Nature's genetic gradients and the clinical phenotype

Molecular Assessment of Methylglyoxal-Induced Toxicity and Therapeutic Approaches in Various Diseases: Exploring the Interplay with the Glyoxalase System

This comprehensive exploration delves into the intricate interplay of methylglyoxal (MG) and glyoxalase 1 (GLO I) in various physiological and pathological contexts. The linchpin of the narrative revolves around the role of these small molecules in age-related issues, diabetes, obesity, cardiovascular diseases, and neurodegenerative disorders. Methylglyoxal, a reactive dicarbonyl metabolite, takes center stage, becoming a principal player in the development of AGEs and contributing to cell and tissue dysfunction. The dual facets of GLO I-activation and inhibition-unfold as potential therapeutic avenues. Activators, spanning synthetic drugs like candesartan to natural compounds like polyphenols and isothiocyanates, aim to restore GLO I function. These molecular enhancers showcase promising outcomes in conditions such as diabetic retinopathy, kidney disease, and beyond. On the contrary, GLO I inhibitors emerge as crucial players in cancer treatment, offering new possibilities in diseases associated with inflammation and multidrug resistance. The symphony of small molecules, from GLO I activators to inhibitors, presents a nuanced understanding of MG regulation. From natural compounds to synthetic drugs, each element contributes to a molecular orchestra, promising novel interventions and personalized approaches in the pursuit of health and wellbeing. The abstract concludes with an emphasis on the necessity of rigorous clinical trials to validate these findings and acknowledges the importance of individual variability in the complex landscape of health.

A Novel DLG1 Variant in a Family with Brugada Syndrome: Clinical Characteristics and In Silico Analysis

BACKGROUND

Brugada syndrome (BrS) is an inherited primary channelopathy syndrome associated to sudden cardiac death. Overall, variants have been identified in eighteen genes encoding for ion channel subunits and seven genes for regulatory proteins. Recently, a missense variant in DLG1 has been found within a BrS phenotype-positive patient. DLG1 encodes for synapse associated protein 97 (SAP97), a protein characterized by the presence of multiple domains for protein-protein interactions including PDZ domains. In cardiomyocytes, SAP97 interacts with Nav1.5, a PDZ binding motif of SCN5A and others potassium channel subunits.

AIM OF THE STUDY

To characterize the phenotype of an Italian family with BrS syndrome carrying a DLG1 variant.

METHODS

Clinical and genetic investigations were performed. Genetic testing was performed with whole-exome sequencing (WES) using the Illumina platform. According to the standard protocol, a variant found by WES was confirmed in all members of the family by bi-directional capillary Sanger resequencing. The effect of the variant was investigated by using in silico prediction of pathogenicity.

RESULTS

The index case was a 74-year-old man with spontaneous type 1 BrS ECG pattern that experienced syncope and underwent ICD implantation. WES of the index case, performed assuming a dominant mode of inheritance, identified a heterozygous variant, c.1556G>A (p.R519H), in the exon 15 of the DLG1 gene. In the pedigree investigation, 6 out of 12 family members had the variant. Carriers of the gene variant all had BrS ECG type 1 drug induced and showed heterogeneous cardiac phenotypes with two patients experiencing syncope during exercise and fever, respectively. The amino acid residue #519 lies near a PDZ domain and in silico analysis suggested a causal role for the variant. Modelling of the resulting protein structure predicted that the variant disrupts an H-bond and a likelihood of being pathogenic. As a consequence, it is likely that a conformational change affects protein functionality and the modulating role on ion channels.

CONCLUSIONS

A DLG1 gene variant identified was associated with BrS. The variant could modify the formation of multichannel protein complexes, affecting ion channels to specific compartments in cardiomyocytes.

Identification of Novel SCN5A Single Nucleotide Variants in Brugada Syndrome: A Territory-Wide Study From Hong Kong

Background

The aim of this study is to report on the genetic composition of Brugada syndrome (BrS) patients undergoing genetic testing in Hong Kong.

