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TK2-Related Mitochondrial DNA Depletion Syndrome via the TK2 Gene

  • Summary and Pricing
  • Clinical Features and Genetics
  • Citations
  • Methods
  • Ordering/Specimens
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TEST METHODS

NGS Sequencing

Test Code Test Copy GenesIndividual Gene PriceCPT Code Copy CPT Codes
7323 TK2$690.00 81405 Add to Order
Pricing Comment

Our most cost-effective testing approach is NextGen sequencing with Sanger sequencing supplemented as needed to ensure sufficient coverage and to confirm NextGen calls that are pathogenic, likely pathogenic or of uncertain significance. If, however, full gene Sanger sequencing only is desired (for purposes of insurance billing or STAT turnaround time for example), please see link below for Test Code, pricing, and turnaround time information.

For Sanger Sequencing click here.
Targeted Testing

For ordering targeted known variants, please proceed to our Targeted Variants landing page.

Turnaround Time

The great majority of tests are completed within 28 days.

Clinical Sensitivity

In a small cohort of 13 patients, biallelic pathogenic variants in the TK2 gene were found in 46% (6/13) of patients with the myopathic form of MDS (Spinazzola et al. 2009).

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Deletion/Duplication Testing via aCGH

Test Code Test Copy GenesIndividual Gene PriceCPT Code Copy CPT Codes
600 TK2$690.00 81479 Add to Order
Pricing Comment

# of Genes Ordered

Total Price

1

$690

2

$730

3

$770

4-10

$840

11-30

$1,290

31-100

$1,670

Over 100

Call for quote

Turnaround Time

The great majority of tests are completed within 28 days.

Clinical Sensitivity

Gross deletions or duplications in the TK2 gene appear to be a rare cause of mtDNA depletion syndrome. Only two families have been reported to harbor gross deletions or complex rearrangements in the TK2 gene to date (Zhang et al. 2010; Stewart et al. 2011).

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Clinical Features

Mitochondrial DNA (mtDNA) Depletion Syndromes (MDSs) are a group of clinically and genetically heterogeneous diseases characterized by a quantitative abnormality of the mitochondrial genome in specific tissues (Suomalainen and Isohanni 2010; El-Hattab et al. 2013). The myopathic form of MDS has a wide clinical spectrum, with a typical age of onset that ranges from infancy to early childhood (Chanprasert et al. 2012).

The most severe presentation of TK2-related MDS is characterized by infantile myopathy with motor regression, resulting in early death from respiratory failure (Oskoui et al. 2006). Clinical features may include hypotonia, proximal muscle weakness, decreased physical stamina, poor feeding, and respiratory difficulties. Other severe phenotypes have been reported, including a spinal muscular atrophy-like presentation, rapidly progressive proximal muscle weakness with encephalopathy and epilepsy, hepatic involvement with elevated transaminases in liver tissue, and progressive myopathy with dystrophic changes (Oskoui et al. 2006; Lesko et al. 2010; Zhang et al. 2010; Collins et al. 2009).

In contrast, milder phenotypes of this disease have also been described, such as late-onset proximal muscle weakness and adult-onset progressive myopathy (Cámara et al. 2015). Although much less frequently reported, TK2 defects may cause autosomal recessive progressive external ophthalmoplegia (arPEO), which is characterized by multiple mtDNA deletions in skeletal muscle (Tyynismaa et al. 2012). This adult-onset disease features ptosis and ophthalmoparesis resulting from progressive weakness of the extraocular eye muscles.

Genetics

The myopathic form of MDS is an autosomal recessive disorder caused by defects in the TK2 gene (Saada et al. 2001). TK2 has 10 exons that encode thymidine kinase, a mitochondrial deoxyribonucleoside kinase required for mtDNA synthesis. Pathogenic missense, nonsense, and splicing site variants, in addition to small deletion and insertion events, have been found across the whole coding region of TK2. Arg130Trp is a pathogenic Finnish founder variant associated with one of the most severe phenotypes (Götz et al. 2008, referred to as R172W). One large intragenic deletion (encompassing exons 1 and 2 of TK2) and one complex rearrangement have also been documented (Zhang et al. 2010; Stewart et al. 2011). Compound heterozygous TK2 pathogenic variants have also been reported to result in a rare form of adult-onset autosomal recessive progressive external ophthalmoplegia (arPEO), which is more frequently caused by defects in the POLG gene (Tyynismaa et al. 2012). Additionally, pathogenic variants in RNASEH1 or RRM2B have also been reported as a rare cause of arPEO (Reyes et al. 2015; Pitceathly et al. 2012).

