Large next-generation sequencing gene panels in genetic heart disease: challenges in clinical practice.

BACKGROUND: Next-generation sequencing gene panels are increasingly used for genetic diagnosis in inherited cardiac diseases. Besides pathogenic variants, multiple variants, variants of uncertain significance (VUS) and incidental findings can be detected. Such test results can be challenging for counselling and clinical decision making. METHODS: We present patient cases to illustrate the challenges that can arise when unclear genetic test results are detected in cardiogenetic gene panels. RESULTS: We identified three types of challenging gene panel results: 1) one or more VUS in combination with a pathogenic variant, 2) variants associated with another genetic heart disease, and 3) variants associated with a syndrome involving cardiac features. CONCLUSION: Large gene panels not only increase the detection rates of pathogenic variants but also of variants with uncertain pathogenicity, multiple variants and incidental findings. Gene panel results can be challenging for genetic counselling and require proper pre-test and post-test counselling. We advise evaluation of challenging cases by a multidisciplinary team.

Large next-generation sequencing gene panels in genetic heart disease: yield of pathogenic variants and variants of unknown significance.

BACKGROUND: Genetic heterogeneity is common in inherited cardiac diseases. Next-generation sequencing gene panels are therefore suitable for genetic diagnosis. We describe the results of implementation of cardiomyopathy and arrhythmia gene panels in clinical care. METHODS: We present detection rates for variants with unknown (class 3), likely (class 4), and certain (class 5) pathogenicity in cardiogenetic gene panels since their introduction into diagnostics. RESULTS: In 936 patients tested on the arrhythmia panel, likely pathogenic and pathogenic variants were detected in 8.8% (4.6% class 5; 4.2% class 4), and one or multiple class 3 variants in 34.8%. In 1970 patients tested on the cardiomyopathy panel, likely pathogenic and pathogenic variants were detected in 19.8% (12.0% class 5; 7.9% class 4), and one or multiple class 3 variants in 40.8%. Detection rates of all different classes of variants increased with the increasing number of genes on the cardiomyopathy gene panel. Multiple variants were detected in 11.7% and 28.5% of patients on the arrhythmia and cardiomyopathy panels respectively. In more recent larger versions of the cardiomyopathy gene panel the detection rate of likely pathogenic and pathogenic variants only slightly increased, but was associated with a large increase of class 3 variants. CONCLUSION: Overall detection rates (class 3, 4, and 5 variants) in a diagnostic setting are 44% and 61% for the arrhythmia and cardiomyopathy gene panel respectively, with only a small minority of likely pathogenic and pathogenic variants (8.8% and 19.8% respectively). Larger gene panels can increase the detection rate of likely pathogenic and pathogenic variants, but mainly increase the frequency of variants of unknown pathogenicity.

Haplotype sharing test maps genes for familial cardiomyopathies.

Identifying a mutation in a heterogeneous disease such as inherited cardiomyopathy is a challenge because classical methods, like linkage analysis, can often not be applied as there are too few meioses between affected individuals. However, if affected individuals share the same causal mutation, they will also share a genomic region surrounding it. High-density genotyping arrays are able to identify such regions shared among affected individuals. We hypothesize that the longest shared haplotype is most likely to contain the disease-causing mutation. We applied this method to two pedigrees: one with arrhythmogenic right ventricular cardiomyopathy (ARVC) and one with dilated cardiomyopathy (DCM), using high-density genome-wide SNP arrays. In the ARVC pedigree, the largest haplotype was on chromosome 12 and contained a causative PKP2 mutation. In the DCM pedigree, a causative MYH7 mutation was present on a large shared haplotype on chromosome 14. We calculated that a pedigree containing at least seven meioses has a high chance of correctly detecting the mutation-containing haplotype as the largest. Our data show that haplotype sharing analysis can assist in identifying causative genes in families with low penetrance Mendelian diseases, in which standard tools cannot be used due to lack of sufficient pedigree information.

[44-year-old woman with elevated liver enzymes and a family history for cholelithiasis]. / 44-jährige Patientin mit unklarer Leberwerterhöhung und familiär gehäuftem Gallensteinleiden.