Methods

Patients with suspected BrS who presented to the Hospital Authority of Hong Kong between 1997 and 2019, and underwent genetic testing, were analyzed retrospectively.

Results

A total of 65 subjects were included (n = 65, 88% male, median presenting age 42 [30–54] years old, 58% type 1 pattern). Twenty-two subjects (34%) showed abnormal genetic test results, identifying the following six novel, pathogenic or likely pathogenic mutations in SCN5A: c.674G > A, c.2024-11T > A, c.2042A > C, c.4279G > T, c.5689C > T, c.429del. Twenty subjects (31%) in the cohort suffered from spontaneous ventricular tachycardia/ventricular fibrillation (VT/VF) and 18 (28%) had incident VT/VF over a median follow-up of 83 [Q1–Q3: 52–112] months. Univariate Cox regression demonstrated that syncope (hazard ratio [HR]: 4.27 [0.95–19.30]; P = 0.059), prior VT/VF (HR: 21.34 [5.74–79.31; P < 0.0001) and T-wave axis (HR: 0.970 [0.944–0.998]; P = 0.036) achieved P < 0.10 for predicting incident VT/VF. After multivariate adjustment, only prior VT/VF remained a significant predictor (HR: 12.39 [2.97–51.67], P = 0.001).

Conclusion

This study identified novel mutations in SCN5A in a Chinese cohort of BrS patients.

Brugada syndrome: A comprehensive review of pathophysiological mechanisms and risk stratification strategies

Brugada syndrome (BrS) is an inherited ion channel channelopathy predisposing to ventricular arrhythmias and sudden cardiac death. Originally believed to be predominantly associated with mutations in SCN5A encoding for the cardiac sodium channel, mutations of 18 genes other than SCN5A have been implicated in the pathogenesis of BrS to date. Diagnosis is based on the presence of a spontaneous or drug-induced coved-type ST segment elevation. The predominant electrophysiological mechanism underlying BrS remains disputed, commonly revolving around the three main hypotheses based on abnormal repolarization, depolarization or current-load match. Evidence from computational modelling, pre-clinical and clinical studies illustrates that molecular abnormalities found in BrS lead to alterations in excitation wavelength (λ), which ultimately elevates arrhythmic risk. A major challenge for clinicians in managing this condition is the difficulty in predicting the subset of patients who will suffer from life-threatening ventricular arrhythmic events. Several repolarization risk markers have been used thus far, but these neglect the contributions of conduction abnormalities in the form of slowing and dispersion. Indices incorporating both repolarization and conduction based on the concept of λ have recently been proposed. These may have better predictive values than the existing markers. Current treatment options include pharmacological therapy to reduce the occurrence of arrhythmic events or to abort these episodes, and interventions such as implantable cardioverter-defibrillator insertion or radiofrequency ablation of abnormal arrhythmic substrate.

Hypertrophic Cardiomyopathy: Genetics, Pathogenesis, Clinical Manifestations, Diagnosis, and Therapy

Hypertrophic cardiomyopathy (HCM) is a genetic disorder that is characterized by left ventricular hypertrophy unexplained by secondary causes and a nondilated left ventricle with preserved or increased ejection fraction. It is commonly asymmetrical with the most severe hypertrophy involving the basal interventricular septum. Left ventricular outflow tract obstruction is present at rest in about one third of the patients and can be provoked in another third. The histological features of HCM include myocyte hypertrophy and disarray, as well as interstitial fibrosis. The hypertrophy is also frequently associated with left ventricular diastolic dysfunction. In the majority of patients, HCM has a relatively benign course. However, HCM is also an important cause of sudden cardiac death, particularly in adolescents and young adults. Nonsustained ventricular tachycardia, syncope, a family history of sudden cardiac death, and severe cardiac hypertrophy are major risk factors for sudden cardiac death. This complication can usually be averted by implantation of a cardioverter-defibrillator in appropriate high-risk patients. Atrial fibrillation is also a common complication and is not well tolerated. Mutations in over a dozen genes encoding sarcomere-associated proteins cause HCM. MYH7 and MYBPC3, encoding β-myosin heavy chain and myosin-binding protein C, respectively, are the 2 most common genes involved, together accounting for ≈50% of the HCM families. In ≈40% of HCM patients, the causal genes remain to be identified. Mutations in genes responsible for storage diseases also cause a phenotype resembling HCM (genocopy or phenocopy). The routine applications of genetic testing and preclinical identification of family members represents an important advance. The genetic discoveries have enhanced understanding of the molecular pathogenesis of HCM and have stimulated efforts designed to identify new therapeutic agents.

