Evaluation of genetic tests for susceptibility to common complex diseases: why, when and how?

Recent research into the human genome has generated a wealth of scientific knowledge and increased both public and professional interest in the concept of personalised medicine. Somewhat unexpectedly, in addition to increasing our understanding about the genetic basis for numerous diseases, these new discoveries have also spawned a burgeoning new industry of ‘consumer genetic testing’. In this paper, we present the principles learnt though the evaluation of tests for single gene disorders and suggest a comparable framework for the evaluation of genetic tests for susceptibility to common complex diseases. Both physicians and the general public will need to be able to assess the claims made by providers of genetic testing services, and ultimately policy-makers will need to decide if and when such tests should be offered through state funded healthcare systems.     

Genetic therapy, identity and the person-regarding reasons

It has been argued that there can be no person-regarding reasons for practising genetic therapy, since it affects identity and causes to exist an individual who would not otherwise have existed. And there can be no such reasons for causing somebody to exist because existing cannot be better for an individual than never existing. In the present paper, both of these claims are denied. It is contended, first, that in practically all significant cases genetic therapy will not affect the identity of beings of our kind. This is so irrespective of whether, essentially, we are beings with minds or beings of a certain biological species, the human one. Second, it is contended that, even if genetic therapy were to affect our identity, there could be person-regarding reasons for conducting it, for existence can be better than non-existence for the individual.     

Ensuring the appropriate use of genetic tests

Ensuring the correct use of genetic tests is an important challenge for health-policy makers. Many new genetic tests will identify susceptibility to common diseases or adverse drug responses. Some will lead to new prevention opportunities, but others will have minimal clinical value. Statutory regulation alone cannot guarantee appropriate use. Other strategies, including resource allocation and matters related to clinical governance - such as practice-guideline development and health-provider education - are also important.     

Multi-dimensional regulation of metabolic networks shaping plant development and performance

The metabolome is an integral part of a plant's life cycle and determines for a large part its external phenotype. It is the final, internal product of chemical interactions, obtained through developmental, genetic, and environmental inputs, and as such, it defines the state of a plant in terms of development and performance. Understanding its regulation will provide knowledge and new insights into the biochemical pathways and genetic interactions that shape the plant and its surroundings. In this review, we will focus on four dimensions that contribute to the huge diversity of metabolomes and we will illustrate how this diversity shapes the plant in terms of development and performance: (i) temporal regulation: the metabolome is extremely dynamic and temporal changes in the environment can have an immense impact on its composition; (ii) spatial regulation: metabolites can be very specific, in both quantitative and qualitative terms, to specialized organs, tissues, and cell types; (iii) environmental regulation: the metabolic profile of plants is highly dependent on environmental signals, such as light, temperature, and nutrients, and very susceptible to biotic and abiotic stresses; and (iv) genetic regulation: the biosynthesis, structure, and accumulation of metabolites have a genetic origin, and there is quantitative and qualitative variation for metabolomes within a species. We will address the contribution of these dimensions to the wide diversity of metabolomes and highlight how the multi-dimensional regulation of metabolism defines the plant's phenotype.     

Ethical considerations in the presymptomatic diagnosis of Alzheimer's disease

Research into the human genome has undoubtedly opened up a new perspective in medicine. The ability to identify the cause of specific diseases, especially neurodegenerative diseases, will definitively change the concepts of disease and treatment, while advances such as antibiotic therapy and anesthesia will be relegated to history. However, the arrival of genome medicine poses major bioethical challenges, many of which remain to be resolved. We review the applicability, results and consequences of predictions based on genetic tests for presymptomatic Alzheimer's disease, as well as the dilemmas and contradictions that are already arising as a result of the commercialization of predictive tests for public use with little or no medical supervision. Given that there is currently no effective treatment of Alzheimer?s disease, the greatest challenge and contradiction lies in managing the results of predictive tests. There are no indications for the performance of predictive genetic tests in late or sporadic Alzheimer's disease or for counselling of persons requesting these tests. The PICOGEN program provides a safe, effective, reliable and satisfactory option for persons requesting these tests who meet the inclusion criteria. Currently, caution should be the norm when considering the performance of predictive tests in presymptomatic dementia.     

