Mutational risks in females: genomic imprinting and maternal molecules

A Model for Human Situs Determination

Alterations of human laterality range from situs reversal or consensus isometry to isolated anomalies of the cardiac, respiratory, gastrointestinal, skeletal, and central nervous systems. A mechanism for human situs determination has been derived from the general model of Brown and Wolpert (1990) , with steps involving A-P axis, D-V axis, midline, global situs, and local situs specification. Comparison with Drosophila segmentation is supported by maternal transmission of certain human situs defects and mutation of appropriate Drosophila gene homologues in the human Waardenburg and Greig syndromes. Anteroposterior gradients in expression of vertebrate homeotic genes may relate to a proposed hierarchy of regional laterality decisions. Early alterations in A-P or D-V axis polarity would produce situs reversal in 100 of individuals, as observed in pure situs inversus, homozygous inv mice or manipulated Xenopus embryos. Later alterations would permit random right-left decisions and account for heterotaxy in 50 of affected individuals, as observed in poly asplenia or homozygous iv mice. Randomisation of brain asymmetry could explain why situs but not brain laterality may be reversed in humans, with forebrain situs reversal or isometry leading to brain anomalies. Homologues of Drosophila genes regulating axis polarity, heart and gut development are attractive candidates in human laterality disorders, but none is uniquely localised to the 6q14-q21 or9q32-q34, 7q22, 10q21-22, 11q13 or 11q25, 12q13, 13qter, or Xq24-q27.1 chromosome regions highlighted by heterotaxic patients or mutant mice.

Gender differences in the induction of chromosomal aberrations and gene mutations in rodent germ cells

Germ cell mutagenicity testing provides experimental data to quantify genetic risk for exposed human populations. The majority of tests are performed with exposure of males, and female data are relatively rare. The reason for this paucity lies in the differences between male and female germ cell biology. Male germ cells are produced throughout reproductive life and all developmental stages can be ascertained by appropriate breeding schemes. In contrast, the female germ cell pool is limited, meiosis begins during embryogenesis and oocytes are arrested over long periods of time until maturation processes start for small numbers of oocytes during the oestrus cycle in mature females. The literature data are reviewed to point out possible gender differences of germ cells to exogenous agents such as chemicals or ionizing radiation. From the limited information, it can be concluded that male germ cells are more sensitive than female germ cells to the induction of chromosomal aberrations and gene mutations. However, exceptions are described which shed doubt on the extrapolation of experimental data from male rodents to the genetic risk of the human population. Furthermore, the female genome may be more sensitive to mutation induction during peri-conceptional stages compared to the male genome of the zygote. With few exceptions, germ cell experiments have been carried out under high acute exposure to optimize the effects and to compensate for the limited sample size in animal experiments. Human exposure to environmental agents, on the other hand, is usually chronic and involves low doses. Under these conditions, gender differences may become apparent that have not been studied so far. Additionally, data are reviewed that suggest a false impression of safety when responses are negative under high acute exposure of male rodents while a mutational response is induced by low chronic exposure. The classical (morphological) germ cell mutation tests are not performed anymore because they are animal and time consuming. Nevertheless, information is needed to place genetic risk extrapolations on more solid grounds and thereby to prevent an increased genetic burden to future generations. It is pointed out that modern molecular methodologies are available now to experimentally address the open questions.

Air pollution induces heritable DNA mutations

Hundreds of thousands of people worldwide live or work in close proximity to steel mills. Integrated steel production generates chemical pollution containing compounds that can induce genetic damage (1, 2). Previous investigations of herring gulls in the Great Lakes demonstrated elevated DNA mutation rates near steel mills (3, 4) but could not determine the importance of airborne or aquatic routes of contaminant exposure, or eliminate possible confounding factors such as nutritional status and disease burden. To address these issues experimentally, we exposed laboratory mice in situ to ambient air in a polluted industrial area near steel mills. Heritable mutation frequency at tandem-repeat DNA loci in mice exposed 1 km downwind from two integrated steel mills was 1.5- to 2.0-fold elevated compared with those at a reference site 30 km away. This statistically significant elevation was due primarily to an increase in mutations inherited through the paternal germline. Our results indicate that human and wildlife populations in proximity to integrated steel mills may be at risk of developing germline mutations more frequently because of the inhalation of airborne chemical mutagens.