Electrophysiological Mechanisms of Brugada Syndrome: Insights from Pre-clinical and Clinical Studies

Brugada syndrome (BrS), is a primary electrical disorder predisposing affected individuals to sudden cardiac death via the development of ventricular tachycardia and fibrillation (VT/VF). Originally, BrS was linked to mutations in the SCN5A, which encodes for the cardiac Na+ channel. To date, variants in 19 genes have been implicated in this condition, with 11, 5, 3, and 1 genes affecting the Na+, K+, Ca2+, and funny currents, respectively. Diagnosis of BrS is based on ECG criteria of coved- or saddle-shaped ST segment elevation and/or T-wave inversion with or without drug challenge. Three hypotheses based on abnormal depolarization, abnormal repolarization, and current-load-mismatch have been put forward to explain the electrophysiological mechanisms responsible for BrS. Evidence from computational modeling, pre-clinical, and clinical studies illustrates that molecular abnormalities found in BrS lead to alterations in excitation wavelength (λ), which ultimately elevates arrhythmic risk. A major challenge for clinicians in managing this condition is the difficulty in predicting the subset of patients who will suffer from life-threatening VT/VF. Several repolarization risk markers have been used thus far, but these neglect the contributions of conduction abnormalities in the form of slowing and dispersion. Indices incorporating both repolarization and conduction and based on the concept of λ have recently been proposed. These may have better predictive values than the existing markers.

Missing heritability of complex diseases: Enlightenment by genetic variants from intermediate phenotypes

Diseases of complex origin have a component of quantitative genetics that contributes to their susceptibility and phenotypic variability. However, after several studies, a major part of the genetic component of complex phenotypes has still not been found, a situation known as "missing heritability." Although there have been many hypotheses put forward to explain the reasons for the missing heritability, its definitive causes remain unknown. Complex diseases are caused by multiple intermediate phenotypes involved in their pathogenesis and, very often, each one of these intermediate phenotypes also has a component of quantitative inheritance. Here we propose that at least part of the missing heritability can be explained by the genetic component of intermediate phenotypes that is not detectable at the level of the main complex trait. At the same time, the identification of the genetic component of intermediate phenotypes provides an opportunity to identify part of the missing heritability of complex diseases.

Clinical applications of molecular genetic discoveries

Genome-wide association studies of complex traits have mapped >15,000 common single nucleotide variants (SNVs). Likewise, applications of massively parallel nucleic acid sequencing technologies often referred to as next-generation sequencing to molecular genetic studies of complex traits have catalogued a large number of rare variants (population frequency of <0.01) in cases with complex traits. Moreover, high-throughput nucleic acid sequencing, variant burden analysis, and linkage studies are illuminating the presence of large number of SNVs in cases and families with single-gene disorders. The plethora of the genetic variants has exposed the formidable challenge of identifying the causal and pathogenic variants from the enormous number of innocuous common and rare variants that exist in the population and in an individual genome. The arduous task of identifying the causal and pathogenic variants is further compounded by the pleiotropic effects of the variants, complexity of cis and trans interactions in the genome, variability in phenotypic expression of the disease, as well as phenotypic plasticity, and the multifarious determinants of the phenotype. Population genetic studies offer the initial roadmaps and have the potential to elucidate novel pathways involved in the pathogenesis of the disease. However, the genome of an individual is unique, rendering unambiguous identification of the causal or pathogenic variant in a single individual exceedingly challenging. Yet, the focus of the practice of medicine is on the individual, as Sir William Osler elegantly expressed in his insightful quotation: "The good physician treats the disease; the great physician treats the patient who has the disease." The daunting task facing physicians, patients, and researchers alike is to apply the modern genetic discoveries to care of the individual with or at risk of the disease.

