Parental origin of chromosomes involved in the translocation t(9;22)

Breast Cancer Genetics: Relevance to Oncology Practice

Our knowledge of genetic mechanisms involved in cancer initiation, promotion, and progression recently bas expanded. In order to benefit from this expansion and to apply genetic discoveries to current protocols for prevention, screening, diagnosis, treatment, prognosis, and monitoring for minimal residual disease, a working knowledge must be developed of the genetic principles, oncogenes, tumor suppressor genes, and genetic models of carcinogenesis. Genetic susceptibility testing for cancer soon will be introduced into oncology practice through established familial risk counseling programs, and the oncologist must be prepared to address the medical, ethical, legal, economic, psychological, and social issues that accompany this testing.

Very high birth weight of offspring is associated with an increased risk of leukemia in their mothers: results of a population-based cohort study

Although the association between birth weight and childhood leukemia is well described, the relation between a child's birth weight and parental risk of leukemia is unknown. We linked data from the Jerusalem Perinatal Study to the Israel Cancer Registry to ascertain the incidence of leukemia in mothers and fathers in relation to their offspring's birth weight. Birth weight >or=4500 g in any of the offspring was associated with a >3-fold risk of leukemia in mothers, but not fathers. Potential mechanisms include shared exposures of high birth weight infants and their mothers, possibly to radiation or growth factors, or genetic pathways leading to both high birth weight and leukemia.

Blockade of the tumor necrosis factor-related apoptosis inducing ligand death receptor DR5 prevents beta-amyloid neurotoxicity

We originally suggested that inhibition of tumor necrosis factor-related apoptosis inducing ligand (TRAIL) death pathway could be taken into consideration as a potential therapeutic strategy for Alzheimer's disease (AD). However, because the critical role of TRAIL in immune surveillance, the neutralization of TRAIL protein by an antibody to prevent its binding to death receptors is definitely a risky approach. Here, we demonstrated that the blockade of the TRAIL death receptor DR5 with a specific antibody completely prevented amyloid beta peptide (A beta) neurotoxicity in both neuronal cell line and primary cortical neurons. DR5 was demonstrated to be a key factor in TRAIL death pathway. In fact, whereas TRAIL expression was enhanced dose-dependently by concentrations of beta amyloid ranging from 10 nM to 1 microM, only the highest toxic dose of A beta (25 microM) induced the increased expression of DR5 and neuronal cell death. In addition, the increased expression of DR5 receptor after beta amyloid treatment was sustained by p53 transcriptional activity, as demonstrated by the data showing that the p53 inhibitor Pifithrin alpha prevented both beta amyloid-induced DR5 induction and cell death. These data suggest a sequential activation of p53 and DR5 upon beta amyloid exposure. Further insight into the key role of DR5 in AD was suggested by data showing a significant increase of DR5 receptor in cortical slices of AD brain. Thus, these findings may give intracellular TRAIL pathway a role in AD pathophysiology, making DR5 receptor a possible candidate as a pharmacological target.

Is embryo-cryopreservation really neutral? A new long-term effect of embryo freezing in mice: protection of adults from induced cancer according to strain and sex

BACKGROUND

Beneficial or harmful effects of embryo freezing have been described in man and animals, raising the question of the neutrality of this technique.

OBJECTIVE

We examined, in mice, the possibility that embryo freezing influences the probability of the emergence, in adults, of an induced urinary bladder cancer.

METHODS

The experiment was conducted in mice derived from embryos of two different genotypes. Females receiving embryos were parsed into two groups according to whether these embryos were cryopreserved or not. The derived adults received the chemical carcinogen N-butyl-N-4hydroxybutylnitrosamine (BBN), in the drinking water. Time to death since the onset of treatment was measured for each animal until the 300th day.

RESULTS

In females from one of the two strains tested, embryo freezing led to a favorable long-term effect on the probability of resistance to induced cancer.

CONCLUSION

This beneficial effect, taken together with other effects reported in the literature be they beneficial or harmful, suggests that embryo freezing in mice may not be neutral.

Natural Selection and the Evolution of Genome Imprinting

Sexual reproduction results from the fusion of gametes in which the chromatin configuration of maternal and paternal chromosomes is distinct at fertilization. Although many of the differences are erased during successive cellular divisions and chromatin modifications, some are retained in both somatic and germline cells. These epigenetic modifications can confer different characteristics on maternal and paternal chromosomes and such differences can be selected during any process that has the ability to distinguish between homologues. The end result of these selective forces are parental origin effects, writ large. The range of effects observed, including transcriptional imprinting and effects on chromosome segregation and heterochromatization, reflects the diversity of selective forces in operation. However, a closer look at these effects suggests that parental origin-dependent differences in chromatin structure might be subject to some common forces and that these forces may explain many of the "nontranscriptional" parental origin effects observed in mammals.

