Genes on chromosomes 1 and 4 in the mouse are associated with repair of radiation-induced chromatin damage

Murine models of chronic lymphocytic leukaemia: role of microRNA-16 in the New Zealand Black mouse model

Mouse models are valuable tools in the study of human chronic lymphocytic leukaemia (CLL). The New Zealand Black (NZB) strain is a naturally occurring model of late-onset CLL characterized by B-cell hyperproliferation and autoimmunity early in life, followed by progression to CLL. Other genetically engineered models of CLL that have been developed include (NZB x NZW) F1 mice engineered to express IL5, mice expressing human TCL1A, and mice overexpressing both BCL2 and a tumour necrosis factor receptor-associated factor. The applicability to human CLL varies with each model, suggesting that CLL is a multifactorial disease. Our work with the de novo NZB model has revealed many similarities to the human situation, particularly familial CLL. In NZB, the malignant clones express CD5, zap-70, and have chromosomal instability and germline Ig sequence. We also identified a point mutation in the 3'-flanking sequence of Mirn16-1, which resulted in decreased levels of the microRNA, miR-16 in lymphoid tissue. Exogenous restoration of miR-16 to an NZB malignant B-1 cell line resulted in cell cycle alterations, suggesting that the altered expression of Mirn15a/16-1 is an important molecular lesion in CLL. Future studies utilizing the NZB mouse could ascertain the role of environmental triggers, such as low dose radiation and organic chemicals in the augmentation of a pre-existing propensity to develop CLL.

Myc translocations in B cell and plasma cell neoplasms

Chromosomal translocations that join the cellular oncogene Myc (c-myc) with immunoglobulin (Ig) heavy-chain (Igh) or light-chain (Igk, Igl) loci are widely believed to be the crucial initiating oncogenic events in the development of B cell and plasma cell neoplasms in three mammalian species: Burkitt lymphoma (BL) in human beings, plasmacytoma (PCT) in mice, and immunocytoma in rats. Among the Myc-Ig translocations found in these neoplasms, mouse PCT T(12;15)(Igh-Myc) is of special interest because it affords a uniquely useful model system to study the fundamental outstanding questions on the mechanisms, genetics, and biological consequences of Myc translocations. Mouse T(12;15) is the direct counterpart of the human BL t(8;14)(q24;q32) translocation and thus of great relevance for human cancer. Mouse T(12;15) is the only cancer-associated translocation in mice that occurs with high incidence, spontaneity, and cell-type specificity. Due to the development of PCR methods for the detection of the underlying reciprocal Myc-Igh junction fragments, it is now known that mouse T(12;15) can be a dynamic process that begins with the genetic exchange of Myc and the Igh switch mu region (Smu), progresses by class switch recombination (CSR) just 3' of the translocation break site, and then undergoes further clonal diversification by micro-deletions in the junction flanks. The molecular pathway that subverts CSR to mediate trans-chromosomal joining of Myc and Smu (translocation origin) and secondary modification of Myc-Igh junctions (translocation "remodeling") has not been elucidated, but recent evidence indicates that it includes CSR factors, such as the activation-induced cytidine deaminase (AID), that may also be involved in the ongoing neoplastic progression of the translocation-bearing tumor precursor. Transgenic mouse models of T(12;15)/t(8;14), including newly developed "iMyc" gene-insertion mice, will be useful in elucidating the role of these CSR factors in the progression of Myc-induced B cell tumors.

The Plasmacytoma Resistance Gene, Pctr2, Delays the Onset of Tumorigenesis and Resides in the Telomeric Region of Chromosome 4

