rs1389777522

Positions:

• chr2-178607428-AT-A

Variant summary

Our verdict is Pathogenic. Variant got 18 ACMG points: 18P and 0B. PVS1 PM2 PP5_Very_Strong

The NM_001267550.2(TTN):​c.53259del​(p.Lys17753AsnfsTer7) variant causes a frameshift change involving the alteration of a non-conserved nucleotide. The variant allele was found at a frequency of 0.00000658 in 152,030 control chromosomes in the GnomAD database, with no homozygous occurrence. Variant has been reported in ClinVar as Likely pathogenic (★★). Variant results in nonsense mediated mRNA decay.

• Frequency: Genomes: 𝑓 0.0000066 (0 hom., cov: 33)
• Consequence: TTN NM_001267550.2 frameshift
• Clinical Significance: Pathogenic/Likely pathogenic criteria provided, multiple submitters, no conflicts P:4
• Conservation: PhyloP100: 2.36

Genes affected

TTN (HGNC:12403): (titin) This gene encodes a large abundant protein of striated muscle. The product of this gene is divided into two regions, a N-terminal I-band and a C-terminal A-band. The I-band, which is the elastic part of the molecule, contains two regions of tandem immunoglobulin domains on either side of a PEVK region that is rich in proline, glutamate, valine and lysine. The A-band, which is thought to act as a protein-ruler, contains a mixture of immunoglobulin and fibronectin repeats, and possesses kinase activity. An N-terminal Z-disc region and a C-terminal M-line region bind to the Z-line and M-line of the sarcomere, respectively, so that a single titin molecule spans half the length of a sarcomere. Titin also contains binding sites for muscle associated proteins so it serves as an adhesion template for the assembly of contractile machinery in muscle cells. It has also been identified as a structural protein for chromosomes. Alternative splicing of this gene results in multiple transcript variants. Considerable variability exists in the I-band, the M-line and the Z-disc regions of titin. Variability in the I-band region contributes to the differences in elasticity of different titin isoforms and, therefore, to the differences in elasticity of different muscle types. Mutations in this gene are associated with familial hypertrophic cardiomyopathy 9, and autoantibodies to titin are produced in patients with the autoimmune disease scleroderma. [provided by RefSeq, Feb 2012]

TTN-AS1 (HGNC:44124): (TTN antisense RNA 1) This gene encodes a non-coding RNA transcribed from the opposite strand to the titin gene. [provided by RefSeq, Aug 2016]

ACMG classification

Verdict is Pathogenic. Variant got 18 ACMG points.

• PVS1: Loss of function variant, product undergoes nonsense mediated mRNA decay. LoF is a known mechanism of disease.
• PM2: Very rare variant in population databases, with high coverage
• PP5: Variant 2-178607428-AT-A is Pathogenic according to our data. Variant chr2-178607428-AT-A is described in ClinVar as [Likely_pathogenic]. Clinvar id is 466636.Status of the report is criteria_provided_multiple_submitters_no_conflicts, 2 stars. Variant chr2-178607428-AT-A is described in Lovd as [Pathogenic].

Transcripts

RefSeq

Gene	Transcript	HGVSc	HGVSp	Effect	#exon/exons	MANE	Protein	UniProt
TTN	NM_001267550.2	c.53259del	p.Lys17753AsnfsTer7	frameshift_variant	277/363	ENST00000589042.5	NP_001254479.2

Ensembl

Gene	Transcript	HGVSc	HGVSp	Effect	#exon/exons	TSL	MANE	Protein	Appris	UniProt
TTN	ENST00000589042.5	c.53259del	p.Lys17753AsnfsTer7	frameshift_variant	277/363	5	NM_001267550.2	ENSP00000467141	P1
TTN-AS1	ENST00000659121.1	n.502+9750del		intron_variant, non_coding_transcript_variant

