NM_021830.5:c.1422G>C

Variant names:

• chr10-100989822-G-C
• rs111033574
• NM_021830.5:c.1422G>C

Variant summary

Our verdict is Pathogenic. The variant received 17 ACMG points: 17P and 0B. PM1 PM2 PP2 PP3_Strong PP5_Very_Strong

The NM_021830.5(TWNK):​c.1422G>C​(p.Trp474Cys) variant causes a missense change involving the alteration of a conserved nucleotide. The variant was absent in control chromosomes in GnomAD project. In-silico tool predicts a pathogenic outcome for this variant. Variant has been reported in ClinVar as Pathogenic (★★). Another variant affecting the same amino acid position, but resulting in a different missense (i.e. W474S) has been classified as Uncertain significance.

• Frequency: Genomes: not found (cov: 33)
• Consequence: TWNK NM_021830.5 missense
• Clinical Significance: Pathogenic criteria provided, multiple submitters, no conflicts P:2
• Conservation: PhyloP100: 9.97
• Publications: 6 publications found

Genes affected

TWNK (HGNC:1160): (twinkle mtDNA helicase) This gene encodes a hexameric DNA helicase which unwinds short stretches of double-stranded DNA in the 5' to 3' direction and, along with mitochondrial single-stranded DNA binding protein and mtDNA polymerase gamma, is thought to play a key role in mtDNA replication. The protein localizes to the mitochondrial matrix and mitochondrial nucleoids. Mutations in this gene cause infantile onset spinocerebellar ataxia (IOSCA) and progressive external ophthalmoplegia (PEO) and are also associated with several mitochondrial depletion syndromes. Alternative splicing results in multiple transcript variants encoding distinct isoforms.[provided by RefSeq, Aug 2009]

TWNK Gene-Disease associations (from GenCC):

• progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal dominant 3

  Inheritance: AD Classification: STRONG Submitted by: Labcorp Genetics (formerly Invitae)
• mitochondrial DNA depletion syndrome 7 (hepatocerebral type)

  Inheritance: AR Classification: STRONG, SUPPORTIVE Submitted by: Orphanet, Labcorp Genetics (formerly Invitae)
• autosomal dominant progressive external ophthalmoplegia

  Inheritance: AD Classification: SUPPORTIVE Submitted by: Orphanet
• mitochondrial DNA depletion syndrome, hepatocerebrorenal form

  Inheritance: AR Classification: SUPPORTIVE Submitted by: Orphanet
• Perrault syndrome

  Inheritance: AR Classification: SUPPORTIVE Submitted by: Orphanet

ACMG classification

Our verdict: Pathogenic. The variant received 17 ACMG points.

• PM1: In a hotspot region, there are 5 aminoacids with missense pathogenic changes in the window of +-8 aminoacids around while only 0 benign, 7 uncertain in NM_021830.5
• PM2: Very rare variant in population databases, with high coverage
• PP2: Missense variant in the gene, where a lot of missense mutations are associated with disease in ClinVar. The gene has 36 curated pathogenic missense variants (we use a threshold of 10). The gene has 6 curated benign missense variants. Gene score misZ: 1.6071 (below the threshold of 3.09). Trascript score misZ: 2.58 (below the threshold of 3.09). GenCC associations: The gene is linked to progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal dominant 3, mitochondrial DNA depletion syndrome 7 (hepatocerebral type), Perrault syndrome, autosomal dominant progressive external ophthalmoplegia, mitochondrial DNA depletion syndrome, hepatocerebrorenal form.
• PP3: MetaRNN computational evidence supports a deleterious effect, 0.972
• PP5: Variant 10-100989822-G-C is Pathogenic according to our data. Variant chr10-100989822-G-C is described in ClinVar as Pathogenic. ClinVar VariationId is 694438.Status of the report is criteria_provided_multiple_submitters_no_conflicts, 2 stars.