Sequencing your genome: what does it mean?

The human genome contains approximately 3.2 billion nucleotides and about 23,500 genes. Each gene has protein-coding regions that are referred to as exons. The human genome contains about 180,000 exons, which are collectively called an exome. An exome comprises about 1% of the human genome and hence is about 30 million nucleotides in size. Today's technologies afford the opportunity to sequence all nucleotides in the human exome and even in the human genome. Given that more than three-quarters of the known disease-causing variants are located in the exome, and considering the cost and technical challenges in analyzing the whole genome sequence data, the focus of present research is primarily on whole exome sequencing (WES). While WES at the medical sequencing level is still expensive, it is becoming more affordable. Cost will not likely be a major barrier in the near future, and the data analysis is becoming less tedious. The most difficult challenge at the heart of medical sequencing is interpreting the findings. Each exome contains about 13,500 single nucleotide variants (SNVs) that affect the amino acid sequence, and a large number are expected to be functional variants. The daunting task is to distinguish the variants that are pathogenic from those that have minimal or no discernible clinical effects. While various algorithms exist, none are sufficiently robust. Thus, in-depth knowledge in genetics and medicine is essential for the proper interpretation of the WES findings. This review will discuss the potential applications of the WES data in the practice of cardiovascular medicine.

Arrhythmogenic cardiomyopathy in a patient with a rare loss-of-function KCNQ1 mutation

BACKGROUND

Ventricular tachycardia (VT) is a common manifestation of advanced cardiomyopathies. In a subset of patients with dilated cardiomyopathy, VT is the initial and the cardinal manifestation of the disease. The molecular genetic basis of this subset of dilated cardiomyopathy is largely unknown.

METHODS AND RESULTS

We identified 10 patients with dilated cardiomyopathy who presented with VT and sequenced 14 common causal genes for cardiomyopathies and arrhythmias. Functional studies included cellular patch clamp, confocal microscopy, and immunoblotting. We identified nonsynonymous variants in 4 patients, including a rare missense p.R397Q mutation in the KCNQ1 gene in a 60-year-old man who presented with incessant VT and had mild cardiac dysfunction. The p.R397Q mutation was absent in an ethnically matched control group, affected a conserved amino acid, and was predicted by multiple algorithms to be pathogenic. Co-expression of the mutant KCNQ1 with its partner unit KCNE1 was associated with reduced tail current density of slowly activating delayed rectifier K(+) current (IKs). The mutation reduced membrane localization of the protein.

CONCLUSIONS

Dilated cardiomyopathy with an initial presentation of VT may be a forme fruste of arrhythmogenic cardiomyopathy caused by mutations in genes encoding the ion channels. The findings implicate KCNQ1 as a possible causal gene for arrhythmogenic cardiomyopathy.

Sortilin and the risk of cardiovascular disease

Plasma low-density lipoprotein cholesterol (LDL-C) levels are a key determinant of the risk of cardiovascular disease, which is why many studies have attempted to elucidate the pathways that regulate its metabolism. Novel latest-generation sequencing techniques have identified a strong association between the 1p13 locus and the risk of cardiovascular disease caused by changes in plasma LDL-C levels. As expected for a complex phenotype, the effects of variation in this locus are only moderate. Even so, knowledge of the association is of major importance, since it has unveiled a new metabolic pathway regulating plasma cholesterol levels. Crucial to this discovery was the work of three independent teams seeking to clarify the biological basis of this association, who succeeded in proving that SORT1, encoding sortilin, was the gene in the 1p13 locus involved in LDL metabolism. SORT1 was the first gene identified as determining plasma LDL levels to be mechanistically evaluated and, although the three teams used different, though appropriate, experimental methods, their results were in some ways contradictory. Here we review all the experiments that led to the identification of the new pathway connecting sortilin with plasma LDL levels and risk of myocardial infarction. The regulatory mechanism underlying this association remains unclear, but its discovery has paved the way for considering previously unsuspected therapeutic targets and approaches.