Frequencies

GnomAD3 genomes AF: 0.00000658 AC: 1 AN: 152030 Hom.: 0 Cov.: 33

Gnomad AFR AF: 0.00

Gnomad AMI AF: 0.00

Gnomad AMR AF: 0.00

Gnomad ASJ AF: 0.00

Gnomad EAS AF: 0.00

Gnomad SAS AF: 0.00

Gnomad FIN AF: 0.00

Gnomad MID AF: 0.00

Gnomad NFE AF: 0.0000147

Gnomad OTH AF: 0.00

GnomAD4 exome Cov.: 31

GnomAD4 genome AF: 0.00000658 AC: 1 AN: 152030 Hom.: 0 Cov.: 33 AF XY: 0.0000135 AC XY: 1 AN XY: 74244

Gnomad4 AFR AF: 0.00

Gnomad4 AMR AF: 0.00

Gnomad4 ASJ AF: 0.00

Gnomad4 EAS AF: 0.00

Gnomad4 SAS AF: 0.00

Gnomad4 FIN AF: 0.00

Gnomad4 NFE AF: 0.0000147

Gnomad4 OTH AF: 0.00

Bravo AF: 0.00000378

ClinVar

Significance: Pathogenic/Likely pathogenic

Submissions summary: Pathogenic:4

Revision: criteria provided, multiple submitters, no conflicts

Submissions by phenotype

Autosomal recessive limb-girdle muscular dystrophy type 2J;C1858763:Dilated cardiomyopathy 1G Pathogenic:1

Likely pathogenic, criteria provided, single submitter

clinical testing

Labcorp Genetics (formerly Invitae), Labcorp

Aug 22, 2022

In summary, the currently available evidence indicates that the variant is pathogenic, but additional data are needed to prove that conclusively. Therefore, this variant has been classified as Likely Pathogenic. This variant is located in the A band of TTN (PMID: 25589632). Truncating variants in this region are significantly overrepresented in patients affected with dilated cardiomyopathy (PMID: 25589632). Truncating variants in this region have also been reported in individuals affected with autosomal recessive centronuclear myopathy (PMID: 23975875). ClinVar contains an entry for this variant (Variation ID: 466636). This variant is also known as c.26640delA. This premature translational stop signal has been observed in individual(s) with dilated cardiomyopathy (PMID: 25163546). This variant is not present in population databases (gnomAD no frequency). This sequence change creates a premature translational stop signal (p.Lys17753Asnfs*7) in the TTN gene. While this is not anticipated to result in nonsense mediated decay, it is expected to create a truncated TTN protein. -

not provided Pathogenic:1

Likely pathogenic, no assertion criteria provided

provider interpretation

Stanford Center for Inherited Cardiovascular Disease, Stanford University

Jun 14, 2017

Given that this is a truncating variant in the A-band, has been reported in another case with DCM and is absent from population databases, we consider this variant likely disease-causing, and we do feel it is suitable for assessing risk in healthy relatives ("predictive genetic testing"). The variant has been seen in at least 1 unrelated case of DCM (not including this patient's family). There is weak case data: this variant is reported in 1 out of 639 northern European patients with DCM (Haas et al, 2015). It is known as c.26640delA or p.Lys8880fs in this paper. The presence of truncating TTN variants in controls indicates that not all such variants can be presumed pathogenic. TTN encodes titin (also known as connectin), the largest protein in humans; titin plays a critical role in the elastic properties of the sarcomere. Two titin molecules span the sarcomere, anchored at the Z-line and M-line. TTN variants have been shown by Herman et al. (2012) to be present in 27% of patients with familial dilated cardiomyopathy (DCM) versus approximately 1% of patients with hypertrophic cardiomyopathy (HCM) and 3% of controls (indicating that not all such variants are disease-causing). In addition, Norton et al. (2013) showed that not all truncating variants in TTN segregate with disease (DCM) in affected families—pointing to the difficulty in determining variant pathogenicity for a specific truncating variant. Norton et al., identified 6 TTN truncating variants carried by individuals affected with DCM in 7 of 17 DCM families (logarithm of odds, 2.99); 2 of these 7 families also had novel missense variants that segregated with disease. Two additional novel truncating TTN variants did not segregate with DCM. Roberts et al, 2015 performed cardiac phenotyping of 5267 affected and unaffected individuals as well as TTN DNA sequencing and RNA and protein analyses heart tissue. They have a resource at cardiodb.org/titin that lists the relative inclusion of TTN exons in different isoforms and provides information to guide assessment of pathogenicity of specific truncation variants in the gene. Variants located in the A-band and present in cardiac isoforms of the protein were enriched in DCM patients versus controls. The genomic coordinates for this variant are chr2:179472156. LRG exon number is 278, N2BA transcript is 277. It is located in the A-band, 100% spliced in, in an IG-like domain. Another variant in the same exon has previously been reported in Herman et al 2012 in their DCM population. The lysine at codon 17753 is completely conserved across species, and neighboring amino acids are highly conserved as well. There is no variation at codon 17753 listed in the Genome Aggregation Consortium Dataset (gnomAD; http://gnomad.broadinstitute.org/), which currently includes variant calls on >140,000 unrelated individuals of African, Asian, European, Latino, and Ashkenazi descent. The average coverage at that site in gnomAD is 85x. -