Mouse plasmacytomas share pathogenetic features in common with both multiple myeloma and Burkitt's lymphoma in humans. Susceptibility to plasmacytoma induction by intraperitoneal pristane in mice is controlled by multiple genes. At least two of these genes reside on mouse chromosome 4 in regions of the genome sharing linkage homology with human chromosomes 9p21, 1p32, and 1p36. A series of congenic strains recombinant for regions of mouse chromosome 4 in the vicinity of the Pctr2 predisposition locus were created and typed for their tumor susceptibility/resistance phenotypes. These strains were derived by introgressively backcrossing alleles from resistant DBA/2 mice onto the susceptible BALB/cAnPt background. Six resistant and two susceptible strains were allelotyped for 10 genes and 49 random DNA markers to identify the smallest region of overlap in the resistant strains. These studies have determined that the Pctr2 locus resides in either a 500-kb interval proximal to Nppa, or in a 1- to 2-centiMorgan (cM) interval distal to Nppa. In these congenic strain analyses, the Nppa and Fv1 loci, in addition to genes within about 1 cM of these loci, have been excluded as candidates for the Pctr2 locus. A relevant locus that may reside in this interval is Rep2; it is associated with the efficiency of repairing X-ray induced DNA damage sustained during the G2 phase of the mitotic cycle. The Pctr2 locus acts in a codominant fashion. F1 hybrids between resistant and susceptible congenic strains exhibit a reduced tumor incidence and a significant delay in the onset of tumorigenesis. Identification and eventual cloning of the Pctr2 locus may assist in the identification of genes involved in many types of cancer showing aberrations in human chromosome 1p36.

Elevated mutant frequencies in gene lacI in splenic lipopolysaccharide blasts after exposure to activated phagocytes in vitro

The interaction of B lymphocytes with phagocytes is critical for shaping the humoral immune response, as well as various aspects of normal and malignant B cell development, and has therefore been studied by immunologists in great detail. However, one potential outcome of this confrontation is often neglected, namely the mutagenicity of phagocytes to B lymphocytes. We are interested in phagocyte-induced B cell mutagenesis and have conducted a feasibility study on the utility of a transgenic reporter assay to evaluate mutant frequencies in B cells that have encountered phagocytes. An in vitro co-incubation system was designed in which splenic lipopolysaccharide (LPS) blasts carrying a phage lambda-derived lacI transgene were exposed to pristane-elicited peritoneal exudate cells (PEC). Mutant frequencies in LPS blasts were significantly increased (up to 6-fold) when the cells were co-incubated with PEC that had been stimulated by phorbol myristate acetate to undergo an oxidative burst. The lacI-based transgenic mutation assay proved also useful for assessing mutagenicity in vivo, as demonstrated by the detection of elevated mutant frequencies in the spleen (3-fold) and the inflammatory granuloma (4.7-fold) obtained from pristane-treated mice. We propose to utilize the lacI-based transgenic mutagenesis assay as a tool to evaluate mutational levels during normal and aberrant B cell differentiation.

Spontaneous development of plasmacytomas in a selected subline of BALB/cJ mice

Sixty per cent of BALB/cAnPt mice injected intraperitoneally (i.p.) with tetramethylpentadecane (pristane) develop plasmacytomas (PCs), whereas less than 10% of BALB/cJ develop such tumours. Most other mouse strains are completely resistant. Resistance is dominant over susceptibility in F1 hybrids between BALB/cAnPt and the resistant non-BALB/c strains, suggesting that susceptibility may be due to some genetic defect. (BALB/cAnPtxBALB/cJ)F1 hybrids have a PC incidence of 36-42%. Previously, BALB/cJ has been shown to harbour at least one resistance gene (Potter et al., Genomics 1988, Vol. 2, pp. 257-262). On the assumption that BALB/cJ may contain a segregating resistance gene, we cross BALB/cJ females with pristane-pretreated BALB/cJ males that were found to be carrying PC cells intraperitoneally 5-7 months after pristane treatment. After two selective crosses, 62% of the BALB/cJ subline BALB/cM2/22 developed PC after pristane and 52% after pristane followed by Abelson virus, while unselected controls had an incidence of 11% and 0%, respectively. Moreover, six spontaneous plasmacytomas developed in untreated females of the selected colony. Five of these carried T(12; 15) (F2; D2/3) translocations. The sixth had a T(1; 10) (G; C1) translocation and an interstitial duplication of segment (C1/E3) on one chromosome 5. It may be concluded that pristane treatment is not a prerequisite for the induction of the PC associated Ig/myc translocations.

