Marieke Teeuw

Consanguineous marriages, pearls and perils: Geneva International Consanguinity Workshop Report.

Approximately 1.1 billion people currently live in countries where consanguineous marriages are customary, and among them one in every three marriages is between cousins. Opinions diverge between those warning of the possible health risks to offspring and others who highlight the social benefits of consanguineous marriages. A consanguinity study group of international experts and counselors met at the Geneva International Consanguinity Workshop from May 3 2010, to May 7, 2010, to discuss the known and presumptive risks and benefits of close kin marriages and to identify important future areas for research on consanguinity. The group highlighted the importance of evidence-based counseling recommendations for consanguineous marriages and of undertaking both genomic and social research in defining the various influences and outcomes of consanguinity. Technological advances in rapid high-throughput genome sequencing and for the identification of copy number variants by comparative genomic hybridization offer a significant opportunity to identify genotype-phenotype correlations focusing on autozygosity, the hallmark of consanguinity. The ongoing strong preferential culture of close kin marriages in many societies, and among migrant communities in Western countries, merits an equivalently detailed assessment of the social and genetic benefits of consanguinity in future studies.

Autosomal recessive disease in children of consanguineous parents: inferences from the proportion of compound heterozygotes.

This short communication deals with the questions of how to calculate the expected proportion of compound heterozygous patients among affected offspring of consanguineous parents, and how, from an observed proportion of compound heterozygotes, to calculate both the proportion of homozygotes not identical by descent and the frequency of pathogenic alleles in the population. This estimate of allele frequency may be useful when dealing with populations with a considerable number of consanguineous matings.

Consanguineous marriage and reproductive risk: attitudes and understanding of ethnic groups practising consanguinity in Western society

Consanguineous couples should be adequately informed about their increased reproductive risk and possibilities for genetic counselling. Information may only be effective if it meets the needs of the target group. This study aimed to gain more insight into: (1) attitudes of people belonging to ethnic groups in Western society towards consanguinity and their understanding of risk for offspring; and (2) their attitudes regarding reproductive information targeted at consanguineous couples. Dutch Moroccans and Turks were invited to complete an online questionnaire by snowball sampling and by placing a link on two popular Dutch Moroccan/Turkish forum websites between September and October 2011. The questionnaire was completed by 201 individuals who were, on average, neither positive nor negative towards consanguinity. Respondents with a consanguineous partner were more positive, estimated the risk for the offspring lower and were less positive about the provision of risk information to consanguineous couples when compared with respondents without a consanguineous partner. Participants of Turkish origin had a more negative attitude towards consanguinity and estimated the reproductive risk higher than Moroccan participants. More than half of the respondents thought that information should be given before marriage, whereas only 10% thought it should never be provided. The general practitioner was most often mentioned (54%) as the designated professional to inform people. Information about genetic risks related to consanguinity should be offered early, preferably before marriage. The diversity of the target population requires various strategies to disseminate information and reach consanguineous couples with the offer of genetic counselling.

Do consanguineous parents of a child affected by an autosomal recessive disease have more DNA identical-by-descent than similarly-related parents with healthy offspring? Design of a case-control study

BackgroundThe offspring of consanguineous relations have an increased risk of congenital/genetic disorders and early mortality. Consanguineous couples and their offspring account for approximately 10% of the global population. The increased risk for congenital/genetic disorders is most marked for autosomal recessive disorders and depends on the degree of relatedness of the parents. For children of first cousins the increased risk is 2-4%. For individual couples, however, the extra risk can vary from zero to 25% or higher, with only a minority of these couples having an increased risk of at least 25%. It is currently not possible to differentiate between high-and low-risk couples. The quantity of DNA identical-by-descent between couples with the same degree of relatedness shows a remarkable variation. Here we hypothesize that consanguineous partners with children affected by an autosomal recessive disease have more DNA identical-by-descent than similarly-related partners who have only healthy children. The aim of the study is thus to establish whether the amount of DNA identical-by-descent in consanguineous parents of children with an autosomal recessive disease is indeed different from its proportion in consanguineous parents who have healthy children only.Methods/DesignThis project is designed as a case-control study. Cases are defined as consanguineous couples with one or more children with an autosomal recessive disorder and controls as consanguineous couples with at least three healthy children and no affected child. We aim to include 100 case couples and 100 control couples. Control couples are matched by restricting the search to the same family, clan or ethnic origin as the case couple. Genome-wide SNP arrays will be used to test our hypothesis.DiscussionThis study contains a new approach to risk assessment in consanguineous couples. There is no previous study on the amount of DNA identical-by-descent in consanguineous parents of affected children compared to the consanguineous parents of healthy children. If our hypothesis proves to be correct, further studies are needed to obtain different risk figure estimates for the different proportions of DNA identical-by-descent. With more precise information about their risk status, empowerment of couples can be improved when making reproductive decisions.

