Patient-Specific TBX5-G125R Variant Induces Profound Transcriptional Deregulation and Atrial Dysfunction.

BACKGROUND: The pathogenic missense variant p.G125R in TBX5 (T-box transcription factor 5) causes Holt-Oram syndrome (also known as hand-heart syndrome) and early onset of atrial fibrillation. Revealing how an altered key developmental transcription factor modulates cardiac physiology in vivo will provide unique insights into the mechanisms underlying atrial fibrillation in these patients. METHODS: We analyzed ECGs of an extended family pedigree of Holt-Oram syndrome patients. Next, we introduced the TBX5-p.G125R variant in the mouse genome (Tbx5G125R) and performed electrophysiologic analyses (ECG, optical mapping, patch clamp, intracellular calcium measurements), transcriptomics (single-nuclei and tissue RNA sequencing), and epigenetic profiling (assay for transposase-accessible chromatin using sequencing, H3K27ac [histone H3 lysine 27 acetylation] CUT&RUN [cleavage under targets and release under nuclease sequencing]). RESULTS: We discovered high incidence of atrial extra systoles and atrioventricular conduction disturbances in Holt-Oram syndrome patients. Tbx5G125R/+ mice were morphologically unaffected and displayed variable RR intervals, atrial extra systoles, and susceptibility to atrial fibrillation, reminiscent of TBX5-p.G125R patients. Atrial conduction velocity was not affected but systolic and diastolic intracellular calcium concentrations were decreased and action potentials were prolonged in isolated cardiomyocytes of Tbx5G125R/+ mice compared with controls. Transcriptional profiling of atria revealed the most profound transcriptional changes in cardiomyocytes versus other cell types, and identified over a thousand coding and noncoding transcripts that were differentially expressed. Epigenetic profiling uncovered thousands of TBX5-p.G125R-sensitive, putative regulatory elements (including enhancers) that gained accessibility in atrial cardiomyocytes. The majority of sites with increased accessibility were occupied by Tbx5. The small group of sites with reduced accessibility was enriched for DNA-binding motifs of members of the SP (specificity protein) and KLF (Krüppel-like factor) families of transcription factors. These data show that Tbx5-p.G125R induces changes in regulatory element activity, alters transcriptional regulation, and changes cardiomyocyte behavior, possibly caused by altered DNA binding and cooperativity properties. CONCLUSIONS: Our data reveal that a disease-causing missense variant in TBX5 induces profound changes in the atrial transcriptional regulatory network and epigenetic state in vivo, leading to arrhythmia reminiscent of those seen in human TBX5-p.G125R variant carriers.

De novo variants in the PABP domain of PABPC1 lead to developmental delay.

PURPOSE: The study aimed to investigate the role of PABPC1 in developmental delay (DD). METHODS: Children were examined by geneticists and pediatricians. Variants were identified using exome sequencing and standard downstream bioinformatics pipelines. We performed in silico molecular modeling and coimmunoprecipitation to test if the variants affect the interaction between PABPC1 and PAIP2. We performed in utero electroporation of mouse embryo brains to enlighten the function of PABPC1. RESULTS: We describe 4 probands with an overlapping phenotype of DD, expressive speech delay, and autistic features and heterozygous de novo variants that cluster in the PABP domain of PABPC1. Further symptoms were seizures and behavioral disorders. Molecular modeling predicted that the variants are pathogenic and would lead to decreased binding affinity to messenger RNA metabolism-related proteins, such as PAIP2. Coimmunoprecipitation confirmed this because it showed a significant weakening of the interaction between mutant PABPC1 and PAIP2. Electroporation of mouse embryo brains showed that Pabpc1 knockdown decreases the proliferation of neural progenitor cells. Wild-type Pabpc1 could rescue this disturbance, whereas 3 of the 4 variants did not. CONCLUSION: Pathogenic variants in the PABP domain lead to DD, possibly because of interference with the translation initiation and subsequently an impaired neurogenesis in cortical development.

Germline variants in HEY2 functional domains lead to congenital heart defects and thoracic aortic aneurysms.