Conformational differences in the 3-D nanostructure of the immunoglobulin heavy-chain locus, a hotspot of chromosomal translocations in B lymphocytes

Spectral precision distance microscopy was utilized to detect small but nonetheless consistently present conformational differences between the immunoglobulin heavy-chain gene clusters (IgH) that reside on the two chromosome 12 homologues in all diploid cells of the mouse. The euclidian distance (i.e., the mean arithmetic three-dimensional [3-D] distance) between the 5' most IgH gene, C mu, and the 3' most IgH gene, C alpha, was used as the indicator to define the co-presence of a condensed IgH domain and a relaxed IgH domain in the same cell. In normal and malignant B cells in which IgH is actively rearranged and transcribed, the C mu/C alpha distance (genomic distance approximately 180 kb) was found to range from 87.5 to 121 nm on the condensed IgH domain and from 154 to 207 nm on the relaxed IgH domain. In non-B cells (fibroblasts, neutrophils, and macrophages), in which IgH is inactive, the C mu/C alpha distance was found to range from 136 to 154 nm on the condensed IgH domain and from 250 to 292 nm on the related IgH domain. These results suggested that conformational differences that may predispose the relaxed IgH domain for illegitimate genetic recombinations, such as chromosomal translocations, are likely to exist in many cell types, including B cells. However, in B lymphocytes this structural predisposition may conspire with the lineage-specific ability to activate proto-oncogenes (after juxtaposition to IgH) to positively affect the preferential involvement of the relaxed IgH domain in chromosomal translocations. Additional studies are warranted to validate this working hypothesis.

Natural selection and the function of genome imprinting: beyond the silenced minority

Most hypotheses of the evolutionary origin of genome imprinting assume that the biochemical character on which natural selection has operated is the expression of the allele from only one parent at an affected locus. We propose an alternative - that natural selection has operated on differences in the chromatin structure of maternal and paternal chromosomes to facilitate pairing during meiosis and to maintain the distinction between homologues during DNA repair and recombination in both meiotic and mitotic cells. Maintenance of differences in chromatin structure in somatic cells can sometimes result in the transcription of only one allele at a locus. This pattern of transcription might be selected, in some instances, for reasons that are unrelated to the original establishment of the imprint. Differences in the chromatin structure of homologous chromosomes might facilitate pairing and recombination during meiosis, but some such differences could also result in non-random segregation of chromosomes, leading to parental-origin-dependent transmission ratio distortion. This hypothesis unites two broad classes of parental origin effects under a single selective force and identifies a single substrate through which Mendel's first and second laws might be violated.

Activation of the L voltage-sensitive calcium channel by mitogen-activated protein (MAP) kinase following exposure of neuronal cells to beta-amyloid. MAP kinase mediates beta-amyloid-induced neurodegeneration

Neuronal degeneration in Alzheimer's disease (AD) has been variously attributed to increases in cytosolic calcium, reactive oxygen species, and phosphorylated forms of the microtubule-associated protein tau. beta-Amyloid (betaA), which accumulates extracellularly in AD brain, induces calcium influx in culture via the L voltage-sensitive calcium channel. Since this channel is normally activated by protein kinase A-mediated phosphorylation, we examined kinase activities recruited following betaA treatment of cortical neurons and SH-SY-5Y neuroblastoma. betaA increased channel phosphorylation; this increase was unaffected by the protein kinase A inhibitor H89 but was reduced by the mitogen-activated protein (MAP) kinase inhibitor PD98059. Pharmacological and antisense oligonucleotide-mediated reduction of MAP kinase activity also reduced betaA-induced accumulation of calcium, reactive oxygen species, phospho-tau immunoreactivity, and apoptosis. These findings indicate that MAP kinase mediates multiple aspects of betaA-induced neurotoxicity and indicates that calcium influx initiates neurodegeneration in AD. betaA increased MAP kinase-mediated phosphorylation of membrane-associated proteins and reduced phosphorylation of cytosolic proteins without increasing overall MAP kinase activity. Increasing MAP kinase activity with epidermal growth factor did not increase channel phosphorylation. These findings indicate that redirection, rather than increased activation, of MAP kinase activity mediates betaA-induced neurotoxicity.

No parental origin bias for the rearranged chromosomes in myeloid leukemias associated with t(9;22), t(8;21) and t(15;17)

We investigated parental origin of rearranged chromosomes 9 and 22 (9q + and 22q -) in five patients with Ph-positive chronic myeloid leukemia (CML) using the C-banding and silver-staining methods of nucleolus organizer regions, respectively; of rearranged chromosome 21 (21q +) in seven patients with t(8;21)-positive acute myeloid leukemia (AML); and of rearranged chromosome 15 (15q +) in six patients with t(15;17)-positive AML. It was found that these rearranged chromosomes can be of either paternal or maternal origin. Although the number of patients examined was small, these results indicate that the genes rearranged as a result of these chromosome translocations (ABL, BCR, AML-1 and PML) are not genomically imprinted.