Protein-DNA interactions at the major and minor promoters of the divergently transcribed dhfr and rep3 genes during the Chinese hamster ovary cell cycle

In mammals, two TATA-less bidirectional promoters regulate expression of the divergently transcribed dihydrofolate reductase (dhfr) and rep3 genes. In CHOC 400 cells, dhfr mRNA levels increase about fourfold during the G1-to-S phase transition of the cell cycle, whereas the levels of rep3 transcripts vary less than twofold during this time. To assess the role of DNA-binding proteins in transcriptional regulation of the dhfr and rep3 genes, the major and minor dhfr-rep3 promoter regions were analyzed by high-resolution genomic footprinting during the cell cycle. At the major dhfr promoter, prominent DNase I footprints over four upstream Sp1 binding sites did not vary throughout G1 and entry into the S phase. Genomic footprinting revealed that a protein is constitutively bound to the overlapping E2F sites throughout the G1-to-S phase transition, an interaction that is most evident on the transcribed template strand. On the nontranscribed strand, multiple changes in the DNase I cleavage pattern are observed during transit through G1 and entry into the S phase. By using gel mobility shift assays and a series of sequence-specific probes, two different species of E2F were shown to interact with the dhfr promoter during the cell cycle. The DNA binding activity of one E2F species, which preferentially recognizes the sequence TTTGGCGC, did not vary significantly during the cell cycle. The DNA binding activity of the second E2F species, which preferentially recognizes the sequence TTTCGCGC, increased during the G1-to-S phase transition. Together, these results indicate that Sp1 and the species of E2F that binds TTTGGCGC participate in the formation of a basal transcription complex, while the species of E2F that binds TTTCGCGC regulates dhfr gene expression during the G1-to-S phase transition. At the minor promoter, DNase I footprints at a consensus c-Myc binding site and three Sp1 binding sites showed little variation during the G1-to-S phase transition. In addition to protein binding at sequences known to be involved in the regulation of transcription, genomic footprinting of the entire promoter region also showed that a protein factor is constitutively bound to the first intron of the rep3 gene.

Influence of prenatal irradiation on second generation mice

Pregnant mice were irradiated with 0.5 and 2 Gy on the 13th gestational day. Offspring were raised until sexual maturity at 2 months of age. Mice from prenatally irradiated mothers and unexposed fathers, or from prenatally irradiated fathers and unexposed mothers, or from parents who had both been prenatally irradiated were mated, resulting in the birth of several mice. These second generation mice were tested as adults for hearing thresholds, with the ABR technique, subjectively to balance tests. and investigated morphologically. Hearing levels were found to be normal and the organ of Corti showed only minor changes of its inner and outer hair cells as shown by transmission and scanning electron microscopy. It is concluded that prenatal irradiation does not seem to cause mutations leading to impaired hearing and balance in second generation mice.

Differences in the molecular structure of c-myc-activating recombinations in murine plasmacytomas and precursor cells

The translocation of c-myc on chromosome (chr.) 15 to an immunoglobulin heavy-chain switch region on chr. 12 is the critical oncogenic step in pristane-induced plasmacytoma (PCT) development in BALB/cAnPt mice. Applying a recently developed PCR method, we have been able to detect the most commonly occurring illegitimate recombinations between alpha-chain switch region (S alpha) and c-myc in preneoplastic B cells residing in mesenteric oil granuloma (OG) tissues 7-30 days postpristane. In this study, we compare the nucleotide sequences at the S alpha/c-myc breaksites on both the c-myc-activating chr. 12+ and the reciprocal chr. 15- from eight transplanted PCTs, seven primary PCTs, and five OGs that contained six B-cell clones. These junction sequences revealed a remarkable diversity of S alpha/c-myc recombinations. In nine cases--four PCTs and five B-cell clones--nearly precise reciprocal exchanges with a loss of only 3-35 bp in c-myc were found. Large deletions in c-myc that removed 369-878 bp were observed in seven PCTs but not in early B cells. Duplications of c-myc ranging from 103 to 229 bp were also restricted to PCTs and noticed in four cases. Clonally related but different reciprocal recombinations, 38 bp apart on chr. 12+ and 15 bp apart on chr. 15-, were isolated from two different specimens of the same OG tissue from a BALB/c mouse 30 days postpristane. A second OG from another 30-day mouse yielded four recombinational fragments--two clonally related chr. 12(+)-specific fragments and two chr. 15(-)-specific fragments--one of which carried a 143-bp insertion of a microsatellite at the breaksite. We suggest that the initial recombinational break-point regions between S alpha and c-myc in plasmacytoma precursor cells at the time of immunoglobulin heavy-chain switching are intrinsically labile and characterized by a persisting instability of c-myc, which can result in large secondary deletions of c-myc.