"Low phospholipid associated cholelithiasis" (LPAC) syndrome is an important differential diagnosis in younger patients with biliary symptoms after cholecystectomy and concomitant elevated serum liver tests. Typical symptoms include recurrence of biliary colics after cholecystectomy, echogenic material in the intrahepatic bile ducts, intrahepatic cholestasis of pregnancy or cholestasis under hormonal contraception and a family history of gallstone disease. Patients with LPAC syndrome can be successfully treated with ursodeoxycholic acid.

Congenital methaemoglobinaemia in a 61-year-old patient with normal haemoglobin levels.

A 61-year-old Ghanaian woman presented with dizziness and low oxygen saturations whereupon a methaemoglobin level of 24.9% was obtained. Initially it was thought to be caused by an unknown toxin. However, failure to normalise spontaneously and a short recurrence following administration of methylene blue suggested a congenital cause. Subsequently a novel variant in the CYB5R3 gene, coding for Cytochrome b5 reductase, was demonstrated. Absence of polycythaemia prompted additional analysis for a concomitant haemoglobinopathy.

A homozygous nonsense mutation in the methylmalonyl-CoA epimerase gene (MCEE) results in mild methylmalonic aciduria.

Methylmalonic aciduria (MMA-uria) is an autosomal recessive inborn error of amino acid metabolism, involving valine, threonine, isoleucine, and methionine. This organic aciduria may present in the neonatal period with life-threatening metabolic acidosis, hyperammonemia, feeding difficulties, pancytopenia, and coma. Most affected patients have mutations in the methylmalonyl-coenzyme A (methylmalonyl-CoA) mutase gene. Mildly affected patients may present in childhood with failure to thrive and recurrent attacks of metabolic acidosis. Both a higher residual activity of methylmalonyl-CoA mutase as well as the vitamin B12-responsive defects (cblA and cblB) may form the basis of the mild disorder. A few patients with moderate MMA-uria are known in whom no defect could be identified. Here we present a 16-year-old female patient with persisting moderate MMA-uria (approximately 50 mmol/mol creatinine). She was born to consanguineous Caucasian parents. Her fibroblast mutase activity was normal and no effect of vitamin B12 supplementation could be established. Reduced incorporation of 14C-propionate into macromolecules suggested a defect in the propionate-to-succinate pathway. We found a homozygous nonsense mutation (c.139C>T) in the methylmalonyl-CoA epimerase gene (MCEE), resulting in an early terminating signal (p.R47X). Both parents were heterozygous for this mutation; they were found to excrete normal amounts of methylmalonic acid (MMA). This is the first report of methylmalonyl-CoA epimerase deficiency, thereby unequivocally demonstrating the biochemical role of this enzyme in human metabolism.

Molecular basis of an adult form of Sandhoff disease: substitution of glutamine for arginine at position 505 of the beta-chain of beta-hexosaminidase results in a labile enzyme.

Sandhoff disease is a lysosomal storage disorder characterized by accumulation of GM2 ganglioside due to mutations in the beta-chain of beta-hexosaminidase. Hexosaminidase activity is negligible in infantile Sandhoff disease whereas residual activity is present in juvenile and adult forms. Here we report the molecular basis of the first described adult form of Sandhoff disease. Southern analysis of chromosomal DNA indicated the absence of chromosomal deletions in the gene encoding the beta-chain. Northern analysis of RNA from cultured fibroblasts demonstrated that at least one of the beta-chain alleles was transcribed into normal-length mRNA. Sequence analysis of the entire cDNA prepared from poly-adenylated RNA showed that only one point mutation was present, consisting of a G-->A transition at nucleotide position 1514. This mutation changes the electric charge at amino acid position 505 by substitution of glutamine for arginine in a highly conserved part of the beta-chain, present even in the slime mold Dictyostelium discoideum. The nucleotide transition generated a new restriction site for DdeI, which was present in only one of the alleles of the patient. Reverse transcription of mRNA followed by restriction with DdeI resulted in complete digestion at the mutation site, demonstrating that the second allele was of an mRNA-negative type. Transfection of COS cells with a cDNA construct containing the mutation but otherwise the normal sequence resulted in the expression of a labile form of beta-hexosaminidase. These results show that the patient's is a genetic compound, and that the lability of beta-hexosaminidase found in this form of Sandhoff disease is based on a single nucleotide transition.