PURPOSE: In this study we aimed to establish the genetic cause of a myriad of cardiovascular defects prevalent in individuals from a genetically isolated population, who were found to share a common ancestor in 1728. METHODS: Trio genome sequencing was carried out in an index patient with critical congenital heart disease (CHD); family members had either exome or Sanger sequencing. To confirm enrichment, we performed a gene-based association test and meta-analysis in two independent validation cohorts: one with 2685 CHD cases versus 4370 . These controls were also ancestry-matched (same as FTAA controls), and the other with 326 cases with familial thoracic aortic aneurysms (FTAA) and dissections versus 570 ancestry-matched controls. Functional consequences of identified variants were evaluated using expression studies. RESULTS: We identified a loss-of-function variant in the Notch target transcription factor-encoding gene HEY2. The homozygous state (n = 3) causes life-threatening congenital heart defects, while 80% of heterozygous carriers (n = 20) had cardiovascular defects, mainly CHD and FTAA of the ascending aorta. We confirm enrichment of rare risk variants in HEY2 functional domains after meta-analysis (MetaSKAT p = 0.018). Furthermore, we show that several identified variants lead to dysregulation of repression by HEY2. CONCLUSION: A homozygous germline loss-of-function variant in HEY2 leads to critical CHD. The majority of heterozygotes show a myriad of cardiovascular defects.

Further delineation of Malan syndrome.

Malan syndrome is an overgrowth disorder described in a limited number of individuals. We aim to delineate the entity by studying a large group of affected individuals. We gathered data on 45 affected individuals with a molecularly confirmed diagnosis through an international collaboration and compared data to the 35 previously reported individuals. Results indicate that height is > 2 SDS in infancy and childhood but in only half of affected adults. Cardinal facial characteristics include long, triangular face, macrocephaly, prominent forehead, everted lower lip, and prominent chin. Intellectual disability is universally present, behaviorally anxiety is characteristic. Malan syndrome is caused by deletions or point mutations of NFIX clustered mostly in exon 2. There is no genotype-phenotype correlation except for an increased risk for epilepsy with 19p13.2 microdeletions. Variants arose de novo, except in one family in which mother was mosaic. Variants causing Malan and Marshall-Smith syndrome can be discerned by differences in the site of stop codon formation. We conclude that Malan syndrome has a well recognizable phenotype that usually can be discerned easily from Marshall-Smith syndrome but rarely there is some overlap. Differentiation from Sotos and Weaver syndrome can be made by clinical evaluation only.

Homozygous DMRT2 variant associates with severe rib malformations in a newborn.

Spondylocostal dysostosis (SCD) is a rare disorder characterized by vertebral segmentation defects and malformations of the ribs. SCD patients have some degree of (kypho)scoliosis, short stature and suffer from respiratory impairment due to the reduced size of their thoracic cage. Mutations in DLL3, MESP2, LFNG, HES7, TBX6, and RIPPLY2 are known to cause different subtypes of SCD. Here, we report on a male neonate with an apparent distinct SCD-like phenotype only partly overlapping the previously described SCD subtypes. The proband presented with severe rib malformations (missing, fused, bifid, and hypoplastic ribs), vertebral malformations (intervertebral fusions of the laminae and irregular ossification of the vertebral bodies), and a mild scoliosis. Clear segmentation defects of the vertebral bodies were lacking. Other dysmorphic features were present as well. Severe respiratory insufficiency was present from birth. Whole exome sequencing identified a homozygous start-loss variant in DMRT2 (NM_006557.6: c.1A > T p.[Met1?]) being a likely cause of the SCD-like phenotype in the proband. Mutations in DMRT2 (OMIM#604935) have not been described in relation to SCD-related phenotypes in humans before. However, Dmrt2 knock-out mice exhibit severe rib and vertebral defects that strikingly overlap with the radiological phenotype of the proband reported here. Therefore, it seems plausible that mutations in DMRT2 are associated with a different (novel) subtype of SCD mainly characterized by severe rib anomalies but lacking clear segmentation defects of the vertebral bodies.

The phenotypic spectrum of Schaaf-Yang syndrome: 18 new affected individuals from 14 families.

