16-66541919-G-A

Position:

• chr16-66541919-G-A

Variant summary

Our verdict is Uncertain significance. Variant got 5 ACMG points: 5P and 0B. PM2 PP3_Moderate PP5

The NM_001272050.2(TK2):​c.-101C>T variant causes a 5 prime UTR premature start codon gain change. The variant allele was found at a frequency of 0.00000205 in 1,461,868 control chromosomes in the GnomAD database, with no homozygous occurrence. In-silico tool predicts a pathogenic outcome for this variant. Variant has been reported in ClinVar as Conflicting classifications of pathogenicity (no stars).

• Frequency:
  • Genomes: not found (cov: 32)
  • Exomes 𝑓: 0.0000021 (0 hom.)
• Consequence: TK2 NM_001272050.2 5_prime_UTR_premature_start_codon_gain
• Clinical Significance: Conflicting classifications of pathogenicity criteria provided, conflicting classifications P:3 U:2
• Conservation: PhyloP100: 4.41

Genes affected

TK2 (HGNC:11831): (thymidine kinase 2) This gene encodes a deoxyribonucleoside kinase that specifically phosphorylates thymidine, deoxycytidine, and deoxyuridine. The encoded enzyme localizes to the mitochondria and is required for mitochondrial DNA synthesis. Mutations in this gene are associated with a myopathic form of mitochondrial DNA depletion syndrome. Alternate splicing results in multiple transcript variants encoding distinct isoforms, some of which lack transit peptide, so are not localized to mitochondria. [provided by RefSeq, Dec 2012]

TK2 (HGNC:):

ACMG classification

Verdict is Uncertain_significance. Variant got 5 ACMG points.

• PM2: Very rare variant in population databases, with high coverage
• PP3: MetaRNN computational evidence supports a deleterious effect, 0.938
• PP5: Variant 16-66541919-G-A is Pathogenic according to our data. Variant chr16-66541919-G-A is described in ClinVar as [Conflicting_classifications_of_pathogenicity]. Clinvar id is 38979.We mark this variant Likely_pathogenic, oryginal submissions are: {Uncertain_significance=2, Pathogenic=1, Likely_pathogenic=2}.

Transcripts

RefSeq

Gene	Transcript	HGVSc	HGVSp	Effect	#exon/exons	MANE	Protein	UniProt
TK2	NM_004614.5	c.191C>T	p.Thr64Met	missense_variant	3/10	ENST00000544898.6	NP_004605.4

Ensembl

Gene	Transcript	HGVSc	HGVSp	Effect	#exon/exons	TSL	MANE	Protein	Appris	UniProt
TK2	ENST00000544898.6	c.191C>T	p.Thr64Met	missense_variant	3/10	1	NM_004614.5	ENSP00000440898.2

Frequencies

GnomAD3 genomes Cov.: 32

GnomAD3 exomes AF: 0.00000795 AC: 2 AN: 251476 Hom.: 0 AF XY: 0.00 AC XY: 0 AN XY: 135910

Gnomad AFR exome AF: 0.00

Gnomad AMR exome AF: 0.0000289

Gnomad ASJ exome AF: 0.00

Gnomad EAS exome AF: 0.00

Gnomad SAS exome AF: 0.00

Gnomad FIN exome AF: 0.00

Gnomad NFE exome AF: 0.00000879

Gnomad OTH exome AF: 0.00

GnomAD4 exome AF: 0.00000205 AC: 3 AN: 1461868 Hom.: 0 Cov.: 31 AF XY: 0.00000138 AC XY: 1 AN XY: 727228