Empowering international canine inherited disorder management

The mapping of the canine genome and the study of canine breed genomic architecture has revolutionized the discovery of genetic tests for inherited disorders in dogs. As the genetics underlying complex disorders are revealed, canine breeders and their registering organisations will be required to understand genetics in a much more sophisticated way. To facilitate the management of genetic disorders in the era of new complex information, we consider how best to apply the results of new research and analytical techniques to benefit the wider canine breeding community with the aims of improving canine health and maintaining benevolent genetic diversity. If this is not done, there is a serious risk that expensive and valuable genetic research will remain unused or be misused to the detriment of breeds. In this review, we make a case for the formation of an international organisation that will exist as a central repository for breed-based genetic analysis and information sharing. This organisation ("Inter-Dog") could be modelled on a similar organisation that is monitoring genetic improvement of dairy cattle. The formation of such an organisation will require the collaboration of international kennel management organisations, researchers, and agencies offering genetic testing services.     

Aspectos éticos del diagnóstico presintomático de la enfermedad de Alzheimer

Research into the human genome has undoubtedly opened up a new perspective in medicine. The ability to identify the cause of specific diseases, especially neurodegenerative diseases, will definitively change the concepts of disease and treatment, while advances such as antibiotic therapy and anesthesia will be relegated to history. However, the arrival of genome medicine poses major bioethical challenges, many of which remain to be resolved. We review the applicability, results and consequences of predictions based on genetic tests for presymptomatic Alzheimer's disease, as well as the dilemmas and contradictions that are already arising as a result of the commercialization of predictive tests for public use with little or no medical supervision. Given that there is currently no effective treatment of Alzheime?s disease, the greatest challenge and contradiction lies in managing the results of predictive tests. There are no indications for the performance of predictive genetic tests in late or sporadic Alzheimer's disease or for counselling of persons requesting these tests. The PICOGEN program provides a safe, effective, reliable and satisfactory option for persons requesting these tests who meet the inclusion criteria. Currently, caution should be the norm when considering the performance of predictive tests in presymptomatic dementia.     

Teleogy and genes

My aim in this paper is to quickly sketch a teleological approach to the problem of isolating the impact of genes on phenotypic characters. I begin by arguing that it is a mistake to think that there will be only one analysis of genetic input suitable for all theoretical interests. My principle focus is Richard Dawkins' argument for genic selectionism. I argue that a teleological analysis of genetic input is what Dawkins requires to establish the right kind of mapping of gene onto phenotype. This comes at a certain cost, however. Accepting the analysis will threaten Dawkins' claims about the teleogogical priority of gene over phenotype.     

The diffusion of new genetic tests for predicting disease.

This paper examines the pathways by which new genetic tests will become available to the public. In view of the scarcity of genetic specialists, the pathway is likely to involve primary care physicians. Other pathways entail state-mandated testing, community-based programs, or testing by laboratories without much involvement of primary care physicians. When testing does become available the "destination" will be either family-centered testing or population-oriented screening. The deterrent to screening will not be the inability to detect disease-causing mutations but the costs and attitudes of providers and the public. When tests are provided primarily to provide information about risks to future children, some people will oppose screening on religious or moral grounds. When there are no inexpensive treatments, some will fear that insurance companies and employers will use tests to deny them health care coverage. Some may not want to know their risks for disorders about which little can be done. For common, multifactorial disorders, genetic tests will have low predictive value. Because of these problems, the decision to be tested, regardless of the destination, requires that "testees" be fully informed and consent to testing. When acceptance rates are low, screening is less likely to be cost-effective; family-centered testing becomes the default destination.     

Molecular diagnostics: hurdles for clinical implementation

In the coming years, molecular diagnostics will continue to be of critical importance to public health worldwide. It will facilitate the detection and characterization of disease, as well as monitoring of the drug response, and will assist in the identification of genetic modifiers and disease susceptibility. A wide range of molecular-based tests is available to assess DNA variation and changes in gene expression. However, there are major hurdles to overcome before the implementation of these tests in clinical laboratories, such as which test to employ, the choice of technology and equipment, and issues such as cost-effectiveness, accuracy, reproducibility, personnel training, reimbursement by third-party payers and intellectual property. At present, PCR-based testing predominates; however, alternative technologies aimed at reducing genome complexity without PCR are anticipated to gain momentum in the coming years. Furthermore, development of integrated chip devices ("lab-on-a-chip") should allow point-of-care testing and facilitate genetic readouts from single cells and molecules. Together with proteomic-based testing, these advances will improve molecular diagnostic testing and will present additional challenges for implementing such testing in health care settings.     