Differential susceptibility of BALB/c and DBA/2 cells to plasmacytoma induction in reciprocal chimeras

Reciprocal chimeras were generated between BALB/c and DBA/2 mice by inoculating newborn recipients of either strain with bone-marrow (BM) cells of the other through the periorbital vein. DBA/2 mice inoculated with the BALB/c with proven chimerism will be referred to as C----D, the reciprocal as D----C. The BALB/c cells carried a Robertsonian 6;15 (Rb6;15) chromosome marker to facilitate identification. The chimeric mice contained between 5% and 70% of donor cells when examined at 4 to 5 weeks of age. Six of 10 C----D developed plasmacytomas (MPC) after 3 x 0.5 ml monthly pristane treatment (incidence 60%) and 8 of 25 (incidence 32%) after 2 to 3 x 0.5 ml pristane followed by Abelson virus (A-MuLV) infection. Seven of 15 D----C developed MPC after pristane treatment (incidence 47%) and 4 of 17 after pristane + A-MuLV (incidence 24%). All tumors that have arisen in both reciprocal chimeras originated from BALB/c cells independently of the degree of chimerism. All tumors contained an Ig/myc translocation. Among the C----D chimeras, 5 carried t(12;15) and I t(6;15) in the pristane-treated group, while 4 carried t(12;15), I t(6;15) and 3 t(15;16) in the pristane + A-MuLV. Among the D----C chimeras 6 carried t(12;15) and I t(6;15) in the pristane-treated group, while 3 t(12;15) and I t(6;15) in the pristane + A-MuLV. No tumors developed in 18 pristane- and 22 pristane + A-MuLV-treated DBA/2 mice nor in 15 pristane- and 17 pristane + A-MuLV-treated (BALB/c x DBA/2)F1 mice. The data indicate that BALB/c and DBA/2 cells differ in their propensity to transform into plasmacytoma in identical host environments after both pristane and pristane + A-MuLV treatment. They also show that the oil granuloma can support MPC development in either type of chimeric host.

A search for strain differences in response of mice to mutagenesis by thio-TEPA

After treatment of mice with thio-TEPA Malashenko and colleagues found differences among inbred strains in yield of dominant lethals and of chromosome aberrations in bone marrow, which they attributed to genes affecting repair. An attempt was made to confirm this work by comparing yields of dominant lethals in different strains of females mated to the same strain of males. However, no differences were found, all strain combinations giving 42-49% dominant lethals after a dose of 2 mg/kg thio-TEPA to late spermatids. Thus, the existence of genetic differences in repair of thio-TEPA induced lesions between strains CBA and C57BL/6J and between C3H/He and 101/H is not confirmed. Possible reasons for the discrepant results are discussed.

Analysis of the mouse Dhfr/Rep-3 major promoter region by using linker-scanning and internal deletion mutations and DNase I footprinting

The mouse dihydrofolate reductase (Dhfr) promoter region is buried within a CpG island (a region rich in unmethylated CpG dinucleotides), has a high G+C content, and lacks CAAT and TATA elements. The region contains four 48-bp repeats, each of which contains an Sp1-binding site. Another gene, Rep-3 (formerly designated Rep-1), shares the same general promoter region with Dhfr, being transcribed in the direction opposite that of Dhfr. Both genes appear to be housekeeping genes and are expressed at relatively low levels in all tissues. The 5' termini of the major Dhfr transcripts are separated from the 5' termini of the Rep-3 transcripts by approximately 140 bp. This curious structural arrangement suggested that the two genes might share common regulatory elements. To investigate the promoter sequences driving bidirectional transcription, a series of promoter mutations was constructed. These mutations were assayed by a replicating minigene system and by promoter fusions to the chloramphenicol acetyltransferase gene. Linker-scanning mutations that spanned the four repeats produced a variety of mRNA transcript phenotypes. The effects were primarily quantitative, generally reducing the abundance of transcripts for one or both genes. Some mutations affected Dhfr in a qualitative manner, such as by changing the startpoint of one of the major Dhfr transcripts or changing the relative abundance of the two major Dhfr transcripts. Additionally, protein transcription factors that bind to sequences in the mouse Dhfr/Rep-3 major promoter region, potentially affecting expression of either or both genes, were investigated by DNase I footprinting. The results indicate that multiple protein-DNA interactions occur in this region, reflecting potentially complex transcriptional control mechanisms that might modulate expression of either or both genes under different physiological conditions.