Testing Strategy

For this Next Generation Sequencing (NGS) test, sequencing is accomplished by capturing specific regions with an optimized solution-based hybridization kit, followed by massively parallel sequencing of the captured DNA fragments. Additional Sanger sequencing is performed for regions not captured or with insufficient number of sequence reads. All reported pathogenic, likely pathogenic, and variants of uncertain significance are confirmed by Sanger sequencing.

For Sanger sequencing, polymerase chain reaction (PCR) is used to amplify targeted regions. After purification of the PCR products, cycle sequencing is carried out using the ABI Big Dye Terminator v.3.0 kit. PCR products are resolved by electrophoresis on an ABI 3730xl capillary sequencer. In nearly all cases, cycle sequencing is performed separately in both the forward and reverse directions.

This test provides full coverage of all coding exons of the TK2 gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing.

Indications for Test

Candidates for this test are patients with the myopathic form of MDS. Testing is also indicated for family members of patients who have known TK2 mutations. Screening for TK2 mutations is also recommended in patients with late-onset PEO with multiple mtDNA deletions (Tyynismaa et al. 2012).

Gene

Official Gene Symbol OMIM ID
TK2 188250
Inheritance Abbreviation
Autosomal Dominant AD
Autosomal Recessive AR
X-Linked XL
Mitochondrial MT

Related Tests

Name
Autism Spectrum Disorders and Intellectual Disability (ASD-ID) Comprehensive Sequencing Panel with CNV Detection
Metabolic Myopathies, Rhabdomyolysis and Exercise Intolerance Sequencing Panel
Mitochondrial Genome Maintenance/Integrity Nuclear Genes Sequencing Panel

CONTACTS

Genetic Counselors
Geneticist
Citations
  • Cámara Y. et al. 2015. Neurology. 84:2286-8. PubMed ID: 25948719
  • Chanprasert S. et al. 2012. TK2-related mitochondrial DNA depletion syndrome, myopathic form. In: Pagon RA, Adam MP, Bird TD, Dolan CR, Fong C-T, Smith RJ, and Stephens K, editors. GeneReviews(®), Seattle (WA): University of Washington, Seattle. PubMed ID: 23230576
  • Collins J. et al. 2009. Neuromuscular Disorders. 19:784-7. PubMed ID: 19736010
  • El-Hattab A.W. and Scaglia F. 2013. Neurotherapeutics. 10:186-98. PubMed ID: 23385875
  • Götz A. et al. 2008. Brain. 131:2841-50. PubMed ID: 18819985
  • Lesko N. et al. 2010. Neuromuscular Disorders. 20:198-203. PubMed ID: 20083405
  • Oskoui M. et al. 2006. Archives of Neurology. 63:1122-6. PubMed ID: 16908738
  • Pitceathly et al. 2012. Brain. 135:3392-403. PubMed ID: 23107649
  • Reyes, A. et al. 2015. American Journal of Human Genetics. 97:186-93. PubMed ID: 26094573
  • Saada A. et al. 2001. Nature Genetics. 29:342-4. PubMed ID: 11687801
  • Soumalainen A. and Isohanni P. 2010. Neuromuscular Disorders. 20:429-37. PubMed ID: 20444604
  • Spinazzola A. et al. 2009. Journal of Inherited Metabolic Diseases. 32:143-58. PubMed ID: 19125351
  • Stewart J.D. et al. 2011. Biochimica et Biophysica Acta. 1812:321-5. PubMed ID: 21138766
  • Tyynismaa H. 2012. et al. Human Molecular Genetics. 21:66-75. PubMed ID: 21937588
  • Zhang S. 2010. et al. Molecular Genetics and Metabolism. 99:53-7. PubMed ID: 19815440
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TEST METHODS

NextGen Sequencing using PG-Select Capture Probes

Test Procedure

We use a combination of Next Generation Sequencing (NGS) and Sanger sequencing technologies to cover the full coding regions of the listed genes plus ~20 bases of non-coding DNA flanking each exon.  As required, genomic DNA is extracted from the patient specimen.  For NGS, patient DNA corresponding to these regions is captured using an optimized set of DNA hybridization probes.  Captured DNA is sequenced using Illumina’s Reversible Dye Terminator (RDT) platform (Illumina, San Diego, CA, USA).  Regions with insufficient coverage by NGS are covered by Sanger sequencing.  All pathogenic, likely pathogenic, or variants of uncertain significance are confirmed by Sanger sequencing.