Transcripts

RefSeq

Gene	Transcript	HGVSc	HGVSp	Effect	Exon rank	MANE	Protein	UniProt
TWNK	NM_021830.5	c.1422G>C	p.Trp474Cys	missense_variant	Exon 2 of 5	ENST00000311916.8	NP_068602.2

Ensembl

Gene	Transcript	HGVSc	HGVSp	Effect	Exon rank	TSL	MANE	Protein	Appris	UniProt
TWNK	ENST00000311916.8	c.1422G>C	p.Trp474Cys	missense_variant	Exon 2 of 5	1	NM_021830.5	ENSP00000309595.2

Frequencies

GnomAD3 genomes Cov.: 33

GnomAD4 exome Cov.: 32

GnomAD4 genome Cov.: 33

ClinVar

Significance: Pathogenic

Submissions summary: Pathogenic:2

Revision: criteria provided, multiple submitters, no conflicts

Submissions by phenotype

Progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal dominant 1 Pathogenic:1

Jul 17, 2019

Laboratory of Inherited Metabolic Diseases, Research centre for medical genetics

Significance: Pathogenic

Review Status: criteria provided, single submitter

Collection Method: clinical testing

- -

not provided Pathogenic:1

May 12, 2022

Labcorp Genetics (formerly Invitae), Labcorp

Significance: Pathogenic

Review Status: criteria provided, single submitter

Collection Method: clinical testing

For these reasons, this variant has been classified as Pathogenic. This variant disrupts the p.Trp474 amino acid residue in TWNK. Other variant(s) that disrupt this residue have been determined to be pathogenic (PMID: 18575922). This suggests that this residue is clinically significant, and that variants that disrupt this residue are likely to be disease-causing. Experimental studies have shown that this missense change affects TWNK function (PMID: 18971204, 20659899). Advanced modeling of protein sequence and biophysical properties (such as structural, functional, and spatial information, amino acid conservation, physicochemical variation, residue mobility, and thermodynamic stability) performed at Invitae indicates that this missense variant is expected to disrupt TWNK protein function. ClinVar contains an entry for this variant (Variation ID: 694438). This missense change has been observed in individuals with autosomal dominant progressive external ophthalmoplegia with mitochondrial DNA deletions (PMID: 11431692; Invitae). This variant is not present in population databases (gnomAD no frequency). This sequence change replaces tryptophan, which is neutral and slightly polar, with cysteine, which is neutral and slightly polar, at codon 474 of the TWNK protein (p.Trp474Cys). -

Computational scores

Source: dbNSFP v4.3

Name	Calibrated prediction	Score	Prediction
AlphaMissense	Pathogenic	0.71
BayesDel_addAF	Pathogenic	0.52	D
BayesDel_noAF	Pathogenic	0.51
CADD	Pathogenic	30
DANN	Uncertain	0.99
DEOGEN2	Pathogenic	0.86	.;D;.;.;.
Eigen	Pathogenic	0.92
Eigen_PC	Pathogenic	0.89
FATHMM_MKL	Pathogenic	0.98	D
LIST_S2	Pathogenic	0.97	D;D;D;D;D
M_CAP	Pathogenic	0.38	D
MetaRNN	Pathogenic	0.97	D;D;D;D;D
MetaSVM	Pathogenic	0.91	D
MutationAssessor	Uncertain	2.8	.;M;M;.;.
PhyloP100		10
PROVEAN	Pathogenic	-8.8	.;D;D;.;.
REVEL	Pathogenic	0.81
Sift	Uncertain	0.0010	.;D;D;.;.
Sift4G	Uncertain	0.0020	.;D;D;.;.
Polyphen		1.0	.;D;D;.;.
Vest4		0.96, 0.96
MutPred		0.87		.;Loss of loop (P = 0.0022);Loss of loop (P = 0.0022);.;.;
MVP		0.95
MPC		1.8
ClinPred		1.0	D
GERP RS		6.0
RBP_binding_hub_radar		0.0
RBP_regulation_power_radar		1.7
Varity_R		0.88
gMVP		0.98
Mutation Taster		=0/100	disease causing (ClinVar)

Splicing

Name	Calibrated prediction	Score	Prediction
SpliceAI score (max)		0.0		Details are displayed if max score is > 0.2

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

Other links and lift over

dbSNP: rs111033574; hg19: chr10-102749579;