PURPOSE: Truncating mutations in the maternally imprinted, paternally expressed gene MAGEL2, which is located in the Prader-Willi critical region 15q11-13, have recently been reported to cause Schaaf-Yang syndrome, a Prader-Willi-like disease that manifests as developmental delay/intellectual disability, hypotonia, feeding difficulties, and autism spectrum disorder. The causality of the reported variants in the context of the patients' phenotypes was questioned, as MAGEL2 whole-gene deletions seem to cause little or no clinical phenotype. METHODS: Here we report a total of 18 newly identified individuals with Schaaf-Yang syndrome from 14 families, including 1 family with 3 individuals found to be affected with a truncating variant of MAGEL2, 11 individuals who are clinically affected but were not tested molecularly, and a presymptomatic fetal sibling carrying the pathogenic MAGEL2 variant. RESULTS: All cases harbor truncating mutations of MAGEL2, and nucleotides c.1990-1996 arise as a mutational hotspot, with 10 individuals and 1 fetus harboring a c.1996dupC (p.Q666fs) mutation and 2 fetuses harboring a c.1996delC (p.Q666fs) mutation. The phenotypic spectrum of Schaaf-Yang syndrome ranges from fetal akinesia to neurobehavioral disease and contractures of the small finger joints. CONCLUSION: This study provides strong evidence for the pathogenicity of truncating mutations of the paternal allele of MAGEL2, refines the associated clinical phenotypes, and highlights implications for genetic counseling for affected families.Genet Med 19 1, 45-52.

LONG-TERM FOLLOW-UP OF PATIENTS WITH RETINITIS PIGMENTOSA TYPE 12 CAUSED BY CRB1 MUTATIONS: A Severe Phenotype With Considerable Interindividual Variability.

PURPOSE: To examine the long-term clinical course and variability in a large pedigree segregating CRB1 type autosomal recessive retinitis pigmentosa. METHODS: An observational case study of 30 patients with CRB1 type autosomal recessive retinitis pigmentosa, homozygous for the CRB1 c.3122T > C; p.(Met1041Thr) mutation from a Dutch genetically isolated population in which the CRB1 gene was originally identified. The authors evaluated medical records, analyzed a questionnaire, and performed a comprehensive ophthalmic examination, including optical coherence tomography. RESULTS: Mean follow-up was 19 years (range 0-45 years, SD 15 years). With aging, patients showed progressive visual decline, deterioration of visual fields, increasing narrowing of the anterior chamber, increased prevalence of cataract, and an increase in the amount of intraretinal pigmentations. Fifty percent of patients had a visual acuity of ≤0.3 at Age 18 and of ≤0.1 at Age 35. Electroretinogram responses were severely reduced or absent already at a young age and optical coherence tomography showed increased retinal thickness with often cystoid maculopathy at young age, and thinning of the retina and disorientation of the photoreceptor layer in the late stages. The clinical course showed considerable interindividual variability, but intraindividual similarity between both eyes was the rule. CONCLUSION: The wide and variable clinical spectrum in patients with the same CRB1 mutation supports the hypothesis that the CRB1 type autosomal recessive retinitis pigmentosa-phenotype is modulated by other factors. The clinical variability will make it harder to evaluate the effect of (upcoming) therapies for retinitis pigmentosa, although because of the intraindividual similarity between both eyes, the contralateral eye can be used as an excellent internal control.

Factors for successful implementation of population-based expanded carrier screening: learning from existing initiatives.

Background: Carrier screening for autosomal recessive disorders aims to facilitate reproductive decision-making by identifying couples with a 1-in-4 risk in every pregnancy of having an affected child. Except for a few countries or regions, carrier screening is not widely offered and is mostly ancestry-based. Technological advances enable carrier screening for multiple diseases simultaneously allowing universal screening regardless of ancestry (population-based expanded carrier screening). It is important to study how this can be successfully implemented. This study therefore aims to identify critical factors involved in successful implementation, from a user perspective, by learning from already implemented initiatives. Methods: Factors associated with successful implementation were identified by: (i) a literature review and (ii) two case studies; studying experiences with carrier screening in two high-risk communities (a Dutch founder population and the Ashkenazi Jewish population), including a survey among community members. Results: Factors identified were familiarity with (specific) genetic diseases and its availability, high perceived benefits of screening (e.g. screening avoids much suffering), acceptance of reproductive options, perceived risk of being a carrier and low perceived social barriers (e.g. stigmatization). In contrast to the Jewish community, the initial demand for screening in the Dutch founder population did not entirely come from the community itself. However, the large social cohesion of the community facilitated the implementation process. Conclusion: To ensure successful implementation of population-based expanded carrier screening, efforts should be made to increase knowledge about genetic diseases, create awareness and address personal benefits of screening in a non-directive way.

Variants in CUL4B are associated with cerebral malformations.

