Allelotype Analysis of Adult T-Cell Leukemia

Microsatellite alterations are also present in the less aggressive types of adult T-cell leukemia-lymphoma

BACKGROUND

Adult T-cell leukemia/lymphoma (ATL) is a mature T-cell neoplasia etiologically linked to HTLV-1. Manifestations of ATL are diverse and different clinical types with different tissue involvement and aggressiveness have been described. The mechanisms that lead to the development of ATL clinical types have not yet been clarified. Considering that in ATL patients HTLV-1 infection generally occurs in childhood, a multistep carcinogenesis model has been proposed. Microsatellite alterations are important genetic events in cancer development and these alterations have been reported in the aggressive types of ATL. Little is known about oncogenesis of the less aggressive types.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we investigated the role of the microsatellite alterations in the pathogenesis mediated by HTLV-1 in the different types of ATL. We examined the presence of microsatellite instability (MSI) and loss of heterozigosity (LOH) in matched pair samples (tumoral and normal) of 24 patients with less aggressive types (smoldering and chronic) and in aggressive types (acute and lymphoma) of ATL. Four microsatellite markers D10S190, D10S191, D1391 and DCC were analyzed. MSI was found in four patients, three smoldering and one chronic, and LOH in four patients, three smoldering and one acute. None of the smoldering patients with microsatellite alterations progressed to aggressive ATL.

CONCLUSIONS/SIGNIFICANCE

To our knowledge, this is the first report describing the presence of MSI and LOH in the less aggressive types of ATL. These results indicate that microsatellite alterations may participate in the development of the less aggressive types of ATL.

Roles of AML1/RUNX1 in T-cell malignancy induced by loss of p53

AML1/RUNX1 is a frequent target of chromosome translocations and mutations in myeloid and B-cell leukemias, and upregulation of AML1 is also observed in some cases of T-cell leukemias and lymphomas. This study shows that the incidence of thymic lymphoma in p53-null mice is less frequent in the Aml1(+/-) than in the Aml1(+/+) background. AML1 is upregulated in p53-null mouse bone-marrow cells and embryonic fibroblasts. In the steady state, p53 binds to and inhibits the distal AML1 promoter. When the cells are exposed to stresses, p53 is released from the distal AML1 promoter, resulting in upregulation of AML1. Overexpression of AML1 stimulates T-lymphocyte proliferation. These results suggest that upregulation of AML1 induced by loss of p53 promotes lymphoid-cell proliferation, thereby inducing lymphoma development.

Impact of p53 aberration on the progression of Adult T-cell Leukemia/Lymphoma

Based on statistical analysis of its age-dependent occurrence, a multi-step carcinogenesis model has been proposed for Adult T-cell Leukemia/Lymphoma (ATLL). We have previously reported that the deletion of the p16 gene is a key event in ATLL progression. In the current study, we report for the first time that the aberrations of p16 and p53 are mutually exclusive in ATLL and either of the two events is sufficient for the ATLL progression. More than half of the patients had one of the two aberrations, and both aberrations emerged as significant markers for a poor prognosis.

Loss of heterozygosity analyzed by single nucleotide polymorphism array in cancer

Neoplastic progression is generally characterized by the accumulation of multiple genetic alterations including loss of tumor suppression gene function. Loss of heterozygosity (LOH) has been used to identify genomic regions that harbor tumor suppressor genes and to characterize different tumor types, pathological stages and progression. LOH pattern has been detected by allelotyping using restriction fragment length polymorphism, and later by simple sequence length polymorphisms (SSLPs or microsatellite) for 10 years. This paper reviews the detection of LOH by recently developed single nucleotide polymorphism (SNP) arrays (all analyzed by Affymetrix array); furthermore, its advantage and disadvantage were analyzed in several kinds of cancer.