Primary familial dilated cardiomyopathy Pathogenic:1

Likely pathogenic, criteria provided, single submitter

clinical testing

Women's Health and Genetics/Laboratory Corporation of America, LabCorp

Jan 22, 2024

Variant summary: TTN c.45555delA (p.Lys15185AsnfsX7) results in a premature termination codon, predicted to cause a truncation of the encoded protein or absence of the protein due to nonsense mediated decay, which are commonly known mechanisms for disease. Nonsense, frameshift, and canonical splice-site variants in TTN are strongly associated with DCM when they affect exons encoding for the A-band region (PMIDs: 22335739, 24503780) and/or exons constitutively expressed (proportion spliced in [PSI]>0.9) in the primary cardiac isoforms (PMIDs: 25589632, 31216868, 32964742, 27869827), which is the case forthis variant (I band with a PSI score of 100%). The variant was absent in 248384 control chromosomes. c.45555delA has been reported in the literature in unspecified individuals affected with Dilated Cardiomyopathy (examples, Haas_2015, Jansweijer_2017). These data do not allow any conclusion about variant significance. To our knowledge, no experimental evidence demonstrating an impact on protein function has been reported. The following publications have been ascertained in the context of this evaluation (PMID: 25163546, 27813223). ClinVar contains an entry for this variant (Variation ID: 466636). Based on the evidence outlined above, the variant was classified as likely pathogenic. -

Cardiovascular phenotype Pathogenic:1

Pathogenic, criteria provided, single submitter

clinical testing

Ambry Genetics

Feb 16, 2024

The c.26064delA pathogenic mutation, located in coding exon 104 of the TTN gene, results from a deletion of one nucleotide at nucleotide position 26064, causing a translational frameshift with a predicted alternate stop codon (p.K8688Nfs*7). This exon is located in the A-band region of the N2-B isoform of the titin protein and is constitutively expressed in TTN transcripts (percent spliced in or PSI 100%). This variant (also referred to as NM_133437:c.26640delA, p.K8880fs) has been detected in individuals reported to have dilated cardiomyopathy (Haas J et al. Eur Heart J, 2015 May;36:1123-35a; Ambry internal data). This variant is considered to be rare based on population cohorts in the Genome Aggregation Database (gnomAD). This alteration is expected to result in loss of function by premature protein truncation or nonsense-mediated mRNA decay. While truncating variants in TTN are present in 1-3% of the general population, truncating variants in the A-band are the most common cause of dilated cardiomyopathy (DCM) (Herman DS et al. N. Engl. J. Med., 2012 Feb;366:619-28; Roberts AM et al. Sci Transl Med, 2015 Jan;7:270ra6). TTN truncating variants encoded in constitutive exons (PSI >90%) have been found to be significantly associated with DCM regardless of their position in titin (Schafer S et al. Nat. Genet., 2017 01;49:46-53). Based on the supporting evidence, this alteration is interpreted as a disease-causing mutation. -

Computational scores

Source: dbNSFP v4.3

Splicing

Name	Calibrated prediction	Score	Prediction
SpliceAI score (max)		0.090		Details are displayed if max score is > 0.2

Find out detailed SpliceAI scores and Pangolin per-transcript scores at spliceailookup.broadinstitute.org

Publications

LitVar

Other links and lift over

dbSNP: rs1389777522; hg19: chr2-179472155;