Mitochondrial disorders are a frequent cause of genetic disease . They comprise a clinically heterogeneous group of diseases caused by mutations of either nuclear or mitochondrial DNA (mtDNA) which may result in decreased cellular energy production due to a dysfunctional mitochondrial respiratory chain. This sequencing assay is available to detect mutations in the mtDNA genome. Mutations in nuclear genes with mitochondrial function will not be detected by this analysis.
Clinical presentation of mtDNA disorders is variable. Most involve multiple organ systems and frequently present with neurologic and myopathic symptoms, which may be intermittent, but disorders may be confined to one organ such as the eye in Leber hereditary optic neuropathy (LHON). Age of onset also varies though symptoms may frequently develop in childhood.
Common clinical features of mtDNA disorders include external ophthalmoplegia, ptosis, cardiomyopathy, diabetes mellitus, sensorineural deafness, optic atrophy, pigmentary retinopathy, myopathy and exercise intolerance . The central nervous system findings are often seizures, dementia, migraine, stroke-like episodes, ataxia, spasticity and encephalopathy. However, due to a significant clinical variability, some individuals do not fit into a specific clinical diagnosis. Heteroplasmy, which is the uneven distribution of mtDNA molecules during cell division, may results in variable penetrance and severity of symptoms, depending on the level of mutant mitochondria .
Some discrete clinical syndromes are well established, for which targeted testing is available:
Kearns-Sayre syndrome (KSS)
Chronic progressive external ophthalmoplegia (CPEO)
Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS)
Myoclonic epilepsy with ragged-red fibers (MERRF)
Retinitis Pigmentosa and Ataxia (NARP)
Leber hereditary optic neuropathy (LHON)
Leigh syndrome (LS)
Sequence analysis of the entire mtDNA genome is available to test for mitochondrial mutations. This test is intended for patients with a diagnosis of a mitochondrial disorder. If applicable, testing for common mutations associated with specific mitochondrial disorders should be performed first. Low levels of heteroplasmy may not be detected.
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This test is indicated for:
- Individual with a clinical diagnosis of a mitochondrial DNA disorder
Methodology: PCR was used to amplify the mitochondrial genome. Direct sequencing of the amplified region was performed using next generation short base pair read sequencing. Sequences are compared to revised Cambridge reference sequence (rCRS) NC_012920.1. Sequence analysis is limited to m.577_m.16023 region of the mitochondrial genome and excludes the highly variable control regions (m.1_m.576 and m.16024_m.16569).
Heteroplasmy: - Heteroplasmy is defined as the co-presence of a reference allele and an alternate allele (variant) at a given position in the mitochondrial genome. For any given variant, the level of heteroplasmy is calculated as a percentage of the total NGS reads (at that particular position) that have the variant. In general, heteroplasmic variants detected at <10% of alleles, are not reported. Known pathogenic or likely pathogenic variants detected at <10% heteroplasmy may be reported with recommendations for additional studies, as appropriate.
Detection is related to the specific condition suspected (refer to the test descriptions for the recognized conditions listed above). Since in general, sequence analysis does not detect low level mutant heteroplasmy. Common mutations must be ruled out before the whole genome sequence is analyzed.
Isolation using the Perkin Elmer™Chemagen™ Chemagen™ Automated Extraction method or Qiagen™ Puregene kit for DNA extraction is recommended.
Infants and Young Children (<2 years of age): 2-3 ml
Children > 2 years of age to 10 years old: 3-5 ml
Older Children & Adults: 5-10 ml
Autopsy: 2-3 ml unclotted cord or cardiac blood
Orangene™ Saliva Collection Kit used according to manufacturer instructions. Please contact EGL for a Saliva Collection Kit for patients that cannot provide a blood sample.
- PCR amplification and restriction enzyme fragment analysis is available to test for Leigh syndrome (QD), Myoclonic epilepsy with ragged-red fibers (QH), and Leber hereditary optic neuropathy (QC).
- Testing for MELAS (Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes) (QA) is available using sequence analysis of the MTTL1 gene and an allele-specific extension assay.
- Testing for Retinitis Pigmentosa and Ataxia (NARP) (QK) is available by PCR amplification with restriction enzyme fragment analysis and an allele-specific extension assay.
- Urine organic acid (OA) with lactic acid and pyruvic acid, and plasma acylcarnitine analysis (AR) may be considered for evaluation of specific mitochondrial disorders.
- Known mutation analysis (KM) is available to family members if mutation is identified in the proband by sequencing.