Exome Sequencing Identifies a Novel LMNA Splice-Site Mutation and Multigenic Heterozygosity of Potential Modifiers in a Family with Sick Sinus Syndrome, Dilated Cardiomyopathy, and Sudden Cardiac Death.

The goals are to understand the primary genetic mechanisms that cause Sick Sinus Syndrome and to identify potential modifiers that may result in intrafamilial variability within a multigenerational family. The proband is a 63-year-old male with a family history of individuals (>10) with sinus node dysfunction, ventricular arrhythmia, cardiomyopathy, heart failure, and sudden death. We used exome sequencing of a single individual to identify a novel LMNA mutation and demonstrated the importance of Sanger validation and family studies when evaluating candidates. After initial single-gene studies were negative, we conducted exome sequencing for the proband which produced 9 gigabases of sequencing data. Bioinformatics analysis showed 94% of the reads mapped to the reference and identified 128,563 unique variants with 108,795 (85%) located in 16,319 genes of 19,056 target genes. We discovered multiple variants in known arrhythmia, cardiomyopathy, or ion channel associated genes that may serve as potential modifiers in disease expression. To identify candidate mutations, we focused on ~2,000 variants located in 237 genes of 283 known arrhythmia, cardiomyopathy, or ion channel associated genes. We filtered the candidates to 41 variants in 33 genes using zygosity, protein impact, database searches, and clinical association. Only 21 of 41 (51%) variants were validated by Sanger sequencing. We selected nine confirmed variants with minor allele frequencies <1% for family studies. The results identified LMNA c.357-2A>G, a novel heterozygous splice-site mutation as the primary mutation with rare or novel variants in HCN4, MYBPC3, PKP4, TMPO, TTN, DMPK and KCNJ10 as potential modifiers and a mechanism consistent with haploinsufficiency.

Intrafamilial variability for novel TAZ gene mutation: Barth syndrome with dilated cardiomyopathy and heart failure in an infant and left ventricular noncompaction in his great-uncle.

BACKGROUND: The tafazzin gene (TAZ) is located at Xq28 and encodes a protein involved in the transacylation of cardiolipin, an essential mitochondrial phospholipid. Mutations in TAZ are associated with Barth syndrome (BTHS), the X-linked recessive condition with dilated cardiomyopathy, skeletal myopathy, growth retardation, neutropenia and organic aciduria. TAZ mutations also contribute to left ventricular noncompaction (LVNC), a cardiomyopathy characterized by loose, trabeculated myocardium. CASE REPORT: We report a family with a novel TAZ mutation and the clinical spectrum from severe BTHS in an infant to skeletal myopathy with LVNC in an adult, the oldest individual with BTHS reported. The proband is a 51-year-old male with muscle weakness since early childhood. He remained stable until the age of 43. His initial evaluations found LVNC and borderline neutropenia with no elevation of urine 3-methylglutaconic acid. The proband's great nephew is a 3-year-old who presented at birth with poor feeding, hypotonia, lactic acidosis and hypoglycemia. At three months he was admitted with failure to thrive, lethargy and respiratory distress due to heart failure. Cardiac studies revealed dilated cardiomyopathy with a spongiform trabeculated pattern of the left ventricle. Laboratory studies showed cyclic neutropenia and elevated urine 3-methylglutaconic and 3-methylglutaric acids. At age 11months the patient had a heart transplant. We conducted sequence analysis of the TAZ gene for two affected individuals, the proband first and then his great-nephew. A novel, hemizygous nonsense mutation in TAZ exon 7 (c.583G>T, p.Gly195X) was detected. CONCLUSION: At his current age of 51years-old, the proband is the oldest surviving individual reported with a confirmed molecular diagnosis and features of Barth syndrome. Further studies will be conducted to identify the genetic modifying factor(s) associated with the wide phenotypic range seen in this family.