Analysis of the mouse Dhfr/Rep-3 major promoter region by using linker-scanning and internal deletion mutations and DNase I footprinting

The mouse dihydrofolate reductase (Dhfr) promoter region is buried within a CpG island (a region rich in unmethylated CpG dinucleotides), has a high G+C content, and lacks CAAT and TATA elements. The region contains four 48-bp repeats, each of which contains an Sp1-binding site. Another gene, Rep-3 (formerly designated Rep-1), shares the same general promoter region with Dhfr, being transcribed in the direction opposite that of Dhfr. Both genes appear to be housekeeping genes and are expressed at relatively low levels in all tissues. The 5' termini of the major Dhfr transcripts are separated from the 5' termini of the Rep-3 transcripts by approximately 140 bp. This curious structural arrangement suggested that the two genes might share common regulatory elements. To investigate the promoter sequences driving bidirectional transcription, a series of promoter mutations was constructed. These mutations were assayed by a replicating minigene system and by promoter fusions to the chloramphenicol acetyltransferase gene. Linker-scanning mutations that spanned the four repeats produced a variety of mRNA transcript phenotypes. The effects were primarily quantitative, generally reducing the abundance of transcripts for one or both genes. Some mutations affected Dhfr in a qualitative manner, such as by changing the startpoint of one of the major Dhfr transcripts or changing the relative abundance of the two major Dhfr transcripts. Additionally, protein transcription factors that bind to sequences in the mouse Dhfr/Rep-3 major promoter region, potentially affecting expression of either or both genes, were investigated by DNase I footprinting. The results indicate that multiple protein-DNA interactions occur in this region, reflecting potentially complex transcriptional control mechanisms that might modulate expression of either or both genes under different physiological conditions.

Loss of alleles from the distal short arm of chromosome 1 occurs late in melanoma tumor progression

The gene for familial malignant melanoma and its precursor lesion, the dysplastic nevus, has been assigned to a region of the distal short arm of chromosome 1, which is frequently involved in karyotypic abnormalities in melanoma cells. We have examined loci on chromosome 1p for loss-of-constitutional heterozygosity in 35 melanomas and 21 melanoma cell lines to analyze the role of these abnormalities in melanocyte transformation. Loss-of-heterozygosity at loci on chromosome 1p was identified in 15/35 (43%) melanomas and 11/21 (52%) melanoma cell lines. Analysis of multiple metastases derived from the same patient and of melanoma and lymphoblastoid samples from a family with hereditary melanoma showed that the loss-of-heterozygosity at loci on distal 1p is a late event in tumor progression, rather than the second mutation that would occur if melanoma were due to a cellular recessive mechanism. Comparisons with neuroblastoma and multiple endocrine neoplasia (MEN2) suggest that the frequent 1p loss-of-heterozygosity in these malignancies is a common late event of neuroectodermal tumor progression.

Sequence and organization of the mouse poly (ADP-ribose) polymerase gene

Using a human cDNA probe, we have isolated murine genomic and cDNA clones corresponding to the nuclear enzyme poly (ADP-ribose) polymerase (ADPRP). Northern analysis with the mouse cDNA clones reveals transcripts of 3.7-3.8 kb corresponding in size to the human ADPRP transcript. DNA sequence comparisons between mouse and human clones reveals extensive amino acid sequence conservation within regions harboring DNA binding, NAD+ binding or automodification domains. A survey among mouse inbred strains for restriction fragment length polymorphism (RFLP) reveals at least three distinct ADPRP alleles. The segregation of alleles among mouse genetic recombinants positions ADPRP on mouse chromosome 1 between the complement receptor-related gene At-3 and the Fc receptor locus FcR. Furthermore, ADPRP is closely associated with the autoimmune locus gld (generalized lymphadenopathy).