Variants in cullin 4B (CUL4B) are a known cause of syndromic X-linked intellectual disability. Here, we describe an additional 25 patients from 11 families with variants in CUL4B. We identified nine different novel variants in these families and confirmed the pathogenicity of all nontruncating variants. Neuroimaging data, available for 15 patients, showed the presence of cerebral malformations in ten patients. The cerebral anomalies comprised malformations of cortical development (MCD), ventriculomegaly, and diminished white matter volume. The phenotypic heterogeneity of the cerebral malformations might result from the involvement of CUL-4B in various cellular pathways essential for normal brain development. Accordingly, we show that CUL-4B interacts with WDR62, a protein in which variants were previously identified in patients with microcephaly and a wide range of MCD. This interaction might contribute to the development of cerebral malformations in patients with variants in CUL4B.

Noninvasive prenatal diagnosis of Huntington disease: detection of the paternally inherited expanded CAG repeat in maternal plasma.

OBJECTIVE: With a shift towards noninvasive testing, we have explored and validated the use of noninvasive prenatal diagnosis (NIPD) for Huntington disease (HD). METHODS: Fifteen couples have been included, assessing a total of n = 20 pregnancies. Fetal paternally inherited CAG repeat length was determined in total cell-free DNA from maternal plasma using a direct approach by PCR and subsequent fragment analysis. RESULTS: All fetal HD (n = 7) and intermediate (n = 3) CAG repeats could be detected in maternal plasma. Detection of repeats in the normal range (n = 10) was successful in n = 5 cases where the paternal repeat size could be distinguished from maternal repeat patterns after fragment analysis. In all other cases (n = 5), the paternal peaks coincided with the maternal peak pattern. All NIPD results were concordant with results from routine diagnostics on fetal genomic DNA from chorionic villi. CONCLUSION: In this validation study, we demonstrated that all fetuses at risk for HD could be identified noninvasively in maternal plasma. Additionally, we have confirmed results from previously described case reports that NIPD for HD can be performed using a direct approach by PCR. For future diagnostics, parental CAG profiles can be used to predict the success rate for NIPD prior to testing.

Counseling couples at risk of having a child with homozygous familial hypercholesterolemia - Clinical experience and recommendations.

Homozygous familial hypercholesterolemia (HoFH) is a rare, potentially life-limiting, inherited disorder of lipoprotein metabolism characterized by extremely high low-density lipoprotein cholesterol levels. When both parents have heterozygous FH, there is a 25% chance they will conceive a child with HoFH. Here we describe our clinical experience with two such prospective parent couples who were counseled regarding reproductive options and prenatal testing for HoFH. These cases showcase how, in consultation with a molecular geneticist and pediatric cardiologist, parents may be informed of the prognosis and treatment outlook of HoFH based on the FH-variants carried, to ultimately make personal decisions on reproductive options. One couple opted for prenatal testing and termination of pregnancy in case HoFH was found, while the other accepted the risk without testing. We review the available literature on preconception counseling for HoFH and provide practical guidance to clinicians counseling at-risk couples. Optimal counseling of prospective parents may help prevent future physical and psychological problems for both parent and child.

[A female with excessive wrinkling of the hands]. / Gerimpelde handpalmen na contact met water.

A 23-year-old female complained of excessive wrinkling of her hands after brief exposure to water. The symptoms were characteristic for the diagnosis aquagenic wrinkling of the palms (AWP). This disorder is frequently associated with cystic fibrosis (CF) or CF carrier state. DNA analysis of the cystic fibrosis gene is indicated.

How will new genetic technologies, such as gene editing, change reproductive decision-making? Views of high-risk couples.

Couples at increased risk of having offspring with a specific genetic disorder who want to avoid having an affected child have several reproductive options including prenatal diagnosis (PND) and preimplantation genetic testing (PGT). In the future, non-invasive prenatal diagnosis (NIPD), germline gene editing (GGE) and somatic gene editing (SGE) might become available. This study explores if, and how, availability of new genetic technologies, including NIPD, GGE, SGE, would change reproductive decision-making of high-risk couples. In 2018, semi-structured interviews were conducted with 25 genetically at-risk couples. Couples previously had received genetic counselling for PND and PGT, and in most cases opted for (one of) these techniques, at one Dutch Clinical Genetics Center between 2013 and 2017. Considerations participants mentioned regarding the hypothetical use of NIPD, GGE and SGE, seem similar to considerations regarding PND and PGT and are reflected in underlying concepts. These include safety and burden for mother and child, and moral considerations. Couples generally favoured NIPD over PND as this would be safe and enables earlier diagnosis. Increased opportunities of having a 'healthy' embryo and less embryo disposal were considerations in favour of GGE. Some regarded GGE as unsafe and feared slippery slope scenarios. Couples were least favourable towards SGE compared to choosing for a genetic reproductive technology, because of the perceived burden for the affected offspring. With the possibly growing number of technological options, understanding high risk couples' perspectives can assist in navigating the reproductive decision-making process. Counsellors should be prepared to counsel on more and complex reproductive options.