Molecular analysis of mutated thyroid peroxidase detected in patients with total iodide organification defects.

Wild-type and mutant thyroid peroxidase (TPO) was expressed in a Semliki Forest Virus (SFV)-based transient expression system in Chinese hamster ovary-K1 cells. Twenty four hours after transfection proteins immunoreactive with TPO antibodies could be detected on a Western blot. Peroxidase activity was assayed using both the guaiacol and the I3- assay. Addition of hematin was necessary to obtain enzymatic active TPO. Thyroid peroxidase complementary DNA constructs containing mutations originally found in patients with hereditary congenital hypothyroidism caused by total iodide organification defects were analyzed using these techniques. In all cases TPO was expressed as shown by Western blotting and immunostaining. Enzymatic activity (measured by guaiacol and iodide oxidation assay) was below the detection level in four out of five mutants. The only mutant yielding TPO with enzymatic activity was G 1858 A (Gly 590 Ser). However, the mutation could affect splicing of TPO messenger RNA, leading to inactive TPO, because it is located at the exon 10/intron 10 border.

Congenital hypothyroidism caused by a premature termination signal in exon 10 of the human thyroid peroxidase gene.

The molecular basis of a total iodide organification defect causing severe congenital hypothyroidism has been elucidated. The defect occurred in a family in which two of five siblings were affected. Thyroid tissue from one patient was available for investigation. The total thyroid peroxidase (TPO) messenger ribonucleic acid level was reduced and consisted mainly of the alternatively spliced form of TPO missing exon 10 (TPO-2). No TPO-1 (wild-type) protein was detected by Western blotting. The TPO-2 translation product of a slightly smaller mol wt was present in thyroid tissue of this patient. TPO activity was absent and thyroglobulin was not iodinated, showing that iodination in vivo did not occur. Denaturing gradient gel electrophoresis and subsequent sequencing revealed in both alleles of the patients a C-->T transition of nucleotide 1708 of the TPO gene, involving a CpG dinucleotide. The mutation introduces a premature termination signal in exon 10 of the TPO gene, preventing the synthesis of enzymatic active peroxidase.

Serum thyroglobulin levels: the physiological decrease in infancy and the absence in athyroidism.

The cord serum thyroglobulin levels of 218 neonates are much higher than the levels after the first year of life and show a wide range. A relation exists between a shorter gestational age and increased thyroglobulin levels. The serum thyroglobulin levels decrease within a few months after birth, but throughout the first year of life, these levels are still higher than the normal values at later ages (5-35 ng/ml). IN 3 athyroid children, thyroglobulin is undetectable in serum.

Two decades of screening for congenital hypothyroidism in The Netherlands: TPO gene mutations in total iodide organification defects (an update).

Presented is a cohort study to assess the nature and frequency of thyroid peroxidase (TPO) mutations in 45 patients (35 families) with congenital hypothyroidism due to a total iodide organification defect; incidence is 1:66,000 in The Netherlands. The presentation is consistently similar with a severe form of congenital hypothyroidism and also characterized by a complete and immediate release of accumulated radioiodide from the thyroid after sodium perchlorate administration. Sixteen different mutations were found, including eight novel mutations; the majority occurs in exons 8, 9, or 10. The GGCC insertion in exon 8 at nucleotide 1277, leading to an early termination signal in exon 9, is the most frequently occurring mutation. These mutations were detected in 29 families in both TPO alleles (13 homozygous and 16 compound heterozygous). In one family, partial maternal isodisomy of 2p was detected, in four families only one mutated TPO allele could be detected, and in one family no inactivating TPO mutation could be found. Because all patients clearly had the clinicopathologic features of a total iodide organification defect, we conclude that in these five families the mutations in the (other) alleles could be either located in the intronic sequences or in the promoter region. Mutations in the TPO gene result in total iodide organification defects.

A 20-basepair duplication in the human thyroid peroxidase gene results in a total iodide organification defect and congenital hypothyroidism.

