No germline ATM mutation in a series of 16 T-cell prolymphocytic leukemias

In their review, Vanasse et al state that up to half of the individuals without ataxia telangiectasia who contract T-cell prolymphocytic leukemia (T-PLL) were heterozygote carriers of mutations with the ATM gene.1 But no evidence for this assertion could be drawn from the cited references.2-5 In the 4 series of patients with T-PLL analyzed to date for ATM mutation, ATM was inactivated by deletion or mutation in at least two-thirds of the leukemias. In our initial work, we reported that the 3 mutations identified in the tumor DNAs were not present in the paired germline DNAs, demonstrating that these mutations were of somatic origin and that no carrier of ATM mutation was present in this series.4 Similarly, Yuille et al reported 2ATM mutations in T-PLL samples which were absent in remission samples.2 

Furthermore, since these initial reports, we thoroughly investigated 16 patients with T-PLL. The loss of heterozygosity (LOH) analysis, performed as previously described,4 identified deletion of one copy of the 11q23 region in 81 percent of the cases (13 of 16). Because missense mutations of ATM were frequent in the tumors, contrasting with the truncating mutations representing most of the germline mutations, we used the fluorescence-assisted mismatch analysis (FAMA) technique to search for mutation.6 Briefly, after reverse transcription of the messenger RNA, ATM transcripts were amplified by the polymerase chain reaction (PCR) using fluorescent primers. The PCR products were denatured and reannealed with germline PCR products to form heteroduplexes, and subjected to conventional cytosine- and thymine-specific modifications. Cleavages occurring on opposite strands were detected by denaturing gel electrophoresis using an automated DNA sequencer. The identified cleavage regions were then sequenced using specific primers. Thirteen aberrations of ATM transcripts were identified in 12 patients. Case TP15, without detectable LOH using polymorphic markers, was found homozygote for an A7499G mutation (single-letter amino acid code), which was absent in germline DNA. This was interpreted as a small-sized allelic deletion of theATM gene. Alterations of ATM by deletion or mutation were found in all but one patient (Table). Considering a model in whichATM is consistently inactivated on both alleles in T-PLL, 18 mutated alleles were expected. The 13 identified mutations were consistent with this model, considering the sensitivity of the FAMA technique to detect ATM mutations, estimated at 72% from the analysis of a series of ataxia telangiectasia patients (Laugé et al, unpublished data). The genomic mutation responsible for the aberrantly spliced transcripts in case TP3 was not identified, precluding the determination of its somatic or germline origin. The 12 ATM mutations in 11 patients, identified in the tumor DNAs, were absent in the corresponding germline DNAs obtained from paired lymphoblastoid cell lines generated from the tumor samples. Thus this analysis of a larger series confirmed the somatic nature of ATM mutations in T-PLL and did not identifyATM heterozygotes suffering from T-PLL.

Considering the potential anxiety and unjustified medical care that could be generated by the statement made by Vanasse et al, it should be stated that, surprisingly, there is no evidence so far for an increased risk of T-PLL in ATM carriers. But our data do not exclude a relative risk (RR) inferior to 33. GivenP_TPLL, the frequency to observe no heterozygote in a series of 11 cases with a probability (P) below .05 estimated at .25 [P = (1 − .25)¹¹ = .042], andP_ATM, the frequency of ATM heterozygote in the general population estimated at .01, the maximal relative risk (RR) in ATM heterozygotes for T-PLL was deduced from the formulaP_TPLL = (P_ATM × RR) / (P_ATM × RR + (1 − P_ATM). But even with such a risk, considering the extremely low incidence of T-PLL in the general population, this risk for ATM carriers would be minor. Finally, the apparent absence of overrepresentation ofATM carriers in the patients suffering from T-PLL and the differences in the mutations observed in ataxia telangiectasia patients (most often truncating mutations) and in T-PLL (often missense mutations in the phosphatidyl-inositol 3 phosphate kinase homology domain) may indicate important functional differences in these mutant ATM proteins.7