For Sanger sequencing, Polymerase Chain Reaction (PCR) is used to amplify targeted regions.  After purification of the PCR products, cycle sequencing is carried out using the ABI Big Dye Terminator v.3.0 kit.  PCR products are resolved by electrophoresis on an ABI 3730xl capillary sequencer.  In nearly all cases, cycle sequencing is performed separately in both the forward and reverse directions.

Patient DNA sequence is aligned to the genomic reference sequence for the indicated gene region(s). All differences from the reference sequences (sequence variants) are assigned to one of five interpretation categories, listed below, per ACMG Guidelines (Richards et al. 2015).

(1) Pathogenic Variants
(2) Likely Pathogenic Variants
(3) Variants of Uncertain Significance
(4) Likely Benign Variants
(5) Benign, Common Variants

Human Genome Variation Society (HGVS) recommendations are used to describe sequence variants (http://www.hgvs.org).  Rare variants and undocumented variants are nearly always classified as likely benign if there is no indication that they alter protein sequence or disrupt splicing.

Analytical Validity

As of March 2016, 6.36 Mb of sequence (83 genes, 1557 exons) generated in our lab was compared between Sanger and NextGen methodologies. We detected no differences between the two methods. The comparison involved 6400 total sequence variants (differences from the reference sequences). Of these, 6144 were nucleotide substitutions and 256 were insertions or deletions. About 65% of the variants were heterozygous and 35% homozygous. The insertions and deletions ranged in length from 1 to over 100 nucleotides.

In silico validation of insertions and deletions in 20 replicates of 5 genes was also performed. The validation included insertions and deletions of lengths between 1 and 100 nucleotides. Insertions tested in silico: 2200 between 1 and 5 nucleotides, 625 between 6 and 10 nucleotides, 29 between 11 and 20 nucleotides, 25 between 21 and 49 nucleotides, and 23 at or greater than 50 nucleotides, with the largest at 98 nucleotides. All insertions were detected. Deletions tested in silico: 1813 between 1 and 5 nucleotides, 97 between 6 and 10 nucleotides, 32 between 11 and 20 nucleotides, 20 between 21 and 49 nucleotides, and 39 at or greater than 50 nucleotides, with the largest at 96 nucleotides. All deletions less than 50 nucleotides in length were detected, 13 greater than 50 nucleotides in length were missed. Our standard NextGen sequence variant calling algorithms are generally not capable of detecting insertions (duplications) or heterozygous deletions greater than 100 nucleotides. Large homozygous deletions appear to be detectable.   

Analytical Limitations

Interpretation of the test results is limited by the information that is currently available.  Better interpretation should be possible in the future as more data and knowledge about human genetics and this specific disorder are accumulated.

When Sanger sequencing does not reveal any difference from the reference sequence, or when a sequence variant is homozygous, we cannot be certain that we were able to detect both patient alleles.  Occasionally, a patient may carry an allele which does not amplify, due to a large deletion or insertion.   In these cases, the report will contain no information about the second allele.  Our Sanger and NGS Sequencing tests are generally not capable of detecting Copy Number Variants (CNVs).

We sequence all coding exons for each given transcript, plus ~20 bp of flanking non-coding DNA for each exon.  Test reports contain no information about other portions of the gene, such as regulatory domains, deep intronic regions or any currently uncharacterized alternative exons.

In most cases, we are unable to determine the phase of sequence variants.  In particular, when we find two likely causative mutations for recessive disorders, we cannot be certain that the mutations are on different alleles.

Our ability to detect minor sequence variants due to somatic mosaicism is limited.  Sequence variants that are present in less than 50% of the patient’s nucleated cells may not be detected.

Runs of mononucleotide repeats (eg (A)n or (T)n) with n >8 in the reference sequence are generally not analyzed because of strand slippage during PCR.

Unless otherwise indicated, DNA sequence data is obtained from a specific cell-type (usually leukocytes from whole blood).   Test reports contain no information about the DNA sequence in other cell-types.

We cannot be certain that the reference sequences are correct.

Rare, low probability interpretations of sequencing results, such as for example the occurrence of de novo mutations in recessive disorders, are generally not included in the reports.