Political legitimacy and research ethics

In democratic theory, “legitimacy” refers to the set of conditions that must be in place in order for the claims to authority of somebody to be deemed appropriate, and for their claims to compliance to be warranted. Though criteria of legitimacy have been elaborated in the context of democratic states, there is no reason for them not to be drawn up, with appropriate amendments, for other kinds of authority structures. This paper examines the claims to authority made over researchers by international bodies governing research ethics, who exercise their authority by the research ethics guidelines they produce (including recent revisions to the Declaration of Helsinki and CIOMS Guidelines). We argue that discussions of such bodies and sets of guidelines often elide questions of justification and questions of legitimacy, and that the grounds that might allow us to mount a strong case for the latter are at present sorely underdeveloped.     

Past, present and future strategies to study the genetics of body weight regulation.

Genetic advances have made remarkable progress towards our understanding of body weight regulation. Much of our current knowledge has come from the cloning and characterisation of the genes responsible for obesity syndromes in the mouse, and the identification of homologous mutations causing rare forms of obesity in humans. Gene targeting experiments in mice have been instrumental in confirming the importance of many genes in the aetiology of obesity, and the existence of a fundamental physiological pathway that controls energy balance is becoming clear. The genetic determinants that underlie common forms of human obesity are largely polygenic, with most genes producing small effects. Thus, elucidating the many genetic determinants of obesity is a current challenge for modern geneticists. Despite the inherent difficulties, progress has been made through linkage/association studies and a genetic map of quantitative trait loci for human obesity is beginning to emerge. Obesity research is now very much in a transition period. Not so long ago, access to high throughput screening, as well as microarray and proteomic techniques, was prohibitively expensive and available only to the few. In recent years, these technologies have become more accessible to the larger scientific community and, in this paper, we will discuss how such technological advances are likely to drive the next wave of progress in obesity research. For example, large-scale mutagenesis screens in rodents coupled with high throughput screening are likely to emerge as important technologies for identifying genes previously unexpected to be involved in body weight regulation. Furthermore, applications of microarray and proteomic techniques will further refine our understanding of currently known peptides as well as identify novel pathways and molecules which are involved in energy homeostasis.     

Adjusting multiple testing in multilocus analyses using the eigenvalues of a correlation matrix

Correlated multiple testing is widely performed in genetic research, particularly in multilocus analyses of complex diseases. Failure to control appropriately for the effect of multiple testing will either result in a flood of false-positive claims or in true hits being overlooked. Cheverud proposed the idea of adjusting correlated tests as if they were independent, according to an 'effective number' (M(eff)) of independent tests. However, our experience has indicated that Cheverud's estimate of the Meff is overly large and will lead to excessively conservative results. We propose a more accurate estimate of the M(eff), and design M(eff)-based procedures to control the experiment-wise significant level and the false discovery rate. In an evaluation, based on both real and simulated data, the M(eff)-based procedures were able to control the error rate accurately and consequently resulted in a power increase, especially in multilocus analyses. The results confirm that the M(eff) is a useful concept in the error-rate control of correlated tests. With its efficiency and accuracy, the M(eff) method provides an alternative to computationally intensive methods such as the permutation test.     

Physiology and genetics of methylotrophic bacteria

Methylotrophic bacteria comprise a broad range of obligate aerobic microorganisms, which are able to proliferate on (a number of) compounds lacking carbon-carbon bonds. This contribution will essentially be limited to those organisms that are able to utilize methanol and will cover the physiological, biochemical and genetic aspects of this still diverse group of organisms. In recent years much progress has been made in the biochemical and genetic characterization of pathways and the knowledge of specific reactions involved in methanol catabolism. Only a few of the genetic loci hitherto found have been matched by biochemical experiments through the isolation or demonstration of specific gene products. Conversely, several factors have been identified by biochemical means and were shown to be involved in the methanol dehydrogenase reaction or subsequent electron transfer. For the majority of these components, their genetic loci are unknown. A comprehensive treatise on the regulation and molecular mechanism of methanol oxidation is therefore presented, followed by the data that have become available through the use of genetic analysis. The assemblage of methanol dehydrogenase enzyme, the role of pyrrolo-quinoline quinone, the involvement of accessory factors, the evident translocation of all these components to the periplasm and the dedicated link to the electron transport chain are now accepted and well studied phenomena in a few selected facultative methylotrophs. Metabolic regulation of gene expression, efficiency of energy conservation and the question whether universal rules apply to methylotrophs in general, have so far been given less attention. In order to expand these studies to less well known methylotrophic species initial results concerning such area as genetic mapping, the molecular characterization of specific genes and extrachromosomal genetics will also pass in review.     