DNA sequence variants and the practice of medicine

PURPOSE OF REVIEW

To discuss potential clinical utility of the DNA sequence variants (DSVs) present in the human genome.

RECENT FINDINGS

Advances in the sequencing technology have led to discovery of a very large number of DSVs in the human genome. Accordingly each genome has approximately 4 million DSVs, of which single-nucleotide polymorphisms (SNPs) dominate in number (about 3 million) but the structural variations, including the copy number variants (CNVs), encompass a much larger number of the nucleotides. The biological and clinical impacts of DSVs are innate to their effect sizes and follow a gradient from negligible to drastic. DSVs responsible for single gene disorders impart the largest effect sizes, whereas those with small or moderate effect sizes modify phenotypic expression of the single gene disorders. In contrast, the common complex disorders result from intricate interactions of a very large number of DSVs, each imparting a modest and often clinically indiscernible effect size, with each other and with the environmental factors. DSVs with large effect sizes, under certain circumstances, might have clinical utility in individualization of therapy, early diagnosis and the risk stratification. In contrast, DSVs with small effect sizes are unlikely to provide useful clinical information.

SUMMARY

DSVs, under certain circumstances, could provide valuable information for genetic-based diagnosis, risk stratification and treatment. However, the primary utility of DSVs is in providing insight into the molecular mechanisms that govern the pathogenesis of the human diseases and applying the mechanistic insight to the cure of such disorders.

Challenges in medical applications of whole exome/genome sequencing discoveries

Despite the well-documented influence of genetics on susceptibility to cardiovascular diseases, delineation of the full spectrum of the risk alleles had to await the development of modern next-generation sequencing technologies. The techniques provide unbiased approaches for identification of the DNA sequence variants (DSVs) in the entire genome (whole genome sequencing [WGS]) or the protein-coding exons (whole exome sequencing [WES]). Each genome contains approximately 4 million DSVs and each exome approximately 13,000 single nucleotide variants. The challenge facing researchers and clinicians alike is to decipher the biological and clinical significance of these variants and harness the information for the practice of medicine. The common DSVs typically exert modest effect sizes, as evidenced by the results of genome-wide association studies, and hence have modest or negligible clinical implications. The focus is on the rare variants with large effect sizes, which are expected to have stronger clinical implications, as in single gene disorders with Mendelian patterns of inheritance. However, the clinical implications of the rare variants for common complex cardiovascular diseases remain to be established. The most important contribution of WES or WGS is in delineation of the novel molecular pathways involved in the pathogenesis of the phenotype, which would be expected to provide for preventive and therapeutic opportunities.

Elements of 'missing heritability'

PURPOSE OF REVIEW

To discuss the basis of 'missing heritability', which has emerged as an enigma in the post-genome-wide association studies (GWAS) era.

RECENT FINDINGS

Alleles identified through GWAS account for a relatively small fraction of heritability of the complex phenotypes. Accordingly, a significant part of heritability of the complex traits remains unaccounted for ('missing heritability'). Recent findings offer several explanations, including overestimation of heritability of the complex traits and underestimation of the effects of alleles identified through GWAS. In addition, yet-to-be identified common as well as rare alleles might in part explain the 'missing heritability'. Moreover, gene-gene (epistasis) and gene-environmental interactions might explain another fraction of heritability of complex traits. Moreover, transgenerational epigenetic changes, regulated in part by microRNAs, might also contribute to the 'missing heritability'.

SUMMARY

The new findings suggest a multifarious nature of the 'missing heritability'. The findings de-emphasize the focus on delineating the basis of 'missing heritability' and shift the focus to elucidation of the molecular mechanisms by which genomic and genetic factors govern the pathogenesis of the complex phenotypes.

