An AC-repeat adjacent to mouse Cdkn2B allows the detection of specific allelic losses in the p15INK4b and p16INK4a tumor suppressor genes

Myeloid, B and T lymphoid and mixed lineage thymic lymphomas in the irradiated mouse

Thymic lymphoma is a very common spontaneous and/or induced malignancy in both inbred mice and in transgenic mouse models of human cancer. Although a thymic lymphoma is defined as thymus-dependent T-cell malignancy, diagnostic criteria vary between studies and considerable heterogeneity has been reported. To define and classify the thymic lymphomas that arose in our study of X-irradiated (CBA/HxC57BL/6)F1, F1 backcross and F1 intercross mice, 66 thymic lymphomas were immunogenotyped for immunoglobulin heavy chain (IgH) and T-cell receptor beta (TCRbeta) gene rearrangements, and/or analysed for expression of lineage-specific markers and allelic loss on chromosome 4. The data indicate that 33% of the thymic lymphomas are very similar to mouse radiation-induced acute myeloid (AML) and mixed lineage (IgH(R), TCRbeta(G)) pre-B lympho-myeloid (L-MLs) leukaemias, 33% are mixed lineage (IgH(R), TCRbeta(R)) B/T lymphoid and <33% can be described as single lineage (IgH(G), TCRbeta(R)) T-cell malignancies. As the myeloid and L-ML leukaemias are not thymus-dependent this suggests that a malignant myeloid or pre-B lympho-myeloid cell can colonize the spleen to give an AML or L-ML leukaemia, or can colonize the thymus where TCRbeta gene rearrangement(s) may be induced to give the mixed lineage thymic lymphomas. Thus, assuming the single lineage T-cell thymic lymphomas fulfil the criteria of a thymus-dependent T-cell malignancy, thymic lymphomas are comprised of at least three distinct malignancies.

A PCR-based clonal analysis of radiation-induced loss of heterozygosity in haemopoietic stem cells

CBA mouse strains have been used for many years as a model of radiation-induced acute myeloid leukaemia and the leukaemias in CBA and their F1 hybrids are characterised by a specific loss of heterozygosity involving one homologue of chromosome 2. Previous cytogenetic studies of transplanted irradiated bone marrow, or of bone marrow obtained from irradiated mice significantly before the appearance of leukaemia, have been interpreted as the chromosome 2 deletion being a high frequency, possibly initiating event. However, these studies had not specifically addressed the question of whether the characteristic deletion was induced at a high frequency in stem cells. Using a PCR-based technique, we have studied the induction of chromosome 2 LOH in the progeny of (CBA/H x C57BL/6)F1 stem cells after a potentially leukaemogenic radiation exposure. Whilst chromosome 2 LOH can be induced directly by irradiation and there is a preferential loss of the CBA allele, the frequency is no greater than LOH induced in other chromosomal regions studied. The data do not support radiation-induced deletion involving one homologue of chromosome 2 in long-term repopulating stem cells (<1 in 200) being as high a frequency event as might be inferred by previous cytogenetic studies of total bone marrow.

Allelic loss on chromosome 4 (Lyr2/TLSR5) is associated with myeloid, B-lympho-myeloid, and lymphoid (B and T) mouse radiation-induced leukemias

The CBA/H mouse model of radiation-induced acute myeloid leukemia (AML) was re-examined using molecular approaches. In addition to the typical promyelocytic AMLs, 34% were reclassified as early pre-B lympho-myeloid leukemias (L-ML) based on leukemic blood cell morphology, immunoglobulin heavy-chain gene re-arrangements (IgH(R)), or expression of both lymphoid (Vpre-B1 and Rag1) and myeloid (myeloperoxidase and lysozyme M) genes. Allelic loss on chromosome 4 was frequently detected in AMLs (53%) and L-MLs (more than 95%), and the preferential loss of the maternally transmitted allele suggests the locus may be imprinted. A minimally deleted region (MDR) maps to a 3.4-cM interval, which is frequently deleted in radiation-induced thymic lymphomas (TLSR5) and contains a recessive, maternally transmitted genetic locus (Lyr2) that confers resistance to spontaneous and radiation-induced pre-B and T cell lymphomas, suggesting they are one and the same. Thus, the Lyr2/TLSR5 locus is frequently implicated in myeloid, lymphoid (B and T), and mixed-lineage mouse leukemias and lymphomas. Epigenetic inactivation of one Lyr2/TLSR5 allele during normal mouse development suggests that only a single hit is required for its inactivation during leukemogenesis, and this may be a significant contributing factor to the efficiency of the leukemogenic process in the mouse.