Butyrylcholinesterase in the life cycle of amyloid plaques

Deposits of diffuse beta-amyloid (Abeta) may exist in the brain for many years before leading to neuritic degeneration and dementia. The factors that contribute to the putative transformation of the Abeta amyloid from a relatively inert to a pathogenic state remain unknown and may involve interactions with additional plaque constituents. Matching brain sections from 2 demented and 4 nondemented subjects were processed for the demonstration of Abeta immunoreactivity, butyrylcholinesterase (BChE) enzyme activity, and thioflavine S binding. Additional sections were processed for the concurrent demonstration of two or three of these markers. A comparative analysis of multiple cytoarchitectonic areas processed with each of these markers indicated that Abeta plaque deposits are likely to undergo three stages of maturation, ie, a "diffuse" thioflavine S-negative stage, a thioflavine S-positive (ie, compact) but nonneuritic stage, and a compact neuritic stage. A multiregional analysis showed that BChE-positive plaques were not found in cytoarchitectonic areas or cortical layers that contained only the thioflavine S-negative, diffuse type of Abeta plaques. The BChE-positive plaques were found only in areas containing thioflavine S-positive compact plaques, both neuritic and nonneuritic. Within such areas, almost all (>98%) BChE-containing plaques bound thioflavine S, and almost all (93%) thioflavine S plaques contained BChE. These results suggest that BChE becomes associated with amyloid plaques at approximately the same time that the Abeta deposit assumes a compact beta-pleated conformation. BChE may therefore participate in the transformation of Abeta from an initially benign form to an eventually malignant form associated with neuritic tissue degeneration and clinical dementia.

The chimeric BCR-ABL gene

The 1982 discovery that in chronic myeloid leukaemia (CML) the ABL proto-oncogene is translocated to the BCR gene located on chromosome 22 initiated many studies on the structural organization and function of these genes. The nucleotide sequence of the entire BCR and major parts of the ABL gene has now been determined. However, the actual cause of the fusion of BCR with ABL remains essentially unknown. Mouse models have been helpful to unravel the normal cellular function of BCR and ABL, as well the activity of BCR-ABL, although a single mechanism explaining the transforming activity of the latter has not been discovered. The cause of progression of the disease remains unknown, and no single genetic abnormality has been linked to the blast phase of CML. Much has been learned concerning the molecular biology of CML, but answers to the fundamental questions above may be expected in the coming years in parallel to increasing knowledge of genome structure, signal transduction and cell cycle control.

Parental imprinting and human disease

Parental imprinting is a process that results in allele-specific differences in transcription, DNA methylation, and DNA replication timing. Imprinting plays an important role in development, and its deregulation can cause certain defined disease states. Absence of a paternal contribution to chromosome 15q11-q13, due to hemizygous deletion or uniparental disomy, results in the Prader-Willi syndrome. The absence of a normal maternal copy of the same region causes Angelman syndrome. The Beckwith-Wiedemann syndrome is associated with the failure of normal biparental inheritance of chromosome 11p15, and loss of imprinting is observed in several cancers including Wilms' tumor. The study of the molecular basis of abnormal imprinting in these disorders will facilitate the identification and characterization of other imprinted human disease loci.

Ig/myc translocations of the plasmacytoma-prone BALB/c strain occur independently of the genetic and parental origin of the affected chromosomes 6, 12, and 15

Virtually all murine plasmacytomas (MPCs) carry chromosomal translocations that juxtapose myc on chromosome 15 (chr 15) to one of the three immunoglobulin loci carrying chr 6, 12, or 16. Only some mouse strains are susceptible to MPC induction, however, with BALB/c as the outstanding example. Most other strains are resistant. Our earlier studies with reciprocal BALB/c<-->DBA/2 chimeras suggested that part of this susceptibility is determined at the level of the target cell itself (DBA/2 is MPC resistant). The probability of the Ig/myc translocation is one of the possibly relevant variables. Because MPC resistance is dominant over susceptibility, it is conceivable that the translocations prevail due to some deficiency of the Ig rearrangement or Ig-associated repair mechanisms in BALB/c cells. This could be determined at the level of the chromosomes that participate in the translocation or by genes on other chromosomes. Here, we show that the substitution of the BALB/c-derived chr 12, 6, and 15, which carry IgH, kappa, and myc, respectively, with their homologs derived from MPC-resistant mice, did not affect MPC susceptibility. The use of Robertsonian 4.12 and 6.15 chromosomes in this study has also provided us with the opportunity to assess the parental derivation of the chromosomes participating in the translocation. In contrast to the human chronic myeloid leukemia (CML)-associated BCR/ABL fusion transcript, where a strong bias was claimed that was attributed to imprinting, we have found that the parental chromosomes were randomly involved in the translocation. We have also shown that the translocations could be of uniparental or biparental origin.

