 2008) on samples collected at 0, 5, 9 months from diagnosis, respectively. This study reveals early events in the clonal evolution of CLL leading to loss of p53 function, which can be found even in cases that have less than 20% leukemia cells harboring del(17p), as assessed by FISH. It also suggests that mutations at codon 179 might be a common hot spot in CLL, which potentially could provide for another means with which to evaluate for cases at risk for developing drug-resistant disease either spontaneously or following standard chemotherapy.