Analysis of loss of heterozygosity in lymphoma and leukaemia arising in F1 hybrid mice locates a common region of chromosome 4 loss

Previous studies have identified five lymphoma-related tumour suppressor gene regions on murine chromosome 4. Using detailed allelotype analysis on a range of lympho-haematopoietic tumour types arising in F1 hybrid mice, we now show a consistent pattern of loss of heterozygosity (LOH) which identifies a common region of loss delineated by microsatellites D4Mit21 and D4Mit53 on proximal chromosome 4. This critical segment corresponds to the thymic lymphoma tumour suppressor region 5 (TLSR5) identified in an earlier study. Tumours of this type have also been reported as showing allelic loss from the Trp53 and Ikaros regions on chromosome 11. In the present study, only a small fraction of tumours showed LOH in the Ikaros region, while a minority of lymphomas, but not acute myeloid leukaemias, showed allelic loss of the chromosome 11 segment encoding Trp53. These and other data indicate strongly that the genomic regions identified as showing recurrent LOH depend on the genetic background of the mice. Overall, the results indicate a key role for a tumour suppressor gene(s) encoded in an approximately 3 cM segment on proximal chromosome 4 and provide an experimental basis for the further investigation of the functional role of candidate genes which include Pax5 and Tgfbr1.

Loss of heterozygosity at the proximal-mid part of mouse chromosome 4 defines two novel tumor suppressor gene loci in T-cell lymphomas

Recent studies in our laboratory reported frequent loss of heterozygosity (LOH) on mouse chromosome 4 in T-cell lymphomas, identifying three candidate tumor suppressor regions (TLSR1-3). To determine the possible existence of other tumor suppressor gene loci on the proximal-mid part of chromosome 4 and to clarify whether the p16(INK4a) (alpha and beta) and p15(INK4b) genes are the inactivation targets of deletion at TLSR1, we have tested 73 gamma-radiation-induced T-cell lymphomas of F1 hybrid mice by LOH analysis. Frequent LOH was found at the INK4a and INK4b loci and the surrounding markers D4Mit77, D4Mit245 and D4Wsm1. In addition, we identified two distinct regions of significant allelic losses in the proximal-mid part of chromosome 4, defined by the markers D4Mit116 (TLSR4) and D4Mit21 (TLSR5). Taken together, this evidence and our previous data indicate the existence of at least five different candidate sites for tumor suppressor genes on chromosome 4, thus revealing a main role for this chromosome in the development of mouse T-cell lymphomas.

Hypermethylation of the cell cycle inhibitor p15INK4b 3'-untranslated region interferes with its transcriptional regulation in primary lymphomas

The cyclin-dependent kinase inhibitor p15INK4b has been shown to be involved in human and rodent tumors and seems to act as a tumor suppressor gene in hematological malignancies. Alterations of this gene in tumors include mainly homozygous deletions and hypermethylation of the CpG island in the promoter region. In this work, we describe a new area sensitive to methylation in the 3' untranslated region (UTR) of the murine p15INK4b gene. This region shows different levels of methylation depending on the tissues, being relatively highly methylated in brain and gut, and weakly methylated in liver, spleen or thymus. DNA methylation and expression is similar in both maternal and paternal alleles indicating no imprinting effect. Although methylation of the p15INK4b 3'-UTR is low in normal thymus, increased levels (up to 100%) of specific methylation in this region are found in up to 30% of radiation- or carcinogen-induced thymic lymphomas, correlating with decreased gene expression. Hypermethylation of the p15INK4b 3'-UTR frequently occurs in tumors with loss of heterozygosity (LOH) but without methylation of the promoter CpG island or intragenic mutations. Furthermore, in vitro CpG methylation of the 3'-UTR produces reduced levels of a luciferase reporter in cultured cells. Methylation of two CpG sites in a 120 bp region is sufficient to interfere with transcription of the reporter gene. These data suggest that although the levels of p15INK4b in normal tissues can be mainly determined by promoter regulatory elements, strong hypermethylation of the 3'-UTR can interfere with transcription. Thus, hypermethylation of the 3'-UTR may explain the lack of p15INK4b gene expression in a subset of tumors with no promoter methylation and could be a new alternative mechanism for tumor suppressor gene inactivation in tumorigenesis.