Affiliation of authors: Laboratory for Leukemia Diagnostics, University Hospital Grosshadern, Ludwig-Maximilians-University, Munich, Germany
Correspondence to: Frank Dicker, PhD, Laboratory for Leukemia Diagnostics, University Hospital Grosshadern, Marchioninistr. 15, 81377 Munich, Germany (e-mail: frank.dicker{at}med.uni-muenchen.de).
High MCL-1 protein expression levels have been associated with failure to achieve complete remission in chronic lymphocytic leukemia (CLL) (1). Moshynska et al. (2) recently reported the presence of short sequence insertions, of 6 and 18 nucleotides, in the MCL-1 promoter in some CLL patients. These insertions were detected in the leukemic cell population in 17 of 58 CLL patients but not in normal tissue of affected patients (n = 2). They were also absent in all 18 analyzed control subjects. The authors concluded that the promoter insertions are associated with increased MCL-1 mRNA and protein and may identify a high-risk group of CD38-negative CLL patients.
To pursue this idea, we analyzed the MCL-1 promoter insertions in samples from 72 CLL patients who presented at our institution and from whom written informed consent was obtained. The promoter region was amplified by PCR, and the resulting fragment sizes were analyzed by GeneScan analysis. Results were confirmed by direct DNA sequencing. In 41 patients (60%), we detected the same insertions of 6 and/or 18 nucleotides at the same site as observed by Moshynska et al. (Table 1). In addition, a 12-nucleotide insertion at the same site was seen in two patients. Because we observed no statistically significant difference in the distribution of insertions between CD38-positive and CD38-negative patients (P = .394, Fisher's exact test), we considered the possibility that the insertions might be polymorphisms. Thus, we conducted an additional study of DNA samples from 40 healthy control persons that were obtained from a human genetics laboratory. Informed consent for participation in anonymized genetic studies was obtained from all individuals. Thirty (75%) of the control subjects contained an MCL-1 promoter insertion, a frequency that was not statistically significantly different from that seen in CLL patients (P = .147, Fisher's exact test). Based on these data, we suggest that these insertions in the MCL-1 promoter are most likely polymorphisms rather than somatic mutations contributing to the leukemic potential of the tumor cells. Indeed, buccal swabs from two CLL patients, one heterozygous for the 6-nucleotide insertion and one heterozygous for the 18-nucleotide insertion, contained the same alleles as the peripheral blood lymphocytes.
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Finally, we wish to point out that the 18-nucleotide promoter insertion has previously been reported and was functionally characterized by Akgul et al. (3) (GenBank accession number AF147742). The genomic fragment analyzed by Akgul et al. (3) was derived from a human chromosome 1 library.
While this manuscript was under consideration, Tobin et al. (4) reported the same MCL-1 promoter insertions in CLL participants and in healthy controls with frequencies that were similar to those reported in our study.
Altogether, these data suggest that the MCL-1 promoter insertions are not somatic mutations in the tumor cell population of CLL but are inherited through germline transmission and thus represent real polymorphisms that are detected with high frequency in the population.
REFERENCES
(1) Kitada S, Andersen J, Akar S, Zapata JM, Takayama S, Krajewski S, et al. Expression of apoptosis-regulating proteins in chronic lymphocytic leukemia: Correlations with in vitro and in vivo chemoresponses. Blood 1998;91:337989.
(2) Moshynska O, Sankaran K, Pahwa P, Saxena A. Prognostic significance of a short sequence insertion in the MCL-1 promoter in chronic lymphocytic leukemia. J Natl Cancer Inst 2004;96:67382.
(3) Akgul C, Turner PC, White MRH, Edwards SW. Functional analysis of the human MCL-1 gene. Cell Mol Life Sci 2000;57:68491.[ISI][Medline]
(4) Tobin G, Skogsberg A, Thunberg U, Laurell A, Aleskog A, Merup M, et al. Mcl-1 gene promoter insertions do not correlate with disease outcome, stage, or VH gene mutation status in chronic lymphocytic leukaemia. Leukemia 2005;19:8713.[CrossRef][ISI][Medline]
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