Estimating Genetic Effects and Quantifying Missing Heritability Explained by Identified Rare-Variant Associations

Next-generation sequencing has led to many complex-trait rare-variant (RV) association studies. Although single-variant association analysis can be performed, it is grossly underpowered. Therefore, researchers have developed many RV association tests that aggregate multiple variant sites across a genetic region (e.g., gene), and test for the association between the trait and the aggregated genotype. After these aggregate tests detect an association, it is only possible to estimate the average genetic effect for a group of RVs. As a result of the "winner’s curse," such an estimate can be biased. Although for common variants one can obtain unbiased estimates of genetic parameters by analyzing a replication sample, for RVs it is desirable to obtain unbiased genetic estimates for the study where the association is identified. This is because there can be substantial heterogeneity of RV sites and frequencies even among closely related populations. In order to obtain an unbiased estimate for aggregated RV analysis, we developed bootstrap-sample-split algorithms to reduce the bias of the winner’s curse. The unbiased estimates are greatly important for understanding the population-specific contribution of RVs to the heritability of complex traits. We also demonstrate both theoretically and via simulations that for aggregate RV analysis the genetic variance for a gene or region will always be underestimated, sometimes substantially, because of the presence of noncausal variants or because of the presence of causal variants with effects of different magnitudes or directions. Therefore, even if RVs play a major role in the complex-trait etiologies, a portion of the heritability will remain missing, and the contribution of RVs to the complex-trait etiologies will be underestimated.     

Testing the generalized neutrality hypothesis

Various tests of the hypothesis of selective neutrality based on gene frequency are now available. These tests take as null hypothesis the concept of “strict neutrality”: all new mutants are required to be selectively identical to each other. For evolutionary questions, however, (as opposed to those of genetic polymorphism), a wider null hypothesis might be of interest. Since deleterious alleles have essentially no evolutionary importance, one might wish to test the null hypothesis that only neutral or deleterious mutations occur. The principal alternative to this hypothesis is that there exists heterotic selection of some form for some alleles tending to maintain a level of genetic polymorphism higher than that under neutrality. In this paper an assessment is made of the usefulness of a test of strict neutrality first proposed by this author (Ewens, 1972) as a test of null hypothesis of “generalized neutrality,” i.e. that only neutral or deleterious alleles occur. At the same time some remarks will be made about estimation of the fundamental parameter θ defining these processes.     

A Note on Two‐Sample Tests for Comparing Intra‐Individual Genetic Sequence Diversity between Populations

Summary Gilbert, Rossini, and Shankarappa (2005 , Biometrics 61 , 106‐117) present four U‐statistic based tests to compare genetic diversity between different samples. The proposed tests improved upon previously used methods by accounting for the correlations in the data. We find, however, that the same correlations introduce an unacceptable bias in the sample estimators used for the variance and covariance of the inter‐sequence genetic distances for modest sample sizes. Here, we compute unbiased estimators for these and test the resulting improvement using simulated data. We also show that, contrary to the claims in Gilbert et al., it is not always possible to apply the Welch–Satterthwaite approximate t‐test, and we provide explicit formulas for the degrees of freedom to be used when, on the other hand, such approximation is indeed possible.     

Developmental apoptosis in C. elegans: a complex CEDnario

Apoptosis, an evolutionarily conserved programme of cellular self-destruction, is essential for the development and survival of most multicellular animals. It is required to ensure functional organ architecture and to maintain tissue homeostasis. During development of the simple nematode Caenorhabditis elegans, apoptosis claims over 10% of the somatic cells that are generated - these cells were healthy but unnecessary. Exciting insights into the regulation and execution of apoptosis in C. elegans have recently been made. These new findings will undoubtedly influence our perception of developmental apoptosis in more complex species, including humans.     

Natural genetic variation in plant photosynthesis

Natural genetic variation in plant photosynthesis is a largely unexplored and as a result an underused genetic resource for crop improvement. Numerous studies show genetic variation in photosynthetic traits in both crop and wild species, and there is an increasingly detailed knowledge base concerning the interaction of photosynthetic phenotypes with their environment. The genetic factors that cause this variation remain largely unknown. Investigations into natural genetic variation in photosynthesis will provide insights into the genetic regulation of this complex trait. Such insights can be used to understand evolutionary processes that affect primary production, allow greater understanding of the genetic regulation of photosynthesis and ultimately increase the productivity of our crops.