First steps in exploring prospective exome sequencing of consanguineous couples.

Consanguinity is one of the most frequent risk factors for congenital disorders. In theory, prospective exome sequencing of consanguineous couples could identify couples who both are carriers of autosomal recessive diseases, and empower such couples to make informed reproductive decisions. To investigate this, we sent blood samples to our laboratory of four pairs of consanguineous parents having one or more children affected by an autosomal recessive disorder, without revealing any diagnostic information. The study was restricted to find identical, previously described, or evidently pathogenic mutations in both parents of each couple, in over 400 genes known to result in severe autosomal recessive disorders. Out of the six autosomal recessive disorders known to the four couples studied, two were correctly identified. Carrier status of one not previously known autosomal recessive disorder was discovered. As expected, given the pipeline used, large deletions, mutations in genes not present in the gene list, mutations outside the exons and consensus splice sites, and mutations that were not evidently pathogenic and previously not reported, were not identified. The restriction to detecting only couples with identical mutations diminishes the risk of revealing unsolicited findings and shortens the time needed for analysis, but also results in missing couples with different mutations in the same gene. In addition to the proposed pipeline, couples should be offered testing for carrier status of frequent disorders that can present themselves by large deletions, non-exonic mutations or compound heterozygous mutations (e.g. thalassemia, spinal muscular atrophy, cystic fibrosis). Even though sensitivity is reduced, offering exome sequencing prospectively will increase reproductive options for consanguineous couples.

Challenges in the care for consanguineous couples: an exploratory interview study among general practitioners and midwives

BackgroundIt is often suggested that an effort must be made to increase awareness among consanguineous couples of their reproductive risk, and to refer them for genetic counseling if needed. Primary care professionals are considered most appropriate for addressing the subject and identifying couples at risk during consultations in their practice. This Dutch study aims to explore the experiences, attitudes and beliefs of such professionals regarding their care for consanguineous couples.MethodsSixteen semi-structured interviews were conducted with midwives and general practitioners.ResultsAlthough most primary care professionals considered it their task to inform couples about the risks of consanguinity, during consultations the topic was generally only briefly touched upon and quickly abandoned. Important reasons for this were professionals’ beliefs about religious and social values of couples, their low perception of the couples’ reproductive risk and expected limited feasibility of referral. Feelings of embarrassment regarding addressing consanguinity did not seem to play a significant role.ConclusionsPrimary care professional beliefs about their clients’ religious and social values, their attitudes toward the risk, and perceived limited options for referral seem to conflict with the professional norm to address the topic of consanguinity.

Consanguinity and endogamy in the Netherlands: demographic and medical genetic aspects.

This paper reviews what is currently known about the presence of consanguinity and endogamy in the Netherlands, in the past and today, and concludes with a discussion of medical genetic aspects. First geographic characteristics, the demographic history, the genetic make-up of the native population, legal aspects and the public opinion are reviewed. Then data on the prevalence of consanguinity in the native population are presented for marriages since 1840, followed by data on consanguineous marriages among immigrants from countries with a tradition of close-kin marriages. It is estimated that approximately 1% of at-risk consanguineous couples are referred to clinical genetic centres for prospective genetic counselling in the Netherlands. This picture will change dramatically if and when next-generation sequencing is introduced to identify couples at ≥25% risk prospectively.

Estimating the Total Pathogenic Allele Frequency of Autosomal Recessive Disorders in Case of Consanguinity

OBJECTIVE Estimating the total allele frequency of all pathogenic alleles of an autosomal recessive disease is not possible if only mutational data of a sample of affected individuals are available. However, if the affected individuals come from a population where consanguinity is not uncommon, this total allele frequency can be estimated by additionally using the positive individual inbreeding coefficients or an estimate of the population inbreeding coefficient. In this paper, we propose two estimators. METHODS/RESULTS We propose to estimate the total allele frequency by a conditional maximum likelihood estimator if a part of the affected individuals in the sample comes from consanguineous marriages with known inbreeding coefficients. A simulation study shows that this estimator is unbiased and robust. We propose a second estimator which is based on an estimate of the population inbreeding coefficient. The method is applied to mutational data and individual inbreeding coefficients of Tunisian patients with congenital adrenal hyperplasia. CONCLUSION Additionally using individual inbreeding coefficients or an estimate of the population inbreeding coefficient makes it possible to estimate the total allele frequency. Since consanguinity is commonly practiced in many parts of the world, the estimators proposed in the paper are of practical importance.