Couples' experiences with expanded carrier screening: evaluation of a university hospital screening offer.

Preconception carrier screening offers couples the possibility to receive information about the risk of having a child with a recessive disorder. Since 2016, an expanded carrier screening (ECS) test for 50 severe autosomal recessive disorders has been available at Amsterdam Medical Center, a Dutch university hospital. This mixed-methods study evaluated the experiences of couples that participated in the carrier screening offer, including high-risk participants, as well as participants with a general population risk. All participants received genetic counselling, and pre- (n = 132) and post-test (n = 86) questionnaires and semi-structured interviews (n = 16) were administered. The most important reason to have ECS was to spare a future child a life with a severe disorder (47%). The majority of survey respondents made an informed decision (86%), as assessed by the Multidimensional Measure of Informed Choice. Among the 86 respondents, 27 individual carriers and no new carrier couples were identified. Turn-around time of the test results was considered too long and costs were perceived as too high. Overall, mean levels of anxiety were not clinically elevated. High-risk respondents (n = 89) and pregnant respondents (n = 13) experienced higher levels of anxiety before testing, which decreased after receiving the test result. Although not clinically significant, distress was on average higher for carriers compared to non-carriers (p < 0.0001). All respondents would opt for the test again, and 80.2% would recommend it to others. The results suggest that ECS should ideally be offered before pregnancy, to minimise anxiety. This study could inform current and future implementation initiatives of preconception ECS.

TMEM218 dysfunction causes ciliopathies, including Joubert and Meckel syndromes.

The Joubert-Meckel syndrome spectrum is a continuum of recessive ciliopathy conditions caused by primary cilium dysfunction. The primary cilium is a microtubule-based, antenna-like organelle that projects from the surface of most human cell types, allowing them to respond to extracellular signals. The cilium is partitioned from the cell body by the transition zone, a known hotspot for ciliopathy-related proteins. Despite years of Joubert syndrome (JBTS) gene discovery, the genetic cause cannot be identified in up to 30% of individuals with JBTS, depending on the cohort, sequencing method, and criteria for pathogenic variants. Using exome and targeted sequencing of 655 families with JBTS, we identified three individuals from two families harboring biallelic, rare, predicted-deleterious missense TMEM218 variants. Via MatchMaker Exchange, we identified biallelic TMEM218 variants in four additional families with ciliopathy phenotypes. Of note, four of the six families carry missense variants affecting the same highly conserved amino acid position 115. Clinical features included the molar tooth sign (N = 2), occipital encephalocele (N = 5, all fetuses), retinal dystrophy (N = 4, all living individuals), polycystic kidneys (N = 2), and polydactyly (N = 2), without liver involvement. Combined with existing functional data linking TMEM218 to ciliary transition zone function, our human genetic data make a strong case for TMEM218 dysfunction as a cause of ciliopathy phenotypes including JBTS with retinal dystrophy and Meckel syndrome. Identifying all genetic causes of the Joubert-Meckel spectrum enables diagnostic testing, prognostic and recurrence risk counseling, and medical monitoring, as well as work to delineate the underlying biological mechanisms and identify targets for future therapies.

A gain-of-function TBX5 mutation is associated with atypical Holt-Oram syndrome and paroxysmal atrial fibrillation.

