Bi-allelic loss-of-function variants in KIF21A cause severe fetal akinesia with arthrogryposis multiplex.

BACKGROUND: Fetal akinesia (FA) results in variable clinical presentations and has been associated with more than 166 different disease loci. However, the underlying molecular cause remains unclear in many individuals. We aimed to further define the set of genes involved. METHODS: We performed in-depth clinical characterisation and exome sequencing on a cohort of 23 FA index cases sharing arthrogryposis as a common feature. RESULTS: We identified likely pathogenic or pathogenic variants in 12 different established disease genes explaining the disease phenotype in 13 index cases and report 12 novel variants. In the unsolved families, a search for recessive-type variants affecting the same gene was performed; and in five affected fetuses of two unrelated families, a homozygous loss-of-function variant in the kinesin family member 21A gene (KIF21A) was found. CONCLUSION: Our study underlines the broad locus heterogeneity of FA with well-established and atypical genotype-phenotype associations. We describe KIF21A as a new factor implicated in the pathogenesis of severe neurogenic FA sequence with arthrogryposis of multiple joints, pulmonary hypoplasia and facial dysmorphisms. This hypothesis is further corroborated by a recent report on overlapping phenotypes observed in Kif21a null piglets.

Biallelic ADAM22 pathogenic variants cause progressive encephalopathy and infantile-onset refractory epilepsy.

Pathogenic variants in A Disintegrin And Metalloproteinase (ADAM) 22, the postsynaptic cell membrane receptor for the glycoprotein leucine-rich repeat glioma-inactivated protein 1 (LGI1), have been recently associated with recessive developmental and epileptic encephalopathy. However, so far, only two affected individuals have been described and many features of this disorder are unknown. We refine the phenotype and report 19 additional individuals harbouring compound heterozygous or homozygous inactivating ADAM22 variants, of whom 18 had clinical data available. Additionally, we provide follow-up data from two previously reported cases. All affected individuals exhibited infantile-onset, treatment-resistant epilepsy. Additional clinical features included moderate to profound global developmental delay/intellectual disability (20/20), hypotonia (12/20) and delayed motor development (19/20). Brain MRI findings included cerebral atrophy (13/20), supported by post-mortem histological examination in patient-derived brain tissue, cerebellar vermis atrophy (5/20), and callosal hypoplasia (4/20). Functional studies in transfected cell lines confirmed the deleteriousness of all identified variants and indicated at least three distinct pathological mechanisms: (i) defective cell membrane expression; (ii) impaired LGI1-binding; and/or (iii) impaired interaction with the postsynaptic density protein PSD-95. We reveal novel clinical and molecular hallmarks of ADAM22 deficiency and provide knowledge that might inform clinical management and early diagnostics.

Recognizable Pattern of Arthrogryposis and Congenital Myopathy Caused by the Recurrent TTN Metatranscript-only c.39974-11T > G Splice Variant.

INTRODUCTION: Arthrogryposis is characterized by the presence of multiple contractures at birth and can be caused by pathogenic variants in TTN (Titin). Exons and variants that are not expressed in one of the three major isoforms of titin are referred to as "metatranscript-only" and have been considered to be only expressed during fetal development. Recently, the metatranscript-only variant (c.39974-11T > G) in TTN with a second truncating TTN variant has been linked to arthrogryposis multiplex congenita and myopathy. METHODS: Via exome sequencing we identified the TTN c.39974-11T > G splice variant in trans with one of three truncating variants (p.Arg8922*, p.Lys32998Asnfs*63, p.Tyr10345*) in five individuals from three families. Clinical presentation and muscle ultrasound as well as MRI images were analyzed. RESULTS: All five patients presented with generalized muscular hypotonia, reduced muscle bulk, and congenital contractures most prominently affecting the upper limbs and distal joints. Muscular hypotonia persisted and contractures improved over time. One individual, the recipient twin in the setting of twin-to-twin transfusion syndrome, died from severe cardiac hypertrophy 1 day after birth. Ultrasound and MRI imaging studies revealed a recognizable pattern of muscle involvement with striking fibrofatty involvement of the hamstrings and calves, and relative sparing of the femoral adductors and anterior segment of the thighs. CONCLUSION: The recurrent TTN c.39974-11T > G variant consistently causes congenital arthrogryposis and persisting myopathy providing evidence that the metatranscript-only 213 to 217 exons impact muscle elasticity during early development and beyond. There is a recognizable pattern of muscle involvement, which is distinct from other myopathies and provides valuable clues for diagnostic work-up.

