Mitochondrial DNA depletion syndrome

Characteristics and causes

At the molecular level MDS results from impaired replication, repair or maintenance of mitochondrial genomes, or from disrupted nucleotide supply required for mtDNA replication. Mutations in a set of nuclear genes that control these processes are implicated, including but not limited to POLG, TK2, RRM2B, DGUOK, SUCLA2 and SUCLG1. Loss of functional protein leads to lower mtDNA copy number and secondary deficiency of respiratory chain complexes, which manifests most severely in tissues with high energy demand.

Clinical forms and presentation

Clinically, MDS is heterogeneous. Three broad categories are commonly used:

• Myopathic forms: primarily affect skeletal muscle, causing weakness, hypotonia and respiratory failure; some individuals survive into adolescence.
• Hepatopathic forms: dominant liver disease with early-onset liver failure, often in infancy; metabolic decompensation, hypoglycemia and jaundice are frequent features.
• Encephalomyopathic forms: combined muscle and brain involvement producing developmental delay, seizures, movement disorders and progressive neurological decline; some genetic subtypes (for example certain SUCLA2 mutations) have been reported in survivors into adulthood.

Diagnosis and management

Diagnosis typically combines clinical evaluation with laboratory tests (elevated lactate, abnormal metabolic profiles), neuroimaging when brain involvement is suspected, and measurement of mitochondrial DNA content in muscle or liver tissue. Definitive identification often relies on genetic testing to detect pathogenic variants in relevant nuclear genes. There is no universal cure. Management focuses on supportive and symptomatic care: nutritional support, management of metabolic crises, physical therapy and respiratory support. Experimental and targeted approaches have shown promise in specific subtypes; for example, nucleotide or deoxynucleoside supplementation has been reported to improve outcomes in some forms associated with thymidine kinase 2 deficiency, and liver transplantation has been attempted in selected hepatopathic cases, although transplant does not address systemic or neurological disease.

History, epidemiology and distinctions

MDS was recognized as a distinct group of disorders as genetic and biochemical techniques enabled measurement of mtDNA content and identification of nuclear gene defects. The conditions are rare; precise incidence varies with population and particular genetic causes. Important distinctions separate mtDNA depