Pregnancies and children affected by birth defects and genomic abnormalities stand to suffer from lifelong mild to severe health consequences associated with high familial and societal costs. This workshop addressed the challenges to understanding the causes of these seemingly “random” genomic defects and to identifying the relevant risk factors so that they might be minimized. Affected children often carry de novo mutations (i.e., those not present in somatic cells of either parent), and thus these genomic changes are likely to have arisen in germ cells of one parent or during early development. In stark contrast to cancer, where associations with environmental exposures have been identified and where differences in genetic susceptibility can dramatically alter an individual’s risks, the causes or individual susceptibilities for human birth defects and heritable diseases are largely unknown. Indeed, the status of the research is such that there is still no direct scientific evidence for the existence of transmissible, environmentally induced, human germ-cell mutations, although the indirect evidence from human and animal studies indicates that they should exist. Recent major advances in genome analysis technologies may provide the tools to obtain such evidence, and these rapidly developing technologies were a major motivation for holding this workshop.
Research into the detection of human germ-cell mutagens and the prevention of associated developmental defects and heritable genetic diseases faces three major challenges:
- Understanding the special biology of male and female gametogenesis as it pertains to mutation susceptibility and the risk of damage transmission;
- Developing effective technologies for detecting the broad spectrum of mutations known or predicted to be associated with germ-cell mutagens, developmental defects, and heritable diseases; and
- Initiating research strategies to investigate the induction of germ-cell mutations in exposed human populations.
Male and female germ cells each have a unique biology that influences their susceptibilities to germ-cell mutagens, and these susceptibilities change dramatically throughout the course of germ-cell development, maturation, and fertilization (). In animal studies, the types of mutations seen in offspring depend on the agent and the exact timing of exposure during germ-cell development. Similar specificities have been noted in sperm studies with patients receiving mutagenic chemotherapy. This critical relationship between agent, dose, timing, and outcome was considered by the workshop participants to be of paramount importance for identifying appropriate exposed human populations for germ-cell mutagenesis studies.
The detection of germ-cell mutagens is complicated by the broad spectrum of chromosomal defects and gene mutations known to be associated with birth defects and heritable diseases. Rodent studies have shown that even exposures limited to one germ-cell mutagen often induce a spectrum of transmissible damage and that mutagens can differ dramatically in the types of transmissible damage they induce. The types of transmissible damage include base-pair alterations, repeat-sequence changes, and a variety of chromosomal abnormalities, e.g., duplications, deletions, rearrangements, and aneuploidies. In addition, recent animal studies have shown that altered imprinting patterns, not associated with mutations, can lead to heritable multigenerational defects. Therefore, Workshop attendees emphasized that approaches to studying germ-cell mutagenesis in humans must remain broad-based, and investigations should include the full spectrum of detectable genetic and chromosomal endpoints.
This workshop highlighted the impressive technological advances for investigating DNA sequence and chromosomal alterations that were developed in conjunction with the HGP (). These technological innovations include major advances in high-throughput DNA sequencing; detection of chromosomal duplications, amplifications, and deletions; gene-transcript profiling; and proteomics. The advantages and limitations of these technologies were discussed in detail.
One of the major issues facing the field of germ-cell mutagenesis is to identify candidate germ-cell mutagens for intensive human study, and several strategies for identifying these were discussed during the workshop. These included animal breeding screens, animal and human gamete analyses (especially defects in sperm genomes, ), and epidemiological pilot data. Consistent with the special biology of germ cells, it was emphasized that data from somatic-cell mutagenicity studies cannot be extrapolated directly to germ-cell risk and that the determination of human germ-cell risk requires direct studies of exposed germ cells.
There was consensus on the importance of mounting coordinated animal and human germ-cell mutagenesis studies to explore the impact of important societal concerns, such as exposure to cancer therapy in childhood cancer survivors. It was recommended that such studies be initiated as soon as possible, both in humans and in parallel animal models, using some of the genomic tools currently available. Cancer survivors represent a unique cohort with well-defined exposures and genetic alterations, including base-pair changes, chromosomal alterations, repeat-sequence and minisatellite mutations, and gene-expression profiles. Other types of genomic alterations can be measured in their offspring, using as references both the parent without cancer as well as the parental cancer survivor.
The need to create a bio-bank of human tissue samples, e.g., from cancer patients and their children, also was advocated by attendees. Such bio-banks will be critical in conducting multi-endpoint, comprehensive, collaborative international efforts aimed at detecting exposure-induced heritable alterations in the human genome.
There was also strong support for using animal models of human germ-cell mutagenesis in parallel with studies in humans to provide insights into the biology and biochemical mechanisms of germ-cell mutagenesis.
In contrast to the germ-cell mutagenicity data from animal studies, the following questions regarding human germ-cell mutagens stand unanswered and remain a challenge for the research community:
- What environmental, occupational, or medical agents increase the risk for human germ-cell mutations, and do they act via direct mechanisms (point mutations, chromosomal alterations, etc.) or indirect, epigenetic mechanisms (methylation, etc.)?
- For both direct and indirect mechanisms, what is the potential for transmissibility of genomic damage from the affected gamete to a zygote, and what is the effect on the live-born child?
- Does individual susceptibility affect parental risk to germ-cell mutations, and can we identify individuals with increased susceptibility?
- What is the differential risk to male vs. female germ cells for the same exposure, and do these risks change with age, diet, and other physiological factors?
The workshop attendees were in strong agreement regarding the need to initiate large-scale collaborative human and animal exposure studies to identify and define the environmental factors that contribute to adverse human pregnancy outcomes and genetic diseases among children. This need is compelling and timely because there is strong animal evidence that exposure to environmental agents can induce germ-cell mutations and heritable genetic disease. There are growing human health concerns from exposures to an increasing complexity of environmental chemicals, and we have, for the first time, new and sensitive tools for genome analyses. We must initiate these studies now rather than wait another 80 years to answer the questions: Are there any human germ-cell mutagens, what risks do they pose to future generations, and are some parents at higher risk for germ-cell mutations than others?