Duplication of chromosome 9 carrying a BCR/ABL chimeric gene in Philadelphia chromosome negative chronic myeloid leukemia

We investigated a patient with Philadelphia chromosome (Ph) negative but BCR positive chronic myeloid leukemia (CML) by fluorescence in situ hybridization (FISH). In the chronic phase one chromosome 9 contained a BCR/ABL fusion gene instead of chromosome 22. Although in blast crisis, both chromosomes 9 had BCR/ABL fusion genes. This could be caused by duplication of the rearranged chromosome 9, which may have a significance similar to a double Ph chromosome. This may suggest that the critical event in CML is the formation of a BCR/ABL chimeric gene regardless of its locus in the genome.

Relaxation of imprinting in carcinogenesis

The study of genes at the heat of tumorigenesis has helped unveil an elaborate biochemical circuitry that governs the proliferation and differentiation of cells. Genomic imprinting is rapidly being recognized as a fundamental process in tumor biology. Aberrant relaxation of imprinted genes have been detected in a wide variety of cancers, of both embryonal and nonembryonal origin. However, despite a vast array of experimental observation, both the purpose and pathogenic mechanism of relaxation of imprinting remain an enigma. Hypotheses are examined in this regard along with speculation for future research.

Why is there no diploid overdose effect in Prader-Willi syndrome due to uniparental disomy?

Due to DNA technology, it is now apparent that the mechanisms of genetic disease are more complex than the model of a gene with biallelic expression in the diploid state. If a gene is imprinted, monoallelic expression is the norm when the chromosomes of a pair are inherited normally from each parent. Uniparental disomy (UPD) is the abnormal situation where both chromosomes of a pair come from the same parent. When the chromosome contains an imprinted gene, UPD may result in nullisomy or disomy for a functional copy of that gene. If there are two imprinted loci on the same chromosome, UPD for that chromosome results in nullisomy for one imprinted gene but functional disomy for the other a "diploid overdose" (DO). This situation has been well demonstrated in the Prader-Willi syndrome (PWS) which is the nullisomic phenotype for the PWS gene(s) on chromosome 15q11-13. Chromosome 15q11-13 also contains the gene for Angelman syndrome (AS) which has a phenotype distinct from PWS. Both loci are subject to imprinting--in PWS, the imprint is on the maternal chromosome 15, in AS it is on the paternal chromosome 15. All individuals with PWS due to maternal UPD, while functionally nullisomic for the PWS locus, are functionally disomic for the AS locus--a DO situation. Assuming that biallelic expression of an imprinted gene is harmful, one would expect DO for an imprinted gene to produce a phenotypic effect. Cases of PWS due to UPD do not appear to differ from those due to deletion (hypopigmentation in deletional cases can be explained by loss of D15S12 downstream from the critical region). There is no good evidence of DO for the AS locus in PWS due to UPD. Why then was it 'necessary' in evolutionary terms to imprint the AS locus and maintain the imprint faithfully for life. A similar situation of two imprinted genes on the same chromosome occurs with IGF2 and H19 on chromosome 11p15. Maternal imprinting for IGF2 and paternal imprinting for H19 is the norm. Paternal UPD in this situation does lead to a DO effect, namely Beckwith-Wiedemann syndrome. The possibility of a DO effect needs to be considered when assessing the phenotypic spectrum of UPD for other chromosomes currently under investigation.

Are ABL and BCR imprinted?

An increasing number of clinical observations and genetic experiments have shown that some parts of the genome behave differently depending on whether they are of paternal or maternal origin. This phenomenon is known as “genomic imprinting” and has been defined as “a reversible process whereby a gamete-specific modification in the parental generation can sometimes lead to functional differences between maternal and paternal genomes in diploid cells of the offspring” [1]. The accumulating evidence for its important role in cancer predisposition syndromes as well as for the pathogenesis of certain types of sporadic tumors prompted us to investigate whether imprinting may also be instrumental in selecting particular parental chromosome regions involved in balanced rearrangements, such as the leukemia-specific translocation t(9;22) [2]. Several reports have analyzed the expression, the methylation and the replication patterns of the two genes, ABL and BCR, on chromosomes 9 and 22, respectively, which are affected by this translocation [3-6]. Although not directly comparable, the results of these studies seem to invalidate our cytogenetic observations. We therefore review this controversial issue and provide some possible explanations for the contradictory results.