Molecular genetic studies of complex phenotypes

The approach to molecular genetic studies of complex phenotypes evolved considerably during the recent years. The candidate gene approach, which is restricted to an analysis of a few single-nucleotide polymorphisms (SNPs) in a modest number of cases and controls, has been supplanted by the unbiased approach of genome-wide association studies (GWAS), wherein a large number of tagger SNPs are typed in many individuals. GWAS, which are designed on the common disease-common variant hypothesis (CD-CV), identified several SNPs and loci for complex phenotypes. However, the alleles identified through GWAS are typically not causative but rather in linkage disequilibrium (LD) with the true causal variants. The common alleles, which may not capture the uncommon and rare variants, account only for a fraction of heritability of the complex traits. Hence, the focus is being shifted to rare variants-common disease (RV-CD) hypothesis, surmising that rare variants exert large effect sizes on the phenotype. In conjunctional with this conceptual shift, technologic advances in DNA sequencing techniques have dramatically enhanced whole genome or whole exome sequencing capacity. The sequencing approach affords identification of not only the rare but also the common variants. The approach-whether used in complementation with GWAS or as a stand-alone approach-could define the genetic architecture of the complex phenotypes. Robust phenotyping and large-scale sequencing studies are essential to extract the information content of the vast number of DNA sequence variants (DSVs) in the genome. To garner meaningful clinical information and link the genotype to a phenotype, the identification and characterization of a large number of causal fields beyond the information content of DNA sequence variants would be necessary. This review provides an update on the current progress and limitations in identifying DSVs that are associated with phenotypic effects.

Determinants of plasma vitamin D levels in patients with acute coronary syndromes

BACKGROUND

Vitamin D is implicated in various biological functions ranging from cellular proliferation to immunity. Vitamin D deficiency is associated with an increased risk of several diseases including coronary atherosclerosis.

MATERIALS AND METHODS

We measured plasma 25(OH)D3 level in 224 patients with acute coronary syndromes (ACS) and 209 control individuals by ELISA. We genotyped the study populations for 11 single nucleotide polymorphisms (SNPs) in seven genes involved in vitamin D biosynthesis and metabolism by 5' nuclease assays.

RESULTS

The mean and median plasma 25(OH)D3 levels were not significantly different between patients with ACS and controls (median: 22·06 vs. 22·24 ng mL(-1) , respectively, P = 0·618). Plasma 25(OH)D3 level was < 20 ng mL(-1) in 175/433 (40%) and < 30 ng mL(-1) in 333/433 (77%) participants. Only four individuals had plasma 25(OH)D3 levels of above 60 ng mL(-1) . African-American and Hispanic populations, women and those with diabetes mellitus had significantly lower plasma 25(OH)D3 levels. In multivariable regression analysis, age, sex, diabetes mellitus, body weight, rs2762933 (CYP24A1) and rs6055987 (PLCB1) SNPs were independent predictors of plasma 25(OH)D3 level in the Caucasian population.

CONCLUSIONS

We found no difference in mean plasma vitamin D levels between patients with ACS and controls. Differences in population characteristics between the two study groups including medications use and the lack of data on vitamin D, calcium and multivitamin supplements intake as well as the relatively small sample size of the populations could confound the results. Ethnic background, sex, age, body weight and SNPs in CYP24A1 and PLCB1 were independent determinants of plasma vitamin D levels.

Medical DNA sequencing

PURPOSE OF REVIEW

To discuss implications of information garnered through whole-genome and exome sequencing in the practice of cardiovascular medicine.