Determining the genome-wide kinship coefficient seems unhelpful in distinguishing consanguineous couples with a high versus low risk for adverse reproductive outcome.

BackgroundOffspring of consanguineous couples are at increased risk of congenital disorders. The risk increases as parents are more closely related. Individuals that have the same degree of relatedness according to their pedigree, show variable genomic kinship coefficients. To investigate whether we can differentiate between couples with high- and low risk for offspring with congenital disorders, we have compared the genomic kinship coefficient of consanguineous parents with a child affected with an autosomal recessive disorder with that of consanguineous parents with only healthy children, corrected for the degree of pedigree relatedness.Methods151 consanguineous couples (73 cases and 78 controls) from 10 different ethnic backgrounds were genotyped on the Affymetrix platform and passed quality control checks. After pruning SNPs in linkage disequilibrium, 57,358 SNPs remained. Kinship coefficients were calculated using three different toolsets: PLINK, King and IBDelphi, yielding five different estimates (IBDelphi, PLINK (all), PLINK (by population), King robust (all) and King homo (by population)). We performed a one-sided Mann Whitney test to investigate whether the median relative difference regarding observed and expected kinship coefficients is bigger for cases than for controls. Furthermore, we fitted a mixed effects linear model to correct for a possible population effect.ResultsAlthough the estimated degrees of genomic relatedness with the different toolsets show substantial variability, correlation measures between the different estimators demonstrated moderate to strong correlations. Controls have higher point estimates for genomic kinship coefficients. The one-sided Mann Whitney test did not show any evidence for a higher median relative difference for cases compared to controls. Neither did the regression analysis exhibit a positive association between case–control status and genomic kinship coefficient.ConclusionsIn this case–control setting, in which we compared consanguineous couples corrected for degree of pedigree relatedness, a higher degree of genomic relatedness was not significantly associated with a higher likelihood of having an affected child. Further translational research should focus on which parts of the genome and which pathogenic mutations couples are sharing. Looking at relatedness coefficients by determining genome-wide SNPs does not seem to be an effective measure for prospective risk assessment in consanguineous parents.

Comment on Gialluisi et al.

As authors of the first paper describing the methodology used by Gialluisi et al in their paper on the high allele frequency for Wilson disease in the Sardinian population, we want to congratulate them with their result.1, 2 The paper clearly shows the strength of this methodology. We were also particularly impressed by their conscientious approach to determine the inbreeding coefficient in this special population. Still we want to draw attention to some inaccuracies in their paper, which–in our opinion–should be avoided by future users of this method, as they may result in underestimation of the gene frequency – ie, the total pathogenic allele frequency–and birth prevalence of the disorder. First of all, the authors disregard 14 mutations with relative frequencies below 1%. By including these, the gene frequency becomes 0.0195, instead of 0.0191. Second, the authors included only patients with unambiguous genotype and detailed geographical provenance of parents. However, leaving out seemingly heterozygous patients with a second, unidentified mutation will have the same effect on the estimation of the gene frequency as disregarding known mutations. Finally, the authors seem to have calculated the birth frequency of the disorder in Sardinia by simply squaring the gene frequency. This can be justified in random mating populations but not in the Sardinian population where the inbreeding coefficient is higher than zero. For instance in the mountains, where the inbreeding coefficient is 0.00112344, and using the gene frequency estimate of Gialluisi et al2 the birth prevalence of Wilson disease will be 1:2585 instead of the 1:2732 calculated by these authors. Using the gene frequency estimate that includes the 14 rare mutations, the prevalence estimate in the mountains even becomes 1:2499, – 11% higher than the original estimate. We admit that each of our proposed corrections separately only has a small effect for the final estimate, but these effects are additive, and thus together not always insignificant. Therefore, we hope that by pointing to these inaccuracies future users of the methodology will be forewarned.