Effects of BCR-ABL antisense oligonucleotides (AS-ODN) on human chronic myeloid leukemic cells: AS-ODN as effective purging agents

We examined phosphorothioate oligodeoxyribonucleotides (ODNs) directed against bcr in exon 3 or exon 2, which are rearranged with exon 2 of abl (B3A2 and B2A2) at t(9;22) of chronic myelogenous leukemia (CML). Since these ODNs are designed to be CML cell specific, we studied their effects on the human CML cell line K562, which is known to have B3A2 rearrangement, and leukemic cells from patients, as well as normal hematopoietic stem cells in vitro. In vitro experiments were performed to determine a potential role of these two ODNs as ex vivo purging agents. Incubation of B3A2 antisense at 40, 80, and 120 micrograms/ml with K562 CML cells for 72 hours at 37 degrees C resulted in 44%, 56%, and 63% reduction of CFU-L as compared to controls. In contrast, B3A2 sense and B2A2 antisense had no significant growth inhibitory effect on K562 cells. Incubation of B3A2 and B2A2 antisense ODNs at concentration of 80 micrograms/ml at 37 degrees C for 36 hours with normal peripheral blood stem/progenitor cells (PBSC) resulted in 124% and 98% CFU-GM formation as compared to untreated controls, respectively. However, incubation of PBSC with B3A2 and B2A2 sense-ODNs resulted in a 22% and 44% reduction in CFU-GM, respectively. In order to determine the ex vivo purging effects of bcr-abl ODNs, the K562 cells were mixed with PBSC from normal donors at a ratio of 1:20 (CML:PBSC). The mixture of cells was then incubated with B3A2 antisense at 80 micrograms/ml for 36 hrs at 37 degrees C. After incubation, no CML cells were detected by fluorescence in situ hybridization (FISH) as compared to untreated controls. These results were confirmed by RT-PCR using bcr-abl primers and mRNA isolated from the mixture of cells. Further, these results support the hypothesis that bcr-abl antisense ODNs are potentially effective agents for ex vivo purging of autologous stem cells before transplantation to eliminate/reduce the burden of leukemic cells. No significant toxicity to normal hematopoietic stem/progenitor cell population by the bcr-abl antisense ODNs was observed. Although unanticipated reductions in normal hematopoietic progenitor cells (CFU-GM) were observed with sense ODNs, no reduction in CFU-GM was observed with unrelated phosphorothioate ODN controls.

Excess of deletions of maternal origin in the DiGeorge/velo-cardio-facial syndromes. A study of 22 new patients and review of the literature

We have determined the parental origin of the deleted chromosome 22 in 29 cases of DiGeorge syndrome (DGS) using a CA-repeat mapping within the commonly deleted region, and in one other case by using a chromosome 22 short arm heteromorphism. The CA-repeat was informative in 21 out of 29 families studied and the deleted chromosome was of maternal origin in 16 cases (72%). When these data are pooled with recent results from the literature, 24 de novo DGS, velo-cardio-facial syndrome (VCFS) and isolated conotruncal cardiac disease deletions are found to be of maternal origin and 8 of paternal origin, yielding a chi 2 of 8 with a probability level lower than 0.01. These data, and review of the literature on familial DGS/VCFS and isolated conotruncal cardiopathies suggest that there is a strong tendency for the 22q11.2 deletions to be of maternal origin.

Regulation of genomic imprinting by gametic and embryonic processes

Parental genomic imprinting refers to the phenomenon by which alleles behave differently depending on the sex of the parent from which they are inherited. In the case of the murine transgene RSVIgmyc, imprinting is manifest in two ways: differential DNA methylation and differential expression. In inbred FVB/N mice, a transgene inherited from a male parent is undermethylated and expressed; a transgene inherited from the female parent is overmethylated and silent. Using a series of RSVIgmyc constructs and transgenic mice, we show that the imprinting of this transgene requires a cis-acting signal that is principally derived from the repeat sequences that make up the 3' portion of the murine immunoglobulin alpha heavy-chain switch region. Such imprinting is relatively independent of the site of transgene insertion but is influenced by the structure of the transgene itself. Imprinting is also modulated by genetic background. Detailed studies indicate that the paternal allele is undermethylated and expressed in inbred FVB/N mice and in heterozygous F1 FVB/N/C57Bl/6J mice but is overmethylated and silent in inbred C57Bl/6J mice. Consequently, the FVB/N genome appears to carry alleles of modulating genes that dominantly block methylation and permit expression of the paternally imprinted transgene. Furthermore, our results suggest that overmethylation is the default status of both parental alleles and that the paternal allele can be marked in trans by polymorphic factors that act in postblastocyst embryos.

