CORRESPONDENCE

Re: Prognostic Significance of a Short Sequence Insertion in the MCL-1 Promoter in Chronic Lymphocytic Leukemia

Scott N. Freeman, Gerold Bepler, Eric Haura, Rebecca Sutphen, W. Douglas Cress

Affiliations of authors: Molecular Oncology Program (SNF, WDC), Thoracic Oncology Program (GB, EH), and Cancer Control Program (RS), The H. Lee Moffitt Cancer Center and Research Institute, University of South Florida College of Medicine, Tampa, FL

Correspondence to: Doug Cress, PhD, Molecular Oncology Program, The H. Lee Moffitt Cancer Center and Research Institute, 12902 Magnolia Drive, Tampa, Florida 33612-9497 (e-mail: cressd{at}moffitt.usf.edu).

A recent article entitled "Prognostic Significance of a Short Sequence Insertion in the MCL-1 Promoter in Chronic Lymphocytic Leukemia" identified two novel sequence variants of the MCL-1 promoter within lymphocytes from chronic lymphocytic leukemia (CLL) patients but not within noncancerous tissue from the same individuals or in lymphocytes from 18 healthy control subjects (1). This result suggested that the variants—insertions of 6 and 18 nucleotides at position –188 relative to the transcription start site as mapped by Akgul et al. (2)—were CLL-related somatic oncogenic mutations. Moshynska et al. (1) also determined that the 6- and 18-nucleotide insertions were associated with higher MCL-1 mRNA and protein expression.

In the course of analyzing the E2F1-mediated transcriptional repression of MCL-1 (3,4), we independently identified and cloned the three observed sequence variants from three human cancer cell lines, H1299(Mcl-1 +0/+0) lung cancer cells, K562(Mcl-1 +6/+6) erythroleukemia cells, and T98G(Mcl-1 +0/+18) glioblastoma cells, representing the MCL-1 +0, +6, and +18 alleles, respectively (see Supplemental Material at http://jncicancerspectrum.oxfordjournals.org/jnci/content/vol97/issue14).

We next used polymerase chain reaction (PCR), followed by resolution of the PCR products on acrylamide gels, to determine MCL-1 promoter status in a large number of cell lines and solid tumors. The +6 and +18 promoters occurred with a high frequency in both breast and lung cancer genomic DNA (Fig. 1A). To determine if these variants were somatic in origin, we analyzed DNA derived from 15 sets of paired lung cancer and adjacent normal lung tissue from patients undergoing routine thoracotomy for surgical resection of their malignancy. All samples were provided in deidentified fashion, and all patients provided informed consent as approved by the Institutional Review Board. In every instance, the MCL-1 promoter profile was identical in cancerous and normal tissue (Fig. 1B and data not shown). To address the possibility that the MCL-1 promoter variants may predispose to malignancy, we screened DNA samples derived from 59 healthy individuals, all of whom had provided informed consent, for the presence of the MCL-1 promoter insertions. Nearly half of the total alleles had one or both insertions, and the insertion alleles were present at frequencies similar to those in cancer cell lines (Fig 1, A and C, and data not shown). Thus, it appears likely that the +6 and +18 MCL-1 promoter variants are common benign polymorphisms.



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Fig. 1. Frequency of MCL-1 promoter variants within DNA derived from various samples. A) Allelic frequencies of the MCL-1 +0, +6, and +18 promoters in DNA derived from cancerous cell lines of breast and lung, paired lung tumor and adjacent normal tissue, and healthy donor genomic DNA were determined by resolving polymerase chain reaction (PCR) products from the MCL-1 promoter on a polyacrylamide gel with subsequent staining and visualization. B) MCL-1 promoter status in genomic DNA from paired lung tumor biopsy and adjacent normal tissue derived from patients undergoing routine thoracotomy. The first lane is the 50-bp ladder, with the visible band representing 100 bp. The second through seventh lanes represent polyacrylamide resolution of PCRs amplifying the MCL-1 promoter in paired lung tumor and adjacent normal tissue. The eighth lane represents a negative control PCR with no template DNA, and the ninth lane is a positive control which consists of a mixture of three separate amplifications of the three variant promoters conducted on plasmid template DNA representing the three observed promoters. C) MCL-1 promoter status within genomic DNA derived from healthy donors. The first lane of each gel represents a 50-bp ladder, with the 100-bp fragment visible. The last two lanes represent the negative and positive controls, respectively, and the middle lanes represent deidentified healthy donor DNA samples.

 
MCL-1 belongs to the BCL-2 family of proteins and may be a potent oncogene due to its ability to block apoptosis. Although we found the +6 and +18 polymorphisms to be common, we considered it possible that they contribute to oncogenesis by increasing the expression of MCL-1. To explore this possibility, we cloned the MCL-1 +0, +6, and +18 promoters into a pGL3 luciferase vector, transfected the constructs into multiple cell lines, and determined promoter activity. Both variant promoters displayed decreased promoter activity, both during normal cellular homeostasis and under conditions that actively induce MCL-1 transcription (i.e., treatment with phorbol 12-myristate 13-acetate), with the +18 promoter displaying approximately half the activity of the MCL-1 +0 promoter (see Supplemental Material at http://jncicancerspectrum.oxfordjournals.org/jnci/content/vol97/issue14).

It is uncertain why our results differed from those of Moshynska et al. One possibility is that their sequencing method is less sensitive to the presence of the insertion alleles than our method of direct resolution of PCR products on gels. Alternatively, it is possible that the local population they studied shares a common genetic background in which the MCL-1 +6 and +18 alleles are rare and indeed correlate with CLL. In contrast, we examined a larger control population recruited in Durham, North Carolina, and Tampa, Florida. Given the fact that we have identified many healthy individuals harboring the MCL-1 +6 or +18 alleles, it seems unlikely that the MCL-1 +6 and +18 alleles contribute to CLL in a large population. Thus, the MCL-1 +6 and +18 alleles probably do not represent a reliable prognostic marker.

NOTES

We thank Dr. Yihong Ma, Dr. Kenneth Wright, Dr. Srikumar Chellappan, and Jose Rodriguez for scientific input and for comments on the manuscript.

This work was supported by funds from the National Cancer Institute (CA90489-01 to WDC) and by the Molecular Biology and Molecular Imaging Core Facilities of the Moffitt Research Institute.

REFERENCES

(1) 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;96:673–82.[Abstract/Free Full Text]

(2) Akgul C, Turner PC, White MRH, Edwards SW. Functional analysis of the human MCL-1 gene. Cell Mol Life Sci 2000;57:684–91.[ISI][Medline]

(3) Croxton RL, Ma Y, Song L, Haura EB, Cress WD. Direct repression of the Mcl-1 promoter by E2F-1. Oncogene 2002;21:1359–69.[CrossRef][ISI][Medline]

(4) Croxton RL, Ma Y, Cress WD. Differences in DNA binding properties between E2F1 and E2F4 specify repression of the Mcl-1 promoter. Oncogene 2002;21:1563–70.[CrossRef][ISI][Medline]



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