A single center experience of prenatal parent-fetus trio exome sequencing for pregnancies with congenital anomalies.

OBJECTIVES: To examine the diagnostic yield of trio exome sequencing in fetuses with multiple structural defects with no pathogenic findings in cytogenetic and microarray analyses. METHODS: We recruited 51 fetuses with two or more defects, non-immune fetal hydrops or fetal akinesia deformation syndrome|or fetal akinesia deformation sequence (FADS). Trio exome sequencing was performed on DNA from chorionic villi samples and parental blood. Detection of genomic variation and prioritization of clinically relevant variants was performed according to in-house standard operating procedures. RESULTS: Median maternal and gestational age was 32.0 years and 21.0 weeks, respectively. Forty-three (84.3%) fetuses had two or more affected organ systems. The remaining fetuses had isolated fetal hydrops or FADS. In total, the exome analysis established the genetic cause for the clinical abnormalities in 22 (43.1%, 95% CI 29.4%-57.8%) pregnancies. CONCLUSIONS: In fetuses with multiple defects, hydrops or FADS and normal standard genetic results, trio exome sequencing has the potential to identify genetic anomalies in more than 40% of cases.

Targeted 46-gene and clinical exome sequencing for mutations causing cardiomyopathies.

With the implementation of high-throughput sequencing protocols, the exhaustive scanning of known and candidate disease genes has become a feasible approach to genetic testing of patients with cardiomyopathy. A primary objective of the present study was to assess the performance characteristics of a 46-gene next-generation sequencing (NGS) assay that targets well-established cardiomyopathy genes. A total of 25 samples were analyzed. Twelve of those had previously been sequenced using resequencing arrays and served as reference samples for the assessment of the assay's performance characteristics. The remaining 13 samples were derived from consecutive patients. Both the analytical sensitivity and the specificity of the assay were 100% and the percentage of low-coverage bases was 0.4%, at an average read depth of 210×. In order to assess the diagnostic yield of the test, 13 consecutive samples representing cases of Dilated (n = 7), Hypertrophic (n = 4) and Left Ventricular Non-Compaction Cardiomyopathy (n = 2), were subjected to the 46-gene NGS assay. Including predicted pathogenic variants in the gene TTN, a total of 22 variants (11 novel) were detected in 10 patients, with a clear preponderance of variants of unknown pathogenicity (class 3 variants, 21/22, 95%). Of the seven DCM cases, two were digenic, involving variants in the genes MYH7 and RBM20 in one case and in DSP and TTN in the other case. Three other patients carried single TTN variants predicted to be pathogenic. Of the four HCM patients, one was trigenic (LAMA4, PKP2 and TTN) and three were digenic (DSP and TTN, MYH7 and NEXN, NEXN and TTN, respectively). As to LVNC, one of the two patients had one variant in the gene ABCC9 and two predicted pathogenic variants in the gene TTN. Strikingly, out of the thirteen investigated cases, only a single case exhibited a likely pathogenic or pathogenic variant justifying a positive test report. The percentage of inconclusive cases thus amounted to 69%. Three cases were devoid of any relevant variant. Two of these "negative" cases were subsequently taken to initially evaluate the use of an alternative NGS assay addressing 4813 genes previously implicated in genetic diseases (the so-called clinical exome). Although showing similar sensitivity and specificity values, the coverage of the 46 established cardiomyopathy genes was less efficient (low-coverage bases: 5%). In a case of DCM, the assay revealed a disruptive variant in the gene encoding the adrenoreceptor beta 2 (ADRB2), a protein implicated in signal transduction and energy metabolism in the heart. In conclusion, the 46 gene assay is applicable to routine genetic diagnostics of cardiomyopathy. The test detects many variants of unknown pathogenicity which need to be followed-up in order to gain benefit for the patients and their families. Samples devoid of any relevant variant may be subjected to a clinical exome assay, in order to identify interesting novel candidate genes.

High-density oligonucleotide-based resequencing assay for mutations causing syndromic and non-syndromic forms of thoracic aortic aneurysms and dissections.