Increased neutrophil respiratory burst in bcr-null mutants

Philadelphia (Ph)-positive leukemias invariably contain a chromosomal translocation fusing BCR to ABL. The BCR-ABL protein is responsible for leukemogenesis. Here we show that exposure of bcr-null mutant mice to gram-negative endotoxin led to severe septic shock and increased tissue injury by neutrophils. Neutrophils of bcr (-/-) mice showed a pronounced increase in reactive oxygen metabolite production upon activation and were more sensitive to priming stimuli. Activated (-/-) neutrophils displayed a 3-fold increased p21rac2 membrane translocation compared with (+/+) neutrophils. These results connect Bcr in vivo with the regulation of Rac-mediated superoxide production by the NADPH-oxidase system of leukocytes and suggest a link between Bcr function and the cell type affected in Ph-positive leukemia.

Diagnosis and the new genetics

The rapid pace of gene discovery has led to new opportunities for clinical diagnosis using molecular genetic technologies. Recent achievements include the culmination of the 10-year search for the Huntington's disease gene, the identification of predisposing genes for certain familial colon cancers, and the characterization of potential genetic risk indicators for Alzheimer's disease, hypertension, and coronary heart disease. These advances, coupled with the previous discoveries of important disease genes (e.g. those for cystic fibrosis, Duchenne muscular dystrophy, and fragile X syndrome) have quickly expanded the capacity of genetic analysis, allowing the design of enhanced and novel approaches for diagnostic testing. The transfer of molecular technology to the area of clinical genetic analysis, although associated with many potential benefits, has raised some concern regarding the possible misuse of genetic tests and information, particularly with regard to presymptomatic diagnosis of disease and population screening.

Progressive de novo DNA methylation at the bcr-abl locus in the course of chronic myelogenous leukemia

De novo methylation of CpG islands is a rare event in mammalian cells. It has been observed in the course of developmental processes, such as X chromosome inactivation and genomic imprinting. The methylation of DNA, an important factor in the epigenetic control of gene expression, may also be involved in tumorigenesis. After the t(9;22) chromosomal translocation and generation of the Philadelphia chromosome, the initiating event in chronic myelogenous leukemia (CML), most of the abl coding sequence is fused to the 5' region of the bcr gene. Expression of the hybrid bcr-abl gene is, therefore, regulated by the bcr promoter. In most cases of CML, one of the two abl promoters (Pa) is nested within the bcr-abl transcriptional unit and should be able to transcribe the type Ia 6-kb normal abl mRNA from the Philadelphia chromosome. However, we have found that the 6-kb transcript is present only in CML cell lines containing a normal abl allele and that the apparent inactivation of the nested Pa promoter is associated with allele-specific methylation. Furthermore, we have noticed that the Pa promoter is contained within a CpG island and undergoes progressive de novo methylation in the course of the disease. This is attested to by the fact that DNA samples from CML patients that are methylation-free at the time of diagnosis invariably become methylated in advanced CML. Since tumor progression in CML cannot always be inferred from the clinical presentation, assessment of de novo CpG methylation may prove to be of critical value in management of the disease. It could herald blastic transformation at a stage when bone marrow transplantation, the only potentially curative therapeutic procedure in CML, is still effective.

On the oncodevelopmental role of human imprinted genes

Genome imprinting has an essential role in normal embryonal mammalian development. Starting early in differentiation, the transcripts of certain human genes, e.g. the paternally-H19 and the maternally-imprinted IGF2, are expressed in specific tissues and organs during fetal life. In several malignant disorders, imprinted genes are, again, unfolded. Characteristically, expression follows the same tissue presentation as during embryogenesis. Clinical paternal disomies, i.e. trophoblastic diseases, and their maternal counterpart, i.e. ovarian teratomas, are associated with apparent relaxation of imprinting once they turn malignant. Paediatric neoplasms, like Wilm's tumor (WT) and rhabdomyosarcoma, often express IGF2 and H19. Recently, we have found H19 expression in invasive urothelial cancer. Evidently, imprinted genes display an oncodevelopmental mode of expression, very much like the classical oncofetal proteins AFP and CEA. Based on available data, including tumor preferential paternal allele retention and chromosome 11 short arm physical linkage with oncogenes like H-ras, we hypothesize that imprinted genes not only accompany cancer but may play a causative role as well.

Imprinting: a gamete's point of view

The recent isolation of imprinted mammalian genes has at last allowed the analysis of the molecular controls that regulate monoparental gene expression. Remarkably, many expectations as to how the imprinting mechanism works have been confounded by these studies. As a result, the time now seems right to reconsider our current understanding of the nature of imprinting, and how best it can be defined. Here, I discuss the past and present understanding of imprinting in the light of results obtained from studies of endogenous imprinted genes, and finally propose that the current multiplicity of imprinting terms and definitions be replaced by a single term and definition.

Familial clustering of malignant germ cell tumors and Langerhans' histiocytosis

Three sisters in a family with seven children whose grandmother had an ovarian embryonal carcinoma experienced development of malignant and a malignant-like situation in childhood. Two were diagnosed as having malignant germ cell tumors of the ovary, and the third was found to have Langerhans' histiocytosis. The two girls with germ cell tumor shared an identical human leukocyte antigen, whereas the sister with histiocytosis shared one identical haplotype with them. All three children have been treated successfully with chemotherapy and are doing well off of treatment.