In this study we present the molecular basis of a total iodide organification defect causing severe congenital hypothyroidism. In the thyroid gland of the patient, thyroid peroxidase (TPO) activity and the iodination degree of thyroglobulin were below detection limits, and no TPO messenger ribonucleic acid was detectable by Northern blot analysis. Denaturing gradient gel electrophoretic analysis of the TPO gene of the patient revealed a homozygous mutation in exon 2. Sequence analysis showed the presence of a 20-basepair duplication, 47 basepairs down-stream of the ATG start codon. This duplication generates a frame shift, resulting in a termination signal in exon 3, compatible with the complete absence of TPO. Both parents of the patient are heterozygous for the same duplication, confirming the recessive mode of inheritance of the mutation.

Maternal isodisomy for chromosome 2p causing severe congenital hypothyroidism.

Severe congenital hypothyroidism (CH) due to a total iodide organification defect (TIOD) is usually due to mutations in the thyroid peroxidase (TPO) gene located at chromosome 2p25. A homozygous deletion [DeltaT2512 (codon 808)] in exon 14 was identified in a patient with classical TIOD. The transmission pattern of the TPO gene in this family was anomalous; the mother was heterozygous for the deletion; and the mutation was absent in the father. Polymorphic short tandem repeat (STR) markers confirmed paternity and demonstrated on chromosome 2 that the propositus was homozygous for most markers on chromosome 2p and that these were identical to one of the maternal 2p homologs. A normal karyotype was found in the propositus, his parents and sister. We conclude that the homozygosity in the patient is due to partial maternal isodisomy of the short arm of chromosome 2, carrying a defective TPO gene. The patient, born small for gestational age, develops and grows well and appears healthy (while being treated with thyroxine) and has a normal phenotype except for a unilateral preauricular skin tag. This shows that partial maternal isodisomy for chromosome 2p (2pter - 2p12) is compatible with a minimal influence on normal development.

Structure and characterization of a 50 bp repeat in intron 10 of the human thyroid peroxidase gene.

We have previously identified an EcoRI polymorphism detected by a human thyroid peroxidase (hTPO) cDNA probe, with alleles varying in size from 3.0 to 4.1 kb. For further characterization of this polymorphism, we have cloned the genomic region containing this polymorphism. Sequence analysis of a 3.2 kb EcoRI fragment containing the polymorphism revealed nine copies of a 50 bp direct repeat located 1.9 kb downstream of exon 10. In 50 unrelated Caucasian individuals, the number of repeats was determined and varied from 9 to 31, with an average of 21. Since exon 10 has been shown to be alternatively spliced in hTPO mRNA, we have also tested whether the number of 50 bp repeats affects alternative splicing of exon 10. We find no correlation between the number of repeats in intron 10 and the ratio of alternatively spliced hTPO-1 and hTPO-2 mRNA in six human thyroid glands.

Recurrent and founder mutations in the Netherlands : Plakophilin-2 p.Arg79X mutation causing arrhythmogenic right ventricular cardiomyopathy/dysplasia.

BACKGROUND: Arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) is an inherited cardiac disease with reduced penetrance and a highly variable expression. Mutations in the gene encoding the plakophilin-2 gene (PKP2) are detected in about 50% of ARVC/D patients. The p.Arg79X mutation in PKP2 has been identified in Europe and North America and has been functionally characterised. We evaluated the prevalence of the p.Arg79X mutation in PKP2 in the Dutch population. METHODS: Twelve index patients and 41 family members were evaluated in three university hospitals in the Netherlands. The diagnosis of ARVC/D was established according to the recently revised Task Force Criteria. Segregation of the p.Arg79X mutation was studied and haplotypes were reconstructed to determine whether the p.Arg79X mutation was a recurrent or a founder mutation. RESULTS: The p.Arg79X mutation in PKP2 was identified in 12 index patients. Haplotype analysis revealed a shared haplotype among Dutch p.Arg79X mutation carriers, indicating a common founder. Six index patients (50%) had a first- or second-degree relative who had died of sudden cardiac death below 40 years of age. At age 60, only 60% of the mutation carriers had experienced any symptoms. There was no significant difference in symptom-free survival and event-free survival between men and women. CONCLUSION: We have identified the largest series of patients with the same desmosome gene mutation in ARVC/D reported to date. This p.Arg79X mutation in PKP2 is a founder mutation in the Dutch population. The phenotypes of PKP2 p.Arg79X mutation carriers illustrate the clinical variability and reduced penetrance often seen in ARVC/D. (Neth Heart J 2010;18:583-91.).