Repression of DNA repair mechanisms in IRF-4-expressing and HTLV-I-infected T lymphocytes

Human T cell leukemia virus (HTLV) is the causative agent of adult T cell leukemia (ATL), an aggressive and fatal leukemia of CD4+ T lymphocytes in which interferon regulatory factor-4 (IRF-4) becomes constitutively expressed, concomitant with major alterations in host gene expression. When constitutively expressed in uninfected T lymphocytes, IRF-4 caused reduced expression of critical DNA repair genes, including Rad51, XRCC1, Ung1, RPA, and proliferative cell nuclear antigen (PCNA), a transcriptional phenotype with striking similarities to the profile observed in HTLV-infected T lymphocytes. Concomitant with the inhibition of gene expression and defects in the DNA repair pathways, increased sensitivity of T lymphocytes to various genotoxic stresses that challenged all major DNA repair pathways were detected. Together, these results support a role for IRF- 4 in the repression of DNA repair activity and an increase in the risk of mutations. IRF-4 may thus represent a previously unidentified endogenous transcriptional repressor of DNA repair mechanisms.

Impact of transforming viruses on cellular mutagenesis, genome stability, and cellular transformation

It is estimated that 15% of all cancers are etiologically linked to viral infection. Specific cancers including adult T-cell leukemia, hepatocellular carcinoma, and uterine cervical cancer are associated with infection by human T-cell leukemia virus type I, hepatitis B virus, and high-risk human papilloma virus, respectively. In these cancers, genomic instability, a hallmark of multistep cancers, has been explicitly linked to the expression of oncoproteins encoded by these viruses. This review discusses mechanisms utilized by these viral oncoproteins, Tax, HBx, and E6/E7, to mediate genomic instability and cellular transformation.

Microsatellite Instability Is Associated with Hypermethylation of the hMLH1 Gene and Reduced Gene Expression in Mycosis Fungoides

Fifty-one mycosis fungoides samples were analyzed for microsatellite instability (MSI) using the panel of markers recommended for hereditary nonpolyposis colorectal cancer kindred and a panel we designed for cutaneous T cell lymphoma in order to compare detection rates and determine if MSI is a genome-wide phenomenon. Samples demonstrating MSI were analyzed for abnormalities of the hMLH1 gene including loss of heterozygosity, mutations, and promoter hypermethylation. MSI was detected in 16% using the hereditary nonpolyposis colorectal cancer panel and 22% with the cutaneous T cell lymphoma panel. Overall, 27% demonstrated MSI and 73% had a stable phenotype. hMLH1 gene studies did not detect loss of heterozygosity or reveal any mutations. Promoter hypermethylation was detected in nine of 14 patients with MSI, however (64%). In addition hMLH1 and hMSH2 protein expression was studied using immunohistochemical techniques. Five of nine patients with MSI and hMLH1 promoter methylation showed abnormal hMLH1 protein expression with normal hMSH2 gene expression. All other patients tested demonstrated normal hMLH1 and hMSH2 protein expression. MSI was found to be more prevalent in tumor stage mycosis fungoides (47%) than early stage disease (20%) and was associated with an older age of onset of mycosis fungoides. MSI may be a consequence of hMLH1 promoter hypermethylation in mycosis fungoides patients and may prevent transcription in a subset of patients. This suggests that the development of a mutator phenotype may contribute to disease progression in mycosis fungoides.

Role of tumor suppressor genes in the development of adult T cell leukemia/lymphoma (ATLL)

Adult T cell leukemia/lymphoma (ATLL) is one of the peripheral T cell malignant neoplasms strongly associated with human T cell leukemia virus type-I (HTLV-I). Although the viral transactivating protein Tax has been proposed to play a critical role in leukemogeneis as shown by its transforming activity in various experimental systems, additional cellular events are required for the development of ATLL. One of the genetic events in ATLL is inactivation of tumor suppressor genes. Among many candidates for tumor suppressor genes, the main genetic events have been reported to center around the cyclin-dependent kinase inhibitors ((CDKIs) p15INK4A, p16INK4B, p18INK4C, p19INK4D, p21WAF1, p27KIP1, and p57KIP2), p53 and Rb genes; all of them play a major regulatory role during G1 to S transition in the cell cycle. Acute/lymphomatous ATLL has frequent alterations of p15 (20%) and p16 (28-67%), while chronic/smoldering ATLL has fewer abnormalities of p15 (0-13%) and p16 (5-26%). Most of these changes are deletion of the genes; fewer samples have mutations. ATLL patients with deleted p15 and/or p16 genes have significantly shorter survival than those individuals with both genes preserved. Although genetic alterations of p18, p19, p21, p27 have rarely been reported, inactivation of these genes may contribute to the development of ATLL because low expression levels of these genes seem to mark ATLL. The p53 gene is mutated in 10-50% of acute/lymphomatous ATLL. Functional impairment of the p53 protein, even if the gene has wild-type sequences, has been suggested in HTLV-I infected cells. Each of these genetic events are mainly found in acute/lymphomatous ATLL, suggesting that alterations of these genes may be associated with transformation to an aggressive phenotype. The Rb tumor suppressor gene is infrequently structurally altered, but one half of ATLL cases have lost expression of this key protein. Notably, alterations of one of the CDKIs, p53 and Rb genes appear to obviate the need for inactivation of other genes in the same pathway. A novel tumor suppressor gene on chromosome 6q may also have a critical role in the pathogenesis of ATLL. Taken together, tumor suppressor genes are frequently altered in acute/lymphomatous ATLL and their alteration is probably the driving force fueling the transition from chronic/smoldering to acute/lymphomatous ATLL.

