Variable degree of mosaicism for tetrasomy 18p in phenotypically discordant monozygotic twins-Diagnostic implications

Background: Phenotypically discordant monozygotic twins (PDMZTs) offer a unique opportunity to study post-zygotic genetic variation and provide insights into the linkage between genotype and phenotype. We report a comprehensive analysis of a pair of PDMZTs. Methods: Dysmorphic features and delayed neuro-motor development were observed in the proband, whereas her twin sister was phenotypically normal. Four tissues (blood, skin, hair follicles, and buccal mucosa) from both twins were studied using four complementary methods, including whole-exome sequencing, karyotyping, array CGH, and SNP array. Results: In the proband, tetrasomy 18p affecting all studied tissues except for blood was identified. Karyotyping of fibroblasts revealed isochromosome 18p [i(18p)] in all metaphases. The corresponding analysis of the phenotypically normal sister surprisingly revealed low-level mosaicism (5.4%) for i(18p) in fibroblasts. Conclusion: We emphasize that when mosaicism is suspected, multiple tissues should be studied and we highlight the usefulness of non-invasive sampling of hair follicles and buccal mucosa as a convenient source of non-mesoderm-derived DNA, which complements the analysis of mesoderm using blood. Moreover, low-level mosaic tetrasomy 18p is well tolerated and such low-level mosaicism, readily detected by karyotyping, can be missed by other methods. Finally, mosaicism for low-level tetrasomy 18p might be more common in the general population than it is currently recognized, due to detection limitations

Titin Truncating Variants in Dilated Cardiomyopathy – Prevalence and Genotype-Phenotype Correlations

<div><p><i>TTN</i> gene truncating variants are common in dilated cardiomyopathy (DCM), although data on their clinical significance is still limited. We sought to examine the frequency of truncating variants in <i>TTN</i> in patients with DCM, including familial DCM (FDCM), and to look for genotype-phenotype correlations. Clinical cardiovascular data, family histories and blood samples were collected from 72 DCM probands, mean age of 34 years, 45.8% FDCM. DNA samples were examined by next generation sequencing (NGS) with a focus on the <i>TTN</i> gene. Truncating mutations were followed up by segregation study among family members. We identified 16 <i>TTN</i> truncating variants (<i>TTN</i> trunc) in 17 probands (23.6% of all cases, 30.3% of FDCM, 17.9% of sporadic DCM). During mean 63 months from diagnosis, there was no difference in adverse cardiac events between probands with and without TTN truncating mutations. Among relatives 29 mutation carriers were identified, nine were definitely affected (31%), eight probably affected (27.6%) one possibly affected (3.4%) and eleven were not affected (37.9%). When relatives with all affected statuses were combined, disease penetrance was still incomplete (62.1%) even after exclusion of unaffected relatives under 40 (82%) and was higher in males versus females. In all mutation carriers, during follow-up, 17.4% had major adverse cardiac events, and prognosis was significantly worse in men than in women. In conclusion, <i>TTN</i> truncating variants were observed in nearly one fourth of young DCM patient population, in vast majority without conduction system disease. Incomplete penetrance suggests possible influence of other genetic and/or environmental factors on the course of cardiotitinopathy. Counseling should take into account sex and incomplete penetrance.</p></div

Clinical characteristics of affected and not affected <i>TTN</i> trunc positive relatives.

<p>Clinical characteristics of affected and not affected <i>TTN</i> trunc positive relatives.</p

Kaplan-Meier cumulative survival curves for SAE (HF death, OHT or LVAD) in 46 carriers of <i>TTN</i> truncating variants, males and females, p = 0.018, log-rank test = -2.37.

<p>Kaplan-Meier cumulative survival curves for SAE (HF death, OHT or LVAD) in 46 carriers of <i>TTN</i> truncating variants, males and females, p = 0.018, log-rank test = -2.37.</p