Thoracic aortic aneurysm and dissection is associated with increasing mortality rate that may occur as part of a syndrome or as an isolated familial condition. Several genes have been implicated in causing TAAD, though an appropriate genetic test for their parallel testing is not yet available. Herein, we describe the novel 117-kb "MFSTAAD chip" that may help to understand the genetic basis of TAAD. A custom duplicate resequencing assay was developed to cover eight genes previously described in TAAD; FBN1, TGFBR1&2, COL3A1, MYH11, ACTA2, SLC2A10 and NOTCH1. GSEQ and SeqC software were used for data analysis. The analytical sensitivity of the assay was validated by the recognition of 182 known mutations (153 point mutations, 21 deletions, 7 insertions and 1 duplication) and a cohort of 28 patients were selected to determine the mutation yield, whereby 18 of them were previously negative for mutations in the genes FBN1 and TGFBR2. The assay had significantly higher sensitivity for point mutations (100%) and the largest deletion of 16 bp was detectable through a decline in the hybridization strength. The overall analytical sensitivity was 85%. Mutation testing of 28 unrelated TAAD patients revealed 4 known and 6 possibly pathogenic mutations with a mutation yield of 32%. The MFSTAAD chip is an alternative tool to next-generation sequencing that allows parallel analysis of several genes on a single platform. Refinements in the probe design and data analysis software will increase the analytical sensitivity of insertions and deletions making this assay even more applicable for clinical testing.

Identification of novel genes including NAV2 associated with isolated tall stature.

Very tall people attract much attention and represent a clinically and genetically heterogenous group of individuals. Identifying the genetic etiology can provide important insights into the molecular mechanisms regulating linear growth. We studied a three-generation pedigree with five isolated (non-syndromic) tall members and one individual with normal stature by whole exome sequencing; the tallest man had a height of 211 cm. Six heterozygous gene variants predicted as damaging were shared among the four genetically related tall individuals and not present in a family member with normal height. To gain insight into the putative role of these candidate genes in bone growth, we assessed the transcriptome of murine growth plate by microarray and RNA Seq. Two (Ift140, Nav2) of the six genes were well-expressed in the growth plate. Nav2 (p-value 1.91E-62) as well as Ift140 (p-value of 2.98E-06) showed significant downregulation of gene expression between the proliferative and hypertrophic zone, suggesting that these genes may be involved in the regulation of chondrocyte proliferation and/or hypertrophic differentiation. IFT140, NAV2 and SCAF11 have also significantly associated with height in GWAS studies. Pathway and network analysis indicated functional connections between IFT140, NAV2 and SCAF11 and previously associated (tall) stature genes. Knockout of the all-trans retinoic acid responsive gene, neuron navigator 2 NAV2, in Xenopus supports its functional role as a growth promotor. Collectively, our data expand the spectrum of genes with a putative role in tall stature phenotypes and, among other genes, highlight NAV2 as an interesting gene to this phenotype.

Clinical and molecular description of 19 patients with GATAD2B-Associated Neurodevelopmental Disorder (GAND).

De novo pathogenic variants in the GATAD2B gene have been associated with a syndromic neurodevelopmental disorder (GAND) characterized by severe intellectual disability (ID), impaired speech, childhood hypotonia, and dysmorphic features. Since its first description in 2013, nine patients have been reported in case reports and a series of 50 patients was recently published, which is consistent with the relative frequency of GATAD2B pathogenic variants in public databases. We report the detailed phenotype of 19 patients from various ethnic backgrounds with confirmed pathogenic GATAD2B variants including intragenic deletions. All individuals presented developmental delay with a median age of 2.5 years for independent walking and of 3 years for first spoken words. GATAD2B variant carriers showed very little subsequent speech progress, two patients over 30 years of age remaining non-verbal. ID was mostly moderate to severe, with one profound and one mild case, which shows a wider spectrum of disease severity than previously reported. We confirm macrocephaly as a major feature in GAND (53%). Most common dysmorphic features included broad forehead, deeply set eyes, hypertelorism, wide nasal base, and pointed chin. Conversely, prenatal abnormalities, non-cerebral malformations, epilepsy, and autistic behavior were uncommon. Other features included feeding difficulties, behavioral abnormalities, and unspecific abnormalities on brain MRI. Improving our knowledge of the clinical phenotype is essential for correct interpretation of the molecular results and accurate patient management.

Genetic testing in patients with aortic aneurysms/dissections: a novel genotype/phenotype correlation?

