Mortality Risk Associated With Truncating Founder Mutations in Titin.

Background Truncating titin variants (TTNtv) are the most prevalent genetic cause of dilated cardiomyopathy, found in ≤25% of familial cases. Moreover, TTNtv associated with dilated cardiomyopathy are estimated to be present in 0.5% of the general population. The prognosis of asymptomatic carriers of TTNtv is poorly understood because TTNtv are associated with a highly variable phenotype. We aim to assess the natural history and clinical relevance of TTNtv by analyzing standardized mortality ratios (SMR) in multigenerational pedigrees and in close relatives of present-day patients. Methods Haplotype and genealogical analyses were performed on 3 recurrent TTNtv. Subsequently, the family tree mortality ratio method was used to compare all-cause mortality of subjects at an a priori 50% risk of carrying TTNtv to the general Dutch population. SMRs were stratified for sex, age, and calendar period. Subgroups were compared with Poisson regression. Similarly, SMRs were calculated in parents of 128 present-day dilated cardiomyopathy probands with TTNtv using the reverse parent-offspring method. Results The TTNtv were established as founder mutations and traced to 18th century ancestors. In 20 522 person-years, overall mortality was not significantly increased (SMR, 1.06; 95% CI, 0.95-1.18; P=0.162). However, mortality was significantly increased in subjects living after 1965 (SMR, 1.27; 95% CI, 1.04-1.53; P=0.009) and aged ≥60 years (SMR, 1.17; 95% CI, 1.01-1.35; P=0.02). The reverse parent-offspring analysis showed overall excess mortality (SMR, 1.26; 95% CI, 1.07-1.48; P=0.003), driven by subjects aged ≥60 years. Conclusions The natural history of the analyzed TTNtv shows a relatively mild disease course with significant excess mortality in elderly patients. With increasing life expectancy, TTNtv-associated morbidity and mortality will likely become more prevalent.

Multiple Small Supernumerary Marker Chromosomes Resulting from Maternal Meiosis I or II Errors.

We present 2 cases with multiple de novo supernumerary marker chromosomes (sSMCs), each derived from a different chromosome. In a prenatal case, we found mosaicism for an sSMC(4), sSMC(6), sSMC(9), sSMC(14) and sSMC(22), while a postnatal case had an sSMC(4), sSMC(8) and an sSMC(11). SNP-marker segregation indicated that the sSMC(4) resulted from a maternal meiosis II error in the prenatal case. Segregation of short tandem repeat markers on the sSMC(8) was consistent with a maternal meiosis I error in the postnatal case. In the latter, a boy with developmental/psychomotor delay, autism, hyperactivity, speech delay, and hypotonia, the sSMC(8) was present at the highest frequency in blood. By comparison to other patients with a corresponding duplication, a minimal region of overlap for the phenotype was identified, with CHRNB3 and CHRNA6 as dosage-sensitive candidate genes. These genes encode subunits of nicotinic acetylcholine receptors (nAChRs). We propose that overproduction of these subunits leads to perturbed component stoichiometries with dominant negative effects on the function of nAChRs, as was shown by others in vitro. With the limitation that in each case only one sSMC could be studied, our findings demonstrate that different meiotic errors lead to multiple sSMCs. We relate our findings to age-related aneuploidy in female meiosis and propose that predivision sister-chromatid separation during meiosis I or II, or both, may generate multiple sSMCs.

Functional assessment of potential splice site variants in arrhythmogenic right ventricular dysplasia/cardiomyopathy.

BACKGROUND: Interpretation of genetic screening results in arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) often is difficult. Pathogenicity of variants with uncertain clinical significance may be predicted by software algorithms. However, functional assessment can unambiguously demonstrate the effect of such variants. OBJECTIVE: The purpose of this study was to perform functional analysis of potential splice site variants in ARVD/C patients. METHODS: Nine variants in desmosomal (PKP2, JUP, DSG2, DSC2) genes with potential RNA splicing effect were analyzed. The variants were found in patients who fulfilled 2010 ARVD/C Task Force Criteria (n = 7) or had suspected ARVD/C (n = 2). Total RNA was isolated from fresh blood samples and subjected to reverse transcriptase polymerase chain reaction. RESULTS: An effect on splicing was predicted by software algorithms for all variants. Of the 9 variants, 5 were intronic and 4 exonic. RNA analysis showed a functional effect on mRNA splicing by exon skipping, generation of new splice sites, or activation of cryptic sites in 6 variants. All 5 intronic variants tested severely impaired splicing. Only 1 of 4 exonic potential splice site variants was shown to have a deleterious effect on splicing. The remaining 3 exonic variants had no detectable effect on splicing, and heterozygous presence in mRNA confirmed biallelic expression. CONCLUSION: Six variants of uncertain clinical significance in the PKP2, JUP, and DSG2 genes showed a deleterious effect on mRNA splicing, indicating these are ARVD/C-related pathogenic splice site mutations. These results highlight the importance of functional assessment of potential splice site variants to improve patient care and facilitate cascade screening.

Clinical and genetic characterization of patients with arrhythmogenic right ventricular dysplasia/cardiomyopathy caused by a plakophilin-2 splice mutation.

OBJECTIVES: Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is characterized by fibrofatty replacement of cardiomyocytes. In around 50% of index patients, a genetic predisposition is demonstrated. The purpose of this study was to examine a plakophilin-2 (PKP2) splice site mutation, c.2489+4A>C, identified in 4 separately ascertained Dutch ARVD/C families. METHODS: Genealogical studies and comprehensive screening of 5 desmosomal genes were undertaken. Reverse transcriptase PCR (RT-PCR) and subsequent sequencing was performed. RESULTS: An A-to-C change (c.2489+4A>C) near the splice donor site of intervening sequence 12 of PKP2 was found in all 4 families. Based on pedigree data and haplotype sharing, a common ancestor should be situated more than 7 generations ago. RT-PCR demonstrated the presence of aberrant messenger RNA. Clinical manifestations ranged from severe disease to nonpenetrance in elderly mutation carriers. CONCLUSIONS: This founder mutation in PKP2 is predicted to lead to the presence of a dysfunctional PKP2 protein, whereas most truncating mutations are expected to lead to loss of protein. Mutation carriers displayed a wide range of disease severity, suggesting that PKP2 mutations alone are not sufficient to cause disease, which results in the variable expression and incomplete penetrance characteristic of ARVD/C mutations.