Gnomad4 AFR exome AF: 0.00

Gnomad4 AMR exome AF: 0.0000224

Gnomad4 ASJ exome AF: 0.00

Gnomad4 EAS exome AF: 0.00

Gnomad4 SAS exome AF: 0.00

Gnomad4 FIN exome AF: 0.0000187

Gnomad4 NFE exome AF: 8.99e-7

Gnomad4 OTH exome AF: 0.00

GnomAD4 genome Cov.: 32

Alfa AF: 0.0000307 Hom.: 0

Bravo AF: 0.00000378

ExAC AF: 0.00000824 AC: 1

ClinVar

Significance: Conflicting classifications of pathogenicity

Submissions summary: Pathogenic:3 Uncertain:2

Revision: criteria provided, conflicting classifications

Submissions by phenotype

not provided Pathogenic:3

Likely pathogenic, criteria provided, single submitter	clinical testing	Clinical Genetics Laboratory, Skane University Hospital Lund	Aug 09, 2023	- -
Likely pathogenic, criteria provided, single submitter	clinical testing	GeneDx	Aug 30, 2023	Not observed at significant frequency in large population cohorts (gnomAD); In silico analysis supports that this missense variant has a deleterious effect on protein structure/function; This variant is associated with the following publications: (PMID: 15907288, 33486010, 29602790) -
Pathogenic, criteria provided, single submitter	clinical testing	Labcorp Genetics (formerly Invitae), Labcorp	Oct 08, 2023	This sequence change replaces threonine, which is neutral and polar, with methionine, which is neutral and non-polar, at codon 64 of the TK2 protein (p.Thr64Met). This variant is present in population databases (rs281865487, gnomAD 0.003%). This missense change has been observed in individual(s) with mitochondrial DNA depletion syndrome (PMID: 15907288, 33486010). In at least one individual the data is consistent with being in trans (on the opposite chromosome) from a pathogenic variant. It has also been observed to segregate with disease in related individuals. ClinVar contains an entry for this variant (Variation ID: 38979). 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 TK2 protein function. For these reasons, this variant has been classified as Pathogenic. -

not specified Uncertain:1

Uncertain significance, criteria provided, single submitter

clinical testing

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

May 30, 2024

Variant summary: TK2 c.191C>T (p.Thr64Met) results in a non-conservative amino acid change located in the Deoxynucleoside kinase domain (IPR031314) of the encoded protein sequence. Five of five in-silico tools predict a damaging effect of the variant on protein function. The variant allele was found at a frequency of 8e-06 in 251476 control chromosomes. c.191C>T has been reported in the literature in compound heterozygous individuals affected with Mitochondrial DNA Depletion Syndrome - TK2 Related (Bychkov_2021, Tulinius_2005). These data indicate that the variant may be associated with disease. 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: 33486010, 29602790, 15907288). ClinVar contains an entry for this variant (Variation ID: 38979). Based on the evidence outlined above, the variant was classified as VUS-possibly pathogenic. -

Mitochondrial DNA depletion syndrome, myopathic form Uncertain:1

Uncertain significance, criteria provided, single submitter

clinical testing

Illumina Laboratory Services, Illumina

May 05, 2017

The TK2 c.191C>T (p.Thr64Met) variant has been reported in at least two studies and is found in two affected siblings with the variant in a compound heterozygous state with a second missense variant (Tulinius et al. 2005; Chanprasert et al. 2013). The siblings were diagnosed with a myopathic form of mitochondrial DNA depletion syndrome, and presented with hypotonia, muscle weakness, and less than 10% mtDNA content in skeletal muscle. The p.Thr64Met variant was absent from 100 control alleles (Tulinius et al. 2005). This variant is reported at a frequency of 0.00003 in the Latino population of the Genome Aggregation Database, but this is based on one allele so the variant is presumed to be rare. The evidence for the p.Thr64Met variant is limited and it is therefore classified as a variant of unknown significance but suspicious for pathogenicity for mitochondrial DNA depletion syndrome. This variant was observed by ICSL as part of a predisposition screen in an ostensibly healthy population. -

Computational scores

Source: dbNSFP v4.3

Name	Calibrated prediction	Score	Prediction
AlphaMissense	Pathogenic	0.79
BayesDel_addAF	Pathogenic	0.50	D
BayesDel_noAF	Pathogenic	0.57
CADD	Pathogenic	33
DANN	Pathogenic	1.0
DEOGEN2	Pathogenic	0.98	D;.;D;.;.;D;.
Eigen	Pathogenic	1.0
Eigen_PC	Pathogenic	0.94
FATHMM_MKL	Uncertain	0.97	D
LIST_S2	Pathogenic	0.99	D;D;.;D;D;D;D
M_CAP	Pathogenic	0.34	D
MetaRNN	Pathogenic	0.94	D;D;D;D;D;D;D
MetaSVM	Pathogenic	1.1	D
MutationAssessor	Pathogenic	3.6	H;H;H;.;.;.;.
PrimateAI	Uncertain	0.64	T
PROVEAN	Pathogenic	-5.3	.;.;.;.;D;D;.
REVEL	Pathogenic	0.91
Sift	Pathogenic	0.0	.;.;.;.;D;D;.
Sift4G	Pathogenic	0.0010	D;D;D;D;D;D;D
Polyphen		1.0	D;.;D;.;D;.;.
Vest4		0.88
MutPred		0.80		.;.;.;Loss of methylation at K105 (P = 0.0303);.;.;Loss of methylation at K105 (P = 0.0303);
MVP		1.0
MPC		1.1
ClinPred		0.99	D
GERP RS		6.1
RBP_binding_hub_radar		0.0
RBP_regulation_power_radar		1.7
Varity_R		0.86
gMVP		0.90

Splicing

Name	Calibrated prediction	Score	Prediction
SpliceAI score (max)		0.040		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: rs281865487; hg19: chr16-66575822;