Low HDL cholesterol levels in type I Gaucher disease do not lead to an increased risk of cardiovascular disease.

OBJECTIVE: A low plasma high-density lipoprotein cholesterol (HDL-c) concentration is an important risk factor for the development of atherosclerotic cardiovascular disease. HDL-c levels are abnormally low in type I Gaucher disease (GD) patients. The aim of this study was to determine whether GD is associated with premature atherosclerosis. METHODS: Lipid profiles, apolipoproteins, and carotid artery intima-media thickness (cIMT) were analyzed in 40 type I GD patients, 34 carriers and 41 control subjects. cIMT is a non-invasive validated biomarker for the status of atherosclerosis and present and future cardiovascular disease risk. RESULTS: Compared to control subjects, patients showed decreased HDL-c (1.1+/-0.3 mmol/L) as well as mildly decreased low-density lipoprotein cholesterol (LDL-c) levels (2.8+/-0.7 mmol/L), with an increased ApoB/ApoA1 ratio. In carriers, HDL-c levels were normal, but LDL-c levels were decreased (2.7+/-0.8 mmol/L). Mean cIMT measurements were not different in the three study groups (patients: 0.63+/-0.1mm versus carriers: 0.64+/-0.1mm versus control subjects: 0.65+/-0.1 mm). CONCLUSION: In Gaucher disease low HDL-c levels do not lead to premature atherosclerosis as assessed by cIMT measurement. This indicates that the inverse relationship between levels of HDL-c and risk of cardiovascular disease in the general population may not be present in all conditions characterised by low HDL-c levels.

Unusual scarcity of restriction site polymorphism in the human thyroglobulin gene. A linkage study suggesting autosomal dominance of a defective thyroglobulin allele.

Chromosomal DNA prepared from 90 unrelated individuals, mainly of Caucasian origin, was screened for restriction fragment length polymorphisms in the 3' 220 kilobase pairs (kb) of the human thyroglobulin (Tg) gene. The probes used were Tg cDNA fragments and subcloned single-copy genomic segments, isolated from a human cosmid library. All in all, 1164 nucleotides were screened using 15 different restriction enzymes. The average number of nucleotides screened was 354 per individual. Only one polymorphism was found in these 1164 nucleotides, with a minor allele frequency of 2.2%. This polymorphism, which is located in an intervening sequence, was found in healthy individuals and in a family with hereditary congenital hypothyroidism due to a defect in the synthesis and structure of thyroglobulin. The Mendelian segregation of polymorphism and goiter in ten family members suggests that the rare variant is linked to a normal Tg allele and provides strong evidence for autosomal dominant inheritance of this Tg synthesis defect.

Identification of five novel inactivating mutations in the human thyroid peroxidase gene by denaturing gradient gel electrophoresis.

Thyroid peroxidase (TPO) is the key enzyme in the synthesis of thyroid hormones. Defects in the TPO gene are reported to be the cause of congenital hypothyroidism due to a Total Iodide Organification Defect (TIOD). This type of defect, where iodide taken up by the thyroid gland cannot be oxidized and bound to protein, is the most common hereditary inborn error causing congenital hypothyroidism in the Netherlands. Denaturing Gradient Gel Electrophoresis (DGGE) of PCR amplified genomic DNA was used to screen for mutations in the TPO gene of TIOD patients from nine apparently unrelated families, and seven different mutations were detected. Three frameshift mutations were found: a 20 bp duplication in exon 2, a 4 bp duplication in exon 8, and an insertion of a single nucleotide (C) at pos. 2505 in exon 14. In addition, four single nucleotide substitutions were identified: one single-base mutation resulted in a premature termination codon (C-->T at pos. 1708 in exon 10), two single-base substitutions changed an amino acid in highly conserved regions of the gene (Tyr-->Asp in exon 9 and Glu-->Lys in exon 14). The fourth single-base mutation located at the exon 10/intron 10 border altered a conserved Gly into Ser and could also affect splicing. Nine TIOD patients from five families were compound heterozygotes and six patients from four families were homozygous for one of the mentioned mutations in the TPO gene.