Parental imprinting of autosomal mammalian genes

The molecular mechanisms introducing epigenetic modifications that lead to differential silencing of some autosomal alleles depending on their parental legacy are still largely unknown, but recent results from studies of endogenously imprinted genes and particular transgenes make DNA methylation a strong candidate. At the same time, these results have raised new questions about the details of the imprinting process.

A growing relationship between genomic imprinting and tumorigenesis

The association between aberrations of the human genome and the development of cancer is well established. Gene imprinting, defined as gene expression based on the gamete of origin, has previously been shown to be involved in this process by the loss of tumor suppressor gene regulation. We suggest that the activation of imprinted proto-oncogenes and growth factors may also play a vital role in tumorigenesis. Mechanistically, this can occur when the gene is removed from its imprinted sequence area, thus escaping repression. Synteny between the human and mouse genome provides the opportunity for targeted studies of imprinted genes suspected to be involved in cancer.

CGG-repeat polymorphism of the BCR gene rules out predisposing alleles leading to the Philadelphia chromosome

The Philadelphia chromosome (Ph) is associated with leukemia, most frequently of the chronic myelogenous variety. The Ph chromosome is a translocation chromosome which gains oncogenic potential through the fusion of the ABL oncogene of chromosome 9 with the BCR gene of chromosome 22. The Ph is believed to arise from random chromosome rearrangement with a subsequent selective advantage of the malignant cell line. However, alleles may be present in the population which predispose toward this specific rearrangement. We used a highly polymorphic CGG-repeat polymorphism within the first exon of the BCR gene to determine BCR allele frequencies among 26 leukemia patients with the Ph chromosome and 63 control individuals. Eight BCR alleles of variable CGG-repeat length were present in both groups at statistically similar frequencies and in Hardy-Weinberg equilibrium. We therefore concluded that there are no alleles of the BCR gene that have a major predisposing influence on the development of the Ph chromosome and subsequent leukemia.

Mechanisms of genomic imprinting in mammals

This chapter can be summarized by the following main points: Genomic imprinting results in the functional nonequivalence of the maternal and paternal genomes, thereby preventing the development of viable parthenogenotes and androgenotes in eutherian mammals. Imprinting may have arisen as a result of the specialized evolutionary requirements of the parental genomes or may have been an obligatory step in the development of placentation. A substantial proportion of transgenes and a smaller number of endogenous genes demonstrate imprinted pattern of expression in mice and humans. An analysis of DNA methylation in somatic tissues and germ cells during embryonic and postnatal development reveals dynamic changes, particularly during gametogenesis and early embryogenesis. The nature and timing of these changes suggest that DNA methylation may be involved in genomic imprinting. Imprinted genes display complex methylation patterns. Many aspects of these patterns are consistent with a role for methylation in the imprinted phenotype, although it is currently unclear whether methylation functions in the establishment of imprinting or plays a secondary role in the maintenance of the imprinted pattern of expression. Studies underway to identify new imprinted genes may help elucidate both the function and mechanism of genomic imprinting.

The role of mitotic recombination in carcinogenesis

Genetic recombination systems are present in all living cells and viruses and generally contribute to their hosts' flexibility with respect to changing environmental conditions. Recombination systems not only help highly developed organisms to protect themselves from microbial attack via an elaborate immune system, but conversely, recombination systems also enable microorganisms to escape from such an immune system. Recombination enzymes act with a high specificity on DNA sequences that either exhibit extended stretches of homology or contain characteristic signal sequences. However, recombination enzymes may rarely act on incorrect alternative target sequences, which may result in the formation of chromosomal deletions, inversions, translocations, or amplifications of defined DNA regions. This review describes the characteristics of several recombination systems and focuses on the implication of aberrant recombination in carcinogenesis. The consequences of mitotic recombination on the inappropriate activation of protooncogenes and on the loss of tumor suppressor genes is discussed. Cases are reported where mitotic recombination clearly has been associated with carcinogenesis in rodents as well as humans. Several test systems able to detect recombinagenic activities of chemical compounds are described.