RECENT FINDINGS

Whole-genome and exome sequencing unveils medical information embedded in individual genomes and exomes, which could be incorporated into the practice of medicine for diagnostic and therapeutic gains. The human, however, has considerable genetic diversity, as each genome encompasses about 4 million DNA sequence variants (DSVs). The challenging task is to identify the variants that have clinical implications. DSVs exert a continuum of effect sizes on the phenotype that ranges from negligible to large. From a clinical perspective, selected categories, in order of their significance, are disease-causing, likely disease-causing, disease-associated, biologically functional but unknown clinical significance, and unknown functional and clinical significance variants. The frequency of DSVs in the genome also follows a gradient from rare for the disease-causing variants to common for variants with unknown clinical and biological significance. A subset of DSVs might have implications in accurate and preclinical diagnosis, prognostication and individualization of therapy. Clinical phenotypes, however, are too complex to be determined solely by a single DSV. Even in the case of disease-causing variants, the severity of the disease is determined by multiple additional genetic and nongenetic factors.

SUMMARY

Medical DNA sequencing is expected to retool clinicians with the information content of DSVs. DSVs with large effect sizes are likely to offer clinical utility in early and preclinical diagnosis, prognostication and individualization of therapy.

Molecular genetic and functional characterization implicate muscle-restricted coiled-coil gene (MURC) as a causal gene for familial dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM) are classic forms of systolic and diastolic heart failure, respectively. Mutations in genes encoding sarcomere and cytoskeletal proteins are major causes of HCM and DCM. MURC, encoding muscle-restricted coiled-coil, a Z-line protein, regulates cardiac function in mice. We investigated potential causal role of MURC in human cardiomyopathies.

METHODS AND RESULTS

We sequenced MURC in 1199 individuals, including 383 probands with DCM, 307 with HCM, and 509 healthy control subjects. We found 6 heterozygous DCM-specific missense variants (p.N128K, p.R140W, p.L153P, p.S307T, p.P324L, and p.S364L) in 8 unrelated probands. Variants p.N128K and p.S307T segregated with inheritance of DCM in small families (χ(2)=8.5, P=0.003). Variants p.N128K, p.R140W, p.L153P, and p.S364L were considered probably or possibly damaging. Variant p.P324L recurred in 3 independent probands, including 1 proband with a TPM1 mutation (p.M245T). A deletion variant (p.L232-R238del) was present in 3 unrelated HCM probands, but it did not segregate with HCM in a family who also had a MYH7 mutation (p.L907V). The phenotype in mutation carriers was notable for progressive heart failure leading to heart transplantation in 4 patients, conduction defects, and atrial arrhythmias. Expression of mutant MURC proteins in neonatal rat cardiac myocytes transduced with recombinant adenoviruses was associated with reduced RhoA activity, lower mRNA levels of hypertrophic markers and smaller myocyte size as compared with wild-type MURC.

CONCLUSIONS

MURC mutations impart loss-of-function effects on MURC functions and probably are causal variants in human DCM. The causal role of a deletion mutation in HCM is uncertain.

Strategic approaches to unraveling genetic causes of cardiovascular diseases

DNA sequence variants are major components of the "causal field" for virtually all medical phenotypes, whether single gene familial disorders or complex traits without a clear familial aggregation. The causal variants in single gene disorders are necessary and sufficient to impart large effects. In contrast, complex traits are attributable to a much more complicated network of contributory components that in aggregate increase the probability of disease. The conventional approach to identification of the causal variants for single gene disorders is genetic linkage. However, it does not offer sufficient resolution to map the causal genes in small families or sporadic cases. The approach to genetic studies of complex traits entails candidate gene or genome-wide association studies. Genome-wide association studies provide an unbiased survey of the effects of common genetic variants (common disease-common variant hypothesis). Genome-wide association studies have led to identification of a large number of alleles for various cardiovascular diseases. However, common alleles account for a relatively small fraction of the total heritability of the traits. Accordingly, the focus has shifted toward identification of rare variants that might impart larger effect sizes (rare variant-common disease hypothesis). This shift is made feasible by recent advances in massively parallel DNA sequencing platforms, which afford the opportunity to identify virtually all common as well as rare alleles in individuals. In this review, we discuss various strategies that are used to delineate the genetic contribution to medically important cardiovascular phenotypes, emphasizing the utility of the new deep sequencing approaches.