Allelotyping in mycosis fungoides and Sézary syndrome: common regions of allelic loss identified on 9p, 10q, and 17p

Allelotyping studies have been extensively used in a wide variety of malignancies to define chromosomal regions of allelic loss and sites of putative tumor suppressor genes; however, until now this technique has not been used in cutaneous lymphoma. We have analyzed 51 samples from patients with mycosis fungoides and 15 with Sézary syndrome using methods to detect loss of heterozygosity. Micro satellite markers were selected on 15 chromosomal arms because of their proximity to either known tumor suppressor genes or chromosomal abnormalities identified in previous cytogenetic studies in cutaneous lymphoma. Allelic loss was present in 45% of patients with mycosis fungoides and 67% with Sézary syndrome. Loss of heterozygosity was found in over 10% of patients with mycosis fungoides on 9p, 10q, 1p, and 17p and was present in 37% with early stage (T1 and T2) and 57% with advanced disease (T3 and T4). Allelic loss on 1p and 9p were found in all stages of mycosis fungoides, whereas losses on 17p and 10q were limited to advanced disease. In Sézary syndrome high rates of loss of heterozygosity were detected on 9p (46%) and 17p (42%) with lower rates on 2p (12%), 6q (7%), and 10q (12%). There was no significant difference in the age at diagnosis or number of treatments received by those with loss of heterozygosity and those without, suggesting that increasing age and multiple treatments do not predispose to allelic loss. These results provide the basis for further studies defining more accurately chromosomal regions of deletions and candidate tumor suppressor genes involved in mycosis fungoides and Sézary syndrome.

Loss of heterozygosity and heterogeneity of its appearance and persisting in the course of acute myeloid leukemia and myelodysplastic syndromes

Screening for loss of heterozygosity (LOH) of the panel of 18 highly polymorphic microsatellite markers, especially from the region 11p15, was carried out on 154 samples from 26 patients with acute myeloid leukemia and eight with myelodysplastic syndromes (MDS). LOH was detected at the majority (72%) of the loci tested: 47% of informative patients displayed LOH for at least one of the microsatellite locus from the region 11p15 and 23.5% of patients displayed LOH among the other markers tested within the study. A longitudinal follow-up of patients showed a remarkable heterogeneity of LOH appearance and its persistance during the course of the disease suggesting an intratumor clonal heterogeneity, or alternatively, presence of LOH in more than one cell clone. The data revealed two regions of high loss of one allele in 11p15.5, defined by markers D11S1363 and D11S1338, indicating that LOH at the subtelomeric region of the short arm of chromosome 11 is a much common event in hematological malignancies than it was previously reported.

Mutation analysis of mitotic checkpoint genes (hBUB1 and hBUBR1) and microsatellite instability in adult T-cell leukemia/lymphoma