Mouse homologues of human hereditary disease

Details are given of 214 loci known to be associated with human hereditary disease, which have been mapped on both human and mouse chromosomes. Forty two of these have pathological variants in both species; in general the mouse variants are similar in their effects to the corresponding human ones, but exceptions include the Dmd/DMD and Hprt/HPRT mutations which cause little, if any, harm in mice. Possible reasons for phenotypic differences are discussed. In most pathological variants the gene product seems to be absent or greatly reduced in both species. The extensive data on conserved segments between human and mouse chromosomes are used to predict locations in the mouse of over 50 loci of medical interest which are mapped so far only on human chromosomes. In about 80% of these a fairly confident prediction can be made. Some likely homologies between mapped mouse loci and unmapped human ones are also given. Sixty six human and mouse proto-oncogene and growth factor gene homologies are also listed; those of confirmed location are all in known conserved segments. A survey of 18 mapped human disease loci and chromosome regions in which the manifestation or severity of pathological effects is thought to be the result of genomic imprinting shows that most of the homologous regions in the mouse are also associated with imprinting, especially those with homologues on human chromosomes 11p and 15q. Useful methods of accelerating the production of mouse models of human hereditary disease include (1) use of a supermutagen, such as ethylnitrosourea (ENU), (2) targeted mutagenesis involving ES cells, and (3) use of gene transfer techniques, with production of 'knockout mutations'.

Genomic imprinting: a new mechanism for disease

Evidence has been accumulating from various fields of research that genomic imprinting, defined as the differential modification of genetic material depending on whether inheritance is from the male or female parent, occurs in mammals as well as in man. Human genetic diseases such as complete hydatidiform moles, triploidy, Prader-Willi syndrome, Angelman syndrome, and various cancers provide strong support for the important role of genomic imprinting in human development and represents a new mechanism for disease.

Molecular genetic aspects of human cancers: the 1993 Frank Rose Lecture

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Translocation of BCR to chromosome 9: a new cytogenetic variant detected by FISH in two Ph-negative, BCR-positive patients with chronic myeloid leukemia

Leukemic cells from two patients with Philadelphia-negative chronic myeloid leukemia (CML) were investigated: 1) Cytogenetics showed a normal 46,XY karyotype in both cases, 2) molecular studies revealed rearrangement of the M-BCR region and formation of BCR-ABL fusion mRNA with b2a2 (patient 1) or b3a2 (patient 2) configuration, and 3) fluorescence in situ hybridization (FISH) demonstrated relocation of the 5' BCR sequences from one chromosome 22 to one chromosome 9. The ABL probe hybridized to both chromosomes 9 at band q34, while two other probes which map centromeric and telomeric of BCR on 22q11 hybridized solely with chromosome 22. For the first time, a BCR-ABL rearrangement is shown to take place on 9q34 instead of in the usual location on 22q11. A rearrangement in the latter site is found in all Ph-positive CML and in almost all investigated CML with variant Ph or Ph-negative, BCR-positive cases. The few aberrant chromosomal localizations of BCR-ABL recombinant genes found previously were apparently the result of complex and successive changes. Furthermore in patient 2, both chromosomes 9 showed positive FISH signals with both ABL and BCR probes. Restriction fragment length polymorphism (RFLP) analysis indicated that mitotic recombination had occurred on the long arm of chromosome 9 and that the rearranged chromosome 9 was of paternal origin. The leukemic cells of this patient showed a duplication of the BCR-ABL gene, analogous to duplication of the Ph chromosome in classic CML. In addition they had lost the maternal alleles of the 9q34 chromosomal region.(ABSTRACT TRUNCATED AT 250 WORDS)

The insulin-like growth factor type-2 receptor gene is imprinted in the mouse but not in humans

In mouse, four genes have been found to undergo genomic imprinting resulting in differential expression of maternally and paternally inherited alleles. To determine whether the cognate genes are also subject to imprinting in humans, we have studied allele-specific expression patterns of insulin-like growth factor 2, IGF2-receptor and H19 in human fetal and adult tissues. In keeping with previous findings in mice, our results indicate that in human fetal tissues the paternal H19 alleles is inactive. IGF2 is monoallelically expressed in various tissues but surprisingly not in adult human liver. The human IGF2R gene, another classic example of imprinting in mice, was found to be expressed from both alleles. We provide the first direct evidence for differential imprinting in the human and murine genome.

Allele-specific replication timing of imprinted gene regions

Several lines of evidence suggest that the paternal and maternal genomes may have different expression patterns in the developing organism and this has been confirmed by the identification of endogenous genes that are parentally imprinted in the mouse. Little is known about the precise mechanisms involved in the process, but structural differences between the two alleles must somehow provide cis-acting signals for directing parental-specific transcription. Cell-cycle replication time is one parameter that has been shown to be associated with both tissue-specific gene expression and the allele-specific transcription patterns of the X chromosomes in female cells. For this reason we have examined the replication timing patterns for the chromosomal regions containing the imprinted genes Igf2, Igf2r, H19 and Snrpn in the mouse. At all of these sites, and their corresponding positions in the human genome, the two homologous alleles replicate asynchronously and it is always the paternal allele that is early-replicating. Thus imprinted genes appear to be embedded in large DNA domains with differential replication patterns, which may provide a structural imprint for parental identity.