Genetic and molecular studies on Om, a locus controlling mouse preimplantation development

Several lines of evidence have accumulated in recent years indicating that nuclear cytoplasmic interactions play an important role in the formation and fate of the developing mouse embryo. Early nuclear transplantation experiments indicated that the ability of nuclei to direct cleavage after transfer into enucleated zygotes falls abruptly with nuclei from more advanced preimplantation stages [1]. Transcriptional activation of the nuclei, which occurs during the second cell cycle probably precludes the reprogramming of nuclei from later cleavage stages [2]. Thus, when an 8-cell nucleus is transferred to an enucleated zygote, such a reconstituted zygote is blocked at the 2-cell stage. However, when identical 8-cell nuclei were transferred into both blastomeres of enucleated 2-cell embryos, they were able to support development to the blastocyst stage and even gave rise to live offspring [2-4]. This indicated the importance of the cytoplasmic environment for the ability of the incoming nucleus to support development. It should be noted that in these experiments, the nuclear cytoplasmic ratio was also an important factor in determining the development of the reconstituted embryos [2]. Similar observations were also made when monitoring the development of haploid embryos [5]. In another study, Latham and Solter [6] examined the ability of androgenones, obtained by replacing the female pronucleus of a zygote by the male pronucleus, to develop to the blastocyst stage. Androgenones generated from C57B1/6 eggs were found to be much more competent to give rise to blastocysts than were DBA/2 androgenones. However, when androgenones were constructed from (DBA/2×C57B1/6)F1, zygotes (genetic constitution of the embryos will hereafter be indicated with the female parent coming first followed by the male parent), by replacing the DBA/2 female pronucleus with a C57B1/6 pronucleus, they also developed poorly. This was not simply due to the lack of some component in DBA/2 cytoplasm, since the impaired development was also observed when C57B1/6 male pronuclei from pairs of (DBA/2×C57B1/6) F1, were transferred to an enucleated C57B1/6 egg.

Down's syndrome as a model for the decisive role of maternal lineage in human evolution

The study of human evolution and the mechanism of this process can be approached from physical anthropology, which examines phenotypic expression and molecular evolution, which investigates genotypic change. Alternatively, we suggest that human evolutional process can also be explained using present day examples of abberations in evolution. Thus, from both genotypic and phenotypic perspectives, we address the question of whether Down's syndrome is an instructive example to look into the decisive role of maternal lineage in human evolution.

A test for uniparental disomy in Saccharomyces cerevisiae

Uniparental disomy is a condition in a diploid organisms where one parental chromosome is absent and its homolog from the other parent duplicated. It can be a cause of genetic somatic disease in mammals because of imprinting. Imprinting creates a sex-specific pattern of epigenetic gene inactivation at least in mammals and, consequently, a complete set of both maternal and paternal chromosomes is required for normal development. Moreover, it has been shown for several types of tumors that recessive tumor alleles originally present in a heterozygous condition in normal somatic tissue have become homozygous in the tumor cells. Homozygosity is frequently caused by uniparental disomy. A similar situation is found in Saccharomyces cerevisiae where the spontaneous or induced expression of linked recessive alleles flanking a common centromere is preponderantly due to isodisomy where one of the homologs is lost and the retained homolog duplicated. In contrast to the situation in Aspergillus nidulans, isodisomy does not appear to be caused by two sequential and independent events of malsegregation resulting first in an unstable trisomic condition from which a normal disomic condition is restored through segregational loss of one supernumerary chromosome. Rather, an as yet unknown mechanism seems to directly generate isodisomy and thus Saccharomyces cerevisiae could provide a short-term test for the detection of this type of genetic change.

Genetic imprinting in human evolution: the decisive role of maternal lineage

The modern study of human evolution must take into account physical anthropology, which examines phenotypic expression, and molecular evolution, which examines genotypic change. Recent independent investigations have shown that the process of genetic imprinting, defined as parental-dependent transmission of genetic traits, plays a pivotal role in human evolution. We draw on data from various scientific disciplines to support the hypothesis that maternal lineage via preferential genetic contribution, plays a decisive role in this regard. This concept is of more than theoretical interest, in that, current human disease states can be better understood and studied in the context of loss of genetically-defined evolutionary advantage.

Genomic imprinting: summary of an NICHD conference

Compelling evidence for genomic imprinting as a pathogenetic mechanism in humans mandates re-evaluation of every genetic or multifactorial disease for parent-of-origin effects. In an expanding list of malformation syndromes, cancers, growth abnormalities, and chromosomal disorders, phenotypes may be determined by source rather than content of transmitted DNA. A multidisciplinary conference held on April 13-14, 1992, reviewed the substantial impact of genomic imprinting in mouse development and discussed in role in human pregnancy, childhood cancers, chromosomal translocations, X-inactivation, and several disorders associated with mental retardation. Topics for future research include the mechanism, timing, reversibility, and homology of the imprinting process.