OBJECTIVE: Mutations in the genes encoding fibrillin-1 (FBN1) and transforming growth factor beta receptor type II (TGFBR2) are known causes of Marfan syndrome (MFS) and related disorders. However, a sound correlation between the genotype and the cardiovascular phenotype has not yet been established. The objective of the present study was to identify novel mutations in FBN1 and TGFBR2 and to assess whether the type of mutation is linked to a particular clinical subtype of the cardiovascular condition. METHODS: The clinical records of 36 patients referred to us for molecular genetic diagnosis were reviewed to assess the course and severity of the vascular deterioration. A semiautomatic protocol was established enabling a rapid and cost-effective screening of the genes FBN1 and TGFBR2 by direct sequencing of all coding exons and flanking intronic regions. RESULTS: Novel mutations in FBN1 and TGFBR2 were detected in 12 and 2 patients, respectively. Four individuals carried a recurrent mutation in FBN1. Throughout the study cohort, the incidence of aortic dissections per se did not depend on the type of mutation. However, we found that mutations affecting the calcium-binding epidermal growth factor-like domain were more frequently associated with a dissection of distal parts of the aorta than mutations that lead to a premature termination codon (chi(1)(2): p=0.013), suggesting that the spatio-temporal pattern of vascular deterioration may vary with the type of mutation. CONCLUSIONS: Detecting a mutation in the genes FBN1 and TGFBR2 proves the genetic origin of vascular findings and allows the identification of family members at risk who should undergo preventive checkups. Routine genetic testing of patients with suspected MFS or thoracic aortic aneurysms/dissections could provide further insight into genotype/phenotype correlations related to aortic dissection.

Low-density DNA microarrays are versatile tools to screen for known mutations in hypertrophic cardiomyopathy.

Familial hypertrophic cardiomyopathy (HCM or CMH) is a myocardial disorder caused by mutations that affect the contractile machinery of heart muscle cells. Genetic testing of HCM patients is hampered by the fact that mutations in at least eight different genes contribute to the disease. An affordable high-throughput mutation detection method is as yet not available. Since a significant number of mutations have been repeatedly found in unrelated families, we consider it feasible to pre-screen patients for known mutations, before more laborious techniques capable of detecting new mutations are applied. Here we demonstrate that the principle of hybridization of DNA to oligonucleotide probes immobilized on chips (glass slides) can be applied for this purpose. We have developed a low-density oligonucleotide probe array capable of detecting 12 different heterozygous mutations (in four different genes), among them single- and double-base exchanges, a single nucleotide insertion, and a trinucleotide deletion. The assay is simple and may be amenable to automation. Detection is achieved with a CCD camera-based fluorescence biochip reader. The technique turned out to be robust: Variations in either the relative position of a mutation, or the amount and size of target-DNA were compatible with mutation detection. Mutations could even be detected in amplicons as long as 800 bp, allowing the screening of more than one exon in one amplicon. Our data suggest that the development of a chip that covers all or most of known HCM-associated mutations is feasible and useful.

Three novel mutations in the ACTA2 gene in German patients with thoracic aortic aneurysms and dissections.

Mutations in the gene encoding smooth muscle cell alpha actin (ACTA2) have recently been shown to cause familial thoracic aortic aneurysms leading to type A dissections (TAAD) and predispose to premature stroke and coronary artery disease. In order to further explore the role of ACTA2 variations in the pathogenesis of TAAD, we sequenced the coding regions of this gene in 40 unrelated German patients with TAAD (with (n=21) or without (n=19) clinical features suggestive of Marfan syndrome). All patients had previously tested negative for mutations in the FBN1 and TGFBR2 genes. We identified three novel ACTA2 mutations and mapped them on a three-dimensional model of actin. Two mutations affect residues within (M49V) or adjacent to (R39C), the DNAse-I-binding loop within subdomain 2 of alpha actin. They were observed in families with recurrent aortic aneurysm (R39C) or aortic dissection (M49V). The third mutation causes an exchange in the vicinity of the ATP-binding site (G304R) in a patient thought to have isolated TAAD. None of the affected individuals had clinical features typical for Marfan syndrome, and no case of premature stroke or coronary artery disease was reported from the affected families. In conclusion, we underscore the role of ACTA2 mutations in nonsyndromic TAAD and suggest that ACTA2 should be included in the genes routinely investigated for syndromic and nonsyndromic TAAD. Detailed clinical investigations of additional families are warranted to further explore the full range of phenotypic signs associated with the three novel mutations described here.