We have confidence in our ability to track a specimen once it has been received by PreventionGenetics.  However, we take no responsibility for any specimen labeling errors that occur before the sample arrives at PreventionGenetics.

Deletion/Duplication Testing Via Array Comparative Genomic Hybridization

Test Procedure

Equal amounts of genomic DNA from the patient and a gender matched reference sample are amplified and labeled with Cy3 and Cy5 dyes, respectively. To prevent any sample cross contamination, a unique sample tracking control is added into each patient sample. Each labeled patient product is then purified, quantified, and combined with the same amount of reference product. The combined sample is loaded onto the designed array and hybridized for at least 22-42 hours at 65°C. Arrays are then washed and scanned immediately with 2.5 µM resolution. Only data for the gene(s) of interest for each patient are extracted and analyzed.

Analytical Validity

PreventionGenetics' high density gene-centric custom designed aCGH enables the detection of relatively small deletions and duplications within a single exon of a given gene or deletions and duplications encompassing the entire gene. PreventionGenetics has established and verified this test's accuracy and precision.

Analytical Limitations

Our dense probe coverage may allow detection of deletions/duplications down to 100 bp; however due to limitations and probe spacing this cannot be guaranteed across all exons of all genes. Therefore, some copy number changes smaller than 100-300 bp within a targeted large exon may not be detected by our array.

This array may not detect deletions and duplications present at low levels of mosaicism or those present in genes that have pseudogene copies or repeats elsewhere in the genome.

aCGH will not detect balanced translocations, inversions, or point mutations that may be responsible for the clinical phenotype.

Breakpoints, if occurring outside the targeted gene, may be hard to define.

The sensitivity of this assay may be reduced when DNA is extracted by an outside laboratory.

Order Kits

Ordering Options


myPrevent - Online Ordering
  • The test can be added to your online orders in the Summary and Pricing section.
  • Once the test has been added log in to myPrevent to fill out an online requisition form.
REQUISITION FORM
  • A completed requisition form must accompany all specimens.
  • Billing information along with specimen and shipping instructions are within the requisition form.
  • All testing must be ordered by a qualified healthcare provider.

SPECIMEN TYPES
WHOLE BLOOD

(Delivery accepted Monday - Saturday)

  • Collect 3 ml -5 ml (5 ml preferred) of whole blood in EDTA (purple top tube) or ACD (yellow top tube). For Test #500-DNA Banking only, collect 10 ml -20 ml of whole blood.
  • For small babies, we require a minimum of 1 ml of blood.
  • Only one blood tube is required for multiple tests.
  • Ship blood tubes at room temperature in an insulated container. Do not freeze blood.
  • During hot weather, include a frozen ice pack in the shipping container. Place a paper towel or other thin material between the ice pack and the blood tube.
  • In cold weather, include an unfrozen ice pack in the shipping container as insulation.
  • At room temperature, blood specimen is stable for up to 48 hours.
  • If refrigerated, blood specimen is stable for up to one week.
  • Label the tube with the patient name, date of birth and/or ID number.

DNA

(Delivery accepted Monday - Saturday)

  • Send in screw cap tube at least 5 µg -10 µg of purified DNA at a concentration of at least 20 µg/ml for NGS and Sanger tests and at least 5 µg of purified DNA at a concentration of at least 100 µg/ml for gene-centric aCGH, MLPA, and CMA tests, minimum 2 µg for limited specimens.
  • For requests requiring more than one test, send an additional 5 µg DNA per test ordered when possible.
  • DNA may be shipped at room temperature.
  • Label the tube with the composition of the solute, DNA concentration as well as the patient’s name, date of birth, and/or ID number.
  • We only accept genomic DNA for testing. We do NOT accept products of whole genome amplification reactions or other amplification reactions.

CELL CULTURE

(Delivery preferred Monday - Thursday)

  • PreventionGenetics should be notified in advance of arrival of a cell culture.
  • Culture and send at least two T25 flasks of confluent cells.
  • Some panels may require additional flasks (dependent on size of genes, amount of Sanger sequencing required, etc.). Multiple test requests may also require additional flasks. Please contact us for details.
  • Send specimens in insulated, shatterproof container overnight.
  • Cell cultures may be shipped at room temperature or refrigerated.
  • Label the flasks with the patient name, date of birth, and/or ID number.
  • We strongly recommend maintaining a local back-up culture. We do not culture cells.
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