A novel mutation in TAZ causes mitochondrial respiratory chain disorder without cardiomyopathy

A high mutation load of m.14597A>G in MT-ND6 causes Leigh syndrome

Leigh syndrome (LS) is an early-onset progressive neurodegenerative disorder associated with mitochondrial deficiency. m.14597A>G (p.Ile26Thr) in the MT-ND6 gene was reported to cause Leberʼs hereditary optic neuropathy (LHON) or dementia/dysarthria. In previous reports, less than 90% heteroplasmy was shown to result in adult-onset disease. Here, by whole mitochondrial sequencing, we identified m.14597A>G mutation of a patient with LS. PCR–RFLP analysis on fibroblasts from the patient revealed a high mutation load (> 90% heteroplasmy). We performed functional assays using cybrid cell models generated by fusing mtDNA-less rho0 HeLa cells with enucleated cells from patient fibroblasts carrying the m.14597A>G variant. Cybrid cell lines bearing the m.14597A>G variant exhibited severe effects on mitochondrial complex I activity. Additionally, impairment of cell proliferation, decreased ATP production and reduced oxygen consumption rate were observed in the cybrid cell lines bearing the m.14597A>G variant when the cells were metabolically stressed in medium containing galactose, indicating mitochondrial respiratory chain defects. These results suggest that a high mutation load of m.14597A>G leads to LS via a mitochondrial complex I defect, rather than LHON or dementia/dysarthria.

TAZ encodes tafazzin, a transacylase essential for cardiolipin formation and central to the etiology of Barth syndrome

Tafazzin, which is encoded by the TAZ gene, catalyzes transacylation to form mature cardiolipin and shows preference for the transfer of a linoleic acid (LA) group from phosphatidylcholine (PC) to monolysocardiolipin (MLCL) with influence from mitochondrial membrane curvature. The protein contains domains and motifs involved in targeting, anchoring, and an active site for transacylase activity. Tafazzin activity affects many aspects of mitochondrial structure and function, including that of the electron transport chain, fission-fusion, as well as apoptotic signaling. TAZ mutations are implicated in Barth syndrome, an underdiagnosed and devastating disease that primarily affects male pediatric patients with a broad spectrum of disease pathologies that impact the cardiovascular, neuromuscular, metabolic, and hematologic systems.

Barth syndrome: mechanisms and management

Objectives: Barth syndrome is an ultra-rare, infantile-onset, X-linked recessive mitochondrial disorder, primarily affecting males, due to variants in TAZ encoding for the cardiolipin transacylase tafazzin. This review aimed to summarize and discuss recent and earlier findings concerning the etiology, pathogenesis, clinical presentation, diagnosis, treatment, and outcome of Barth syndrome.

Method: A literature review was undertaken through a MEDLINE search.

Results: The phenotype of Barth syndrome is highly variable but most frequently patients present with hypertrophic/dilated/non-compaction cardiomyopathy, fibroelastosis, arrhythmias, neutropenia, mitochondrial myopathy, growth retardation, dysmorphism, cognitive impairment, and other, rarer features. Lactic acid and creatine kinase, and blood and urine organic acids, particularly 3-methylglutaconic acid and monolysocardiolipin, are often elevated. Cardiolipin is decreased. Biochemical investigations may show decreased activity of various respiratory chain complexes. The diagnosis is confirmed by documentation of a causative TAZ variant. Treatment is symptomatic and directed toward treating heart failure, arrhythmias, neutropenia, and mitochondrial myopathy.

Conclusions: Although Barth syndrome is still an orphan disease, with fewer than 200 cases described so far, there is extensive ongoing research with regard to its pathomechanism and new therapeutic approaches. Although most of these approaches are still experimental, it can be expected that causative strategies will be developed in the near future.

Cardiomyopathy in children with mitochondrial disease: Prognosis and genetic background

BACKGROUND

Cardiomyopathy is a reported indicator of poor prognosis in children with mitochondrial disease. However, the association between prognosis and the genetic background of cardiomyopathy in children with mitochondrial disease has yet to be fully elucidated.

METHODS AND RESULTS

Of 137 children with mitochondrial disease whose genetic diagnosis was made between 2004 and 2018, 29 had mitochondrial cardiomyopathy (21%). After a median follow-up of 35 months, the overall survival rate was significantly lower in patients with cardiomyopathy than in those without (p < 0.001). Ten-year Kaplan-Meier estimates of overall survival were 18 and 67%, respectively. Among the 21 cardiomyopathy patients who died, two died within one month of birth (COQ4 in one patient, and COX10 in one patient), ten died within one year (BOLA3 in three patients, QRSL1 in two patients, large chromosomal deletions in two patients, MT-ATP6/8 in one patient, MT-TL1 in one patient, and TAZ gene in one patient), and nine died after one year (MT-ND5 in three patients, MT-TL1 in three patients, ACAD9 in one patient, KARS in one patient, and MT-TV in one patient). In the three patients with mitochondrial DNA mutations whose cardiac tissues were available, high heteroplasmy rates in the cardiac tissue were observed for m.8528T>C (90%, died at 2 months of age) and m.3243A>G (90 and 80%, died at 12 and 13 years of age, respectively).

CONCLUSIONS

In children with mitochondrial disease, cardiomyopathy was common (21%) and was associated with increased mortality. Genetic analysis coupled with detailed phenotyping could be useful for prognosis.

Barth Syndrome: Different Approaches to Diagnosis

The diagnosis of Barth syndrome is challenging owing to the wide phenotypic spectrum with allelic heterogeneity. Here we report 3 cases of Barth syndrome with phenotypic and allelic heterogeneity that were diagnosed by different approaches, including whole exome sequencing and final confirmation by reverse-transcription polymease chain reaction.