Adult T-cell leukemia/lymphoma (ATLL) is a neoplasm of T-lymphocytes, and human T-cell lymphotropic virus type-I (HTLV-I) is etiologically considered as the causative virus of ATLL. The karyotypes of ATLL are very complex in both number and structure, although no specific karyotype abnormalities have been identified. HTLV-I is thought to integrate its provirus into random sites in host chromosomal DNA and induces chromosomal instability. The BUB gene is a component of the mitotic checkpoint in budding yeast. Recently, human homologues of the BUB were identified and mutant alleles of hBUB1 and hBUBR1 were detected in two colorectal tumor cell lines, which showed microsatellite instability (MIN). In vitro, BUB proteins form a complex of monomers. These proteins interact with the human MAD1 gene product, a target of the HTLV-1 tax oncogene. We examined the role of checkpoint gene in the chromosomal abnormalities of ATLL by investigating mutations of hBUB1 and hBUBR1, and MIN of replication errors of BAX, insulin-like growth factor, and transforming growth factor beta type II. We analyzed ten cases with ATLL and eight B-cell lymphomas (five diffuse large cell lymphomas, three follicular lymphomas). Complex chromosomal abnormalities were detected in ATLL, while B-cell lymphomas showed only simple or minimal chromosomal abnormalities. Significant mutations/deletion of hBUB1 or hBUBR1 were detected in four of ten cases with ATLL, including two heterozygous point mutations, one homozygous point mutation, and one with a 47 bp deletion. In contrast, only one of eight B-cell lymphomas showed nonsense mutation of hBUBR1. None of the ATLL and B-cell lymphomas showed MIN. In the multistage process of leukemogenesis of ATLL, our findings indicate that mutations of mitotic checkpoint genes may play an important role in the induction of complex chromosomal abnormalities.

Genome-wide detection of allelic imbalance using human SNPs and high-density DNA arrays

Most human cancers are characterized by genomic instability, the accumulation of multiple genetic alterations and allelic imbalance throughout the genome. Loss of heterozygosity (LOH) is a common form of allelic imbalance and the detection of LOH has been used to identify genomic regions that harbor tumor suppressor genes and to characterize tumor stages and progression. Here we describe the use of high-density oligonucleotide arrays for genome-wide scans for LOH and allelic imbalance in human tumors. The arrays contain redundant sets of probes for 600 genetic loci that are distributed across all human chromosomes. The arrays were used to detect allelic imbalance in two types of human tumors, and a subset of the results was confirmed using conventional gel-based methods. We also tested the ability to study heterogeneous cell populations and found that allelic imbalance can be detected in the presence of a substantial background of normal cells. The detection of LOH and other chromosomal changes using large numbers of single nucleotide polymorphism (SNP) markers should enable identification of patterns of allelic imbalance with potential prognostic and diagnostic utility.

Detailed Deletion Mapping of the Long Arm of Chromosome 6 in Adult T-Cell Leukemia

Previously, we have found that the loss of heterozygosity (LOH) was frequently observed on chromosome 6q in acute/lymphoma-type adult T-cell leukemia (ATL), suggesting a putative tumor-suppressor gene for ATL may be present on chromosome 6q. To further define a region containing this gene, we performed fine-scale deletional mapping of chromosome 6q in 22 acute/lymphomatous ATL samples using 24 highly informative microsatellite markers. LOH was found in 9 samples (40.9%) at 1 or more of the loci examined. Of the 9 samples, 8 shared the same smallest commonly deleted region flanked by D6S1652 and D6S1644 (6q15-21). The genetic distance between these two loci is approximately 4 cM. These results suggest that a putative tumor-suppressor gene on chromosome 6q15-21 probably plays a very important role in the evolution of acute/lymphomatous ATL. Our map provides key information toward cloning the gene.

Detailed Deletion Mapping of the Long Arm of Chromosome 6 in Adult T-Cell Leukemia

Previously, we have found that the loss of heterozygosity (LOH) was frequently observed on chromosome 6q in acute/lymphoma-type adult T-cell leukemia (ATL), suggesting a putative tumor-suppressor gene for ATL may be present on chromosome 6q. To further define a region containing this gene, we performed fine-scale deletional mapping of chromosome 6q in 22 acute/lymphomatous ATL samples using 24 highly informative microsatellite markers. LOH was found in 9 samples (40.9%) at 1 or more of the loci examined. Of the 9 samples, 8 shared the same smallest commonly deleted region flanked by D6S1652 and D6S1644 (6q15-21). The genetic distance between these two loci is approximately 4 cM. These results suggest that a putative tumor-suppressor gene on chromosome 6q15-21 probably plays a very important role in the evolution of acute/lymphomatous ATL. Our map provides key information toward cloning the gene.