Among lymphoid malignancies of NK-lineage, nasal NK/T-cell lymphoma is the most well known. It presents with a destructive lesion in the nasal cavity and nasopharynx and is highly associated with Epstein-Barr virus (EBV). Tumors with similar histologic and immunophenotypic features to those of nasal lymphoma occur less frequently in extranasal sites including skin, gastrointestinal tract, and soft tissue (1, 2). These nasal and extranasal NK/T-cell lymphomas were classified as extranodal NK/T-cell lymphoma, nasal type, in the World Health Organization (WHO) classification (3).
Although the clinicopathologic features of nasal-type NK/T-cell lymphoma have been well defined, little is known about the pathogenetic mechanism associated with lymphomagenesis. A few reports describe the common deletion of chromosome 6q by conventional cytogenetic study and p53 alteration detected by immunohistochemistry and molecular methods (4–8). Lymphomas arise from a clonal expansion of lymphoid cells that are transformed by the accumulation of genetic lesions affecting oncogenes and tumor suppressor genes. During the past decade, recurring cytogenetic abnormalities closely associated with morphologically and clinically distinct subsets of lymphoma led to the identification of the genes involved in lymphomagenesis.
In NK/T-cell lymphoma, the cytogenetic study has been hampered because of the rarity of this type of lymphoma and the difficulty in applying conventional banding technique on the small biopsy from the nasal cavity. Comparative genomic hybridization (CGH) provides an opportunity to scan the entire human genome and localize genetic alterations to a specific chromosomal region in a single experiment using a small amount of tumor tissue without cell culture (9).
In this study, we applied CGH to seven nasal-type NK/T-cell lymphomas to define the chromosomal regions that are putatively involved in lymphoma development. Additionally, analyses for loss of heterozygosity (LOH) of chromosomes 6q, 1p, and 17p and p53 gene mutation were carried out to confirm their association with nasal-type NK/T-cell lymphoma.
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- MATERIALS AND METHODS
- LITERATURE CITED
Expression of CD56 is a common denominator of NK/T-cell lymphomas. Tumors arising in the nasal cavity are associated with EBV in more than 80% of cases. Although nasal NK/T-cell lymphomas are mostly of NK lineage, a few cases demonstrate TCR gene rearrangement (10, 13) and share similar histologic characteristics and high association with EBV (13, 14). NK/T-cell lymphomas arising in the extranasal sites are diagnosed as “nasal-type” when they are of NK lineage and positive for EBV (10).
The genetic information in nasal-type NK/T-cell lymphomas has been limited. Conventional cytogenetic study has been available in only 14 cases (4–6). An aberration involving chromosome 6q was reported in 5 of 14 cases. Other abnormalities including i(1q), del(7q), del(12q), del(17p), and 11q23 rearrangements were sporadically described. Although a limitation of our study is the number of cases (nine), the data show nonrandom alterations of specific chromosomal segments, some of them corresponding to the sites reported by conventional cytogenetic study.
Chromosomal losses and deletions are indicative of the involvement of tumor suppressor genes in tumorigenesis (15). The deletions in our study frequently involved chromosomes 1p, 17p, and 12q. Deletion of chromosome 1p, especially involving segment 1p36, is one of the common secondary cytogenetic changes detected in 12% of non-Hodgkin's lymphoma cases (16). Chromosomal segments spanning 1p31-p36 contain many candidate tumor suppressor genes including the MTS1 gene. Likewise, 17p is a chromosomal region containing several tumor suppressor genes including the p53 gene and one commonly deleted segment by conventional cytogenetic study in non-Hodgkin's lymphoma (17). Using conventional cytogenetics, alterations of chromosome 1p and 17p in nasal-type NK/T-cell lymphomas manifested as a deletion or as i(17)(q10) or i(1)(q10). The formation of an isochromosome results in loss of genetic material. Therefore, i(17)(q10) or i(1)(q10) represents one more way through which loss of one allele of p53 and other tumor suppressor genes is achieved. In nasal-type NK/T-cell lymphomas, alteration of the p53 gene is common. Abnormal expression of the p53 protein and mutation of the p53 gene were detected in 21.4–88.9% and in 22.2–60% of cases in studies conducted on Chinese and Japanese populations, respectively (7). In present study the mutation rate was lower than previous report, but high frequency of p53 protein expression and common loss of 17p by CGH suggested pathogenetic implication of tumor suppressor genes including p53 gene in nasal-type NK/T-cell lymphoma.
In our study, the low frequency of the 6q deletion by CGH is surprising. The 6q deletion is the most common alteration reported in 5 of 14 cases by conventional and molecular cytogenetic study (4–6, 18, 19). This discrepancy could be explained by the technical limitation of CGH. Although conventional cytogenetic study can detect changes in individual cells, these changes should be present in at least 50% of the cell samples used for CGH study. Because NK/T-cell lymphomas contain many inflammatory cells, the positive small fluorescence signal detected by CGH analysis could be blunted by admixed non-neoplastic cells (9). On the other hand, failure to detect more alterations of chromosome 6q by additional LOH analysis supports the result by CGH study.
The significance of the 6q deletion on the pathogenesis of nasal NK/T-cell lymphoma remains unclear. In non-Hodgkin's lymphoma, loss of 6q, particularly at the regions 6q21-23 and 6q25-27, has been well studied in B-lineage lymphomas. It is the most common secondary aberration. Loss of 6q is detected in up to 40% of cases after the tumor has been established, reflecting clonal evolution. Occasionally, it has been correlated with clinical features of non-Hodgkin's lymphoma such as tumor progression, transformation, and survival (20–22). In our study, most cases were primary tumors in stage I. Therefore, the low frequency of the 6q alteration might reflect the bias in case selection that included cases of early clinical stage before additional chromosomal alterations occurred.
Although DNA losses suggest secondary genetic alteration involving tumor suppressor genes, gene amplification including DNA gain is an essential mechanism of oncogene activation. The characterization of chromosomal amplicon areas will provide the means to discover mechanisms that activate several cellular oncogenes and other genes (23). Generally, conventional cytogenetic analysis detects more losses than gains of chromosomal material, whereas CGH is a more powerful method to detect gains over losses. In our series, DNA gains are more common than losses and involve chromosomal regions 2q, 13q, 10q, 21q, 3q, 5q, and 17q in the decreased order of frequency. Gains in 13q, 2q, 10q, and 21q are nonrandom, affecting three or more of cases.
Gains in 13q and 10q have not been described before in cytogenetic studies of NK/T-cell lymphoma. In our series, a gain of chromosome 13q mapped to bands 13q21 and 13q31-33, which contain several unidentified proto-oncogenes including the G-protein–coupled receptor 18 (24). Chromosome 2q also contains a few oncogenes including TCL4 rearranged in the human leukemic T-cell line (25). The long arm of chromosome 10 contains several known genes such as NFκB and RET, which have been implicated in tumorigenesis (26, 27).
In summary, nonrandom genetic alterations affecting specific chromosomal regions in nasal-type NK/T-cell lymphoma are detected by CGH. Despite the limitation of the CGH technique in detecting minor alterations, it provides information that can be used to discover the pathogenetic mechanism of nasal-type NK/T-cell lymphomas.