Holt-Oram syndrome (HOS) is a heart/hand syndrome clinically characterized by upper limb and cardiac malformations. Mutations in T-box transcription factor 5 (TBX5) underlie this syndrome. Here, we describe a large atypical HOS family in which affected patients have mild skeletal deformations and paroxysmal atrial fibrillation, but few have congenital heart disease. Sequencing of TBX5 revealed a novel mutation, c.373G>A, resulting in the missense mutation p.Gly125Arg, in all investigated affected family members, cosegregating with the disease. We demonstrate that the mutation results in normal Nkx2-5 interaction, is correctly targeted to the nucleus, has significantly enhanced DNA binding and activation of both the Nppa(Anf) and Cx40 promoter, and significantly augments expression of Nppa, Cx40, Kcnj2, and Tbx3 in comparison with wild-type TBX5. Thus, contrary to previously published HOS mutations, the p.G125R TBX5 mutation results in a gain-of-function. We speculate that the gain-of-function mechanism underlies the mild skeletal phenotype and paroxysmal atrial fibrillation and suggest a possible role of TBX5 in the development of (paroxysmal) atrial fibrillation based on a gain-of-function either through a direct stimulation of target genes via TBX5 or indirectly via TBX5 stimulated TBX3. These findings may warrant a renewed look at the phenotypes of families and individuals hitherto not classified as HOS or as atypical but presenting with paroxysmal atrial fibrillation, because these may possibly be the result of additional TBX5 gain-of-function mutations.

Preconception carrier screening for multiple disorders: evaluation of a screening offer in a Dutch founder population.

Technological developments have enabled carrier screening for multiple disorders. This study evaluated experiences with a preconception carrier screening offer for four recessive disorders in a Dutch founder population. Questionnaires were completed by 182 attendees pretesting and posttesting and by 137 non-attendees. Semistructured interviews were conducted with seven of the eight carrier couples. Attendees were mainly informed about the existence of screening by friends/colleagues (49%) and family members (44%). Familiarity with the genetic disorders was high. Knowledge after counseling increased (p < 0.001); however, still 9%, compared to 29% before counseling, wrongly mentioned an increased risk of having an affected child if both parents are carriers of different disorders. Most attendees (97%) recalled their test results correctly, but two couples reported being carrier of another disorder than reported. Overall, 63% felt worried while waiting for results but anxiety levels returned to normal afterwards. In all, 2/39 (5%) carriers felt less healthy. Screened individuals were very satisfied; they did not regret testing (97%) and would recommend testing to others (97%). The majority (94%) stated that couples should always have a pretest consultation, preferably by a genetic counselor rather than their general practitioner (83%). All carrier couples made reproductive decisions based on their results. Main reason for non-attendance was unawareness of the screening offer. With expanded carrier screening, adequately informing couples pretest and posttesting is of foremost importance. Close influencers (family/friends) can be used to raise awareness of a screening offer. Our findings provide lessons for the implementation of expanded carrier screening panels in other communities and other settings.

Biallelic loss of function variants in COASY cause prenatal onset pontocerebellar hypoplasia, microcephaly, and arthrogryposis.

Pontocerebellar hypoplasia (PCH) is a heterogeneous neurodegenerative disorder with a prenatal onset. Using whole-exome sequencing, we identified variants in the gene Coenzyme A (CoA) synthase (COASY) gene, an enzyme essential in CoA synthesis, in four individuals from two families with PCH, prenatal onset microcephaly, and arthrogryposis. In family 1, compound heterozygous variants were identified in COASY: c.[1549_1550delAG]; [1486-3 C>G]. In family 2, all three affected siblings were homozygous for the c.1486-3 C>G variant. In both families, the variants segregated with the phenotype. RNA analysis showed that the c.1486-3 C>G variant leads to skipping of exon 7 with partial retention of intron 7, disturbing the reading frame and resulting in a premature stop codon (p.(Ala496Ilefs*20)). No CoA synthase protein was detected in patient cells by immunoblot analysis and CoA synthase activity was virtually absent. Partial CoA synthase defects were previously described as a cause of COASY Protein-Associated Neurodegeneration (CoPAN), a type of Neurodegeneration and Brain Iron Accumulation (NBIA). Here we demonstrate that near complete loss of function variants in COASY are associated with lethal PCH and arthrogryposis.

With expanded carrier screening, founder populations run the risk of being overlooked.

Genetically isolated populations exist worldwide. Specific genetic disorders, including rare autosomal recessive disorders may have high prevalences in these populations. We searched for Dutch genetically isolated populations and their autosomal recessive founder mutations. We investigated whether these founder mutations are covered in the (preconception) expanded carrier screening tests of five carrier screening providers. Our results show that the great majority of founder mutations are not covered in these screening panels, and these panels may thus not be appropriate for use in founder populations. It is therefore important to be aware of founder mutations in a population when offering carrier tests.