Cardiac alpha-myosin (MYH6) is the predominant sarcomeric disease gene for familial atrial septal defects.

Secundum-type atrial septal defects (ASDII) account for approximately 10% of all congenital heart defects (CHD) and are associated with a familial risk. Mutations in transcription factors represent a genetic source for ASDII. Yet, little is known about the role of mutations in sarcomeric genes in ASDII etiology. To assess the role of sarcomeric genes in patients with inherited ASDII, we analyzed 13 sarcomeric genes (MYH7, MYBPC3, TNNT2, TCAP, TNNI3, MYH6, TPM1, MYL2, CSRP3, ACTC1, MYL3, TNNC1, and TTN kinase region) in 31 patients with familial ASDII using array-based resequencing. Genotyping of family relatives and control subjects as well as structural and homology analyses were used to evaluate the pathogenic impact of novel non-synonymous gene variants. Three novel missense mutations were found in the MYH6 gene encoding alpha-myosin heavy chain (R17H, C539R, and K543R). These mutations co-segregated with CHD in the families and were absent in 370 control alleles. Interestingly, all three MYH6 mutations are located in a highly conserved region of the alpha-myosin motor domain, which is involved in myosin-actin interaction. In addition, the cardiomyopathy related MYH6-A1004S and the MYBPC3-A833T mutations were also found in one and two unrelated subjects with ASDII, respectively. No mutations were found in the 11 other sarcomeric genes analyzed. The study indicates that sarcomeric gene mutations may represent a so far underestimated genetic source for familial recurrence of ASDII. In particular, perturbations in the MYH6 head domain seem to play a major role in the genetic origin of familial ASDII.

Novel correlations between the genotype and the phenotype of hypertrophic and dilated cardiomyopathy: results from the German Competence Network Heart Failure.

AIMS: Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) can both be due to mutations in the genes encoding ß-myosin heavy chain (MYH7) or cardiac myosin-binding protein C (MYBPC3). The aim of the present study was to determine the prevalence and spectrum of mutations in both genes in German HCM and DCM patients and to establish novel genotype-to-phenotype correlations. METHODS AND RESULTS: Coding exons and intron flanks of the two genes MYH7 and MYBPC3 of 236 patients with HCM and 652 patients with DCM were sequenced by conventional and array-based means. Clinical records were established following standard protocols. Mutations were detected in 41 and 11% of the patients with HCM and DCM, respectively. Differences were observed in the frequency of splice site and frame-shift mutations in the gene MYBPC3, which occurred more frequently (P< 0.02, P< 0.001, respectively) in HCM than in DCM, suggesting that cardiac myosin-binding protein C haploinsufficiency predisposes to hypertrophy rather than to dilation. Additional novel genotype-to-phenotype correlations were found in HCM, among these a link between MYBPC3 mutations and a particularly large thickness of the interventricular septum (P= 0.04 vs. carriers of a mutation in MYH7). Interestingly, this correlation and a link between MYH7 mutations and a higher degree of mitral valve regurgitation held true for both HCM and DCM, indicating that the gene affected by a mutation may determine the magnitude of structural and functional alterations in both HCM and DCM. CONCLUSION: A large clinical-genetic study has unravelled novel genotype-to-phenotype correlations in HCM and DCM which warrant future investigation of both the underlying mechanisms and the prognostic use.

Array-based resequencing assay for mutations causing hypertrophic cardiomyopathy.

BACKGROUND: Dissecting the complex genetic basis of hypertrophic cardiomyopathy (HCM) may be key to both better understanding and optimally managing this most prevalent genetic cardiovascular disease. An array-based resequencing (ABR) assay was developed to facilitate genetic testing in HCM. METHODS: An Affymetrix resequencing array and a single long-range PCR protocol were developed to cover the 3 most commonly affected genes in HCM, MYH7 (myosin, heavy chain 7, cardiac muscle, beta), MYBPC3 (myosin binding protein C, cardiac), and TNNT2 [troponin T type 2 (cardiac)]. RESULTS: The assay detected the underlying point mutation in 23 of 24 reference samples and provided pointers toward identifying a G insertion and a 3-bp deletion. The comparability of array-based assay results to conventional capillary sequencing was > or =99.9%. Both techniques detected 1 heterozygous variant that was missed by the other method. CONCLUSIONS: The data provide evidence that ABR can substantially reduce the high workload previously associated with a genetic test for HCM. Therefore, the HCM array could facilitate large-scale studies aimed at broadening the understanding of the genetic and phenotypic diversity of HCM and related cardiomyopathies.

Noncompaction of the ventricular myocardium is associated with a de novo mutation in the beta-myosin heavy chain gene.

Noncompaction of the ventricular myocardium (NVM) is the morphological hallmark of a rare familial or sporadic unclassified heart disease of heterogeneous origin. NVM results presumably from a congenital developmental error and has been traced back to single point mutations in various genes. The objective of this study was to determine the underlying genetic defect in a large German family suffering from NVM. Twenty four family members were clinically assessed using advanced imaging techniques. For molecular characterization, a genome-wide linkage analysis was undertaken and the disease locus was mapped to chromosome 14ptel-14q12. Subsequently, two genes of the disease interval, MYH6 and MYH7 (encoding the alpha- and beta-myosin heavy chain, respectively) were sequenced, leading to the identification of a previously unknown de novo missense mutation, c.842G>C, in the gene MYH7. The mutation affects a highly conserved amino acid in the myosin subfragment-1 (R281T). In silico simulations suggest that the mutation R281T prevents the formation of a salt bridge between residues R281 and D325, thereby destabilizing the myosin head. The mutation was exclusively present in morphologically affected family members. A few members of the family displayed NVM in combination with other heart defects, such as dislocation of the tricuspid valve (Ebstein's anomaly, EA) and atrial septal defect (ASD). A high degree of clinical variability was observed, ranging from the absence of symptoms in childhood to cardiac death in the third decade of life. The data presented in this report provide first evidence that a mutation in a sarcomeric protein can cause noncompaction of the ventricular myocardium.

Novel deletions in MYH7 and MYBPC3 identified in Indian families with familial hypertrophic cardiomyopathy.

Mutations causing familial hypertrophic cardiomyopathy (HCM) have been described in at least 11 genes encoding cardiac sarcomeric proteins. In this study, three previously unknown deletions have been identified in the human cardiac genes coding for beta-myosin heavy chain (MYH7 on chromosome 14) and myosin-binding protein-C (MYBPC3 on chromosome 11). In family MM, a 3-bp deletion in MYH7 was detected to be associated with loss of glutamic acid in position 927 (DeltaE927) of the myosin rod. In two other families (HH and NP, related by a common founder) a 2-bp loss in codon 453 (exon 16) of MYBPC3 was identified as the presumable cause of a translation reading frame shift. Taken together 15 living mutation carriers were investigated. Six deceased family members (with five cases of premature sudden cardiac death (SCD) in families MM and NP) were either obligate or suspected mutation carriers. In addition to these mutations a 25-bp deletion in intron 32 of MYBPC3 was identified in family MM (five carriers) and in a fourth family (MiR, one HCM patient, three deletion carriers). In agreement with the loss of the regular splicing branch point in the altered intron 32, a splicing deficiency was observed in an exon trapping experiment using MYBPC3 exon 33 as a test substrate. Varying disease profiles assessed using standard clinical, ECG and echocardiographic procedures in conjunction with mutation analysis led to the following conclusions: (1) In family MM the DeltaE927 deletion in MYH7 was assumed to be associated with complete penetrance. Two cases of reported SCD might have been related to this mutation. (2) The two families, HH and NP, distantly related by a common founder, and both suffering from a 2-bp deletion in exon 16 of MYBPC3 differed in their average phenotypes. In family NP, four cases of cardiac death were documented, whereas no cardiac-related death was reported from family HH. These results support the notion that mutations in HCM genes may directly determine disease penetrance and severity; however, a contribution of additional, unidentified factors (genes) to the HCM phenotype can-at least in some cases-not be excluded. (3) The deletion in intron 32 of MYBPC3 was seen in two families, but in both its relation to disease was not unequivocal. In addition, this deletion was observed in 16 of 229 unrelated healthy individuals of the population of the South Indian states of Kerala and Tamil Nadu. It was not seen in 270 Caucasians from Russia and western Europe. Hence, it is considered to represent a regional genetic polymorphism restricted to southern India. The association of the deletion with altered splicing in transfected cells suggests that this deletion may create a "modifying gene", which is per se not or only rarely causing HCM, but which may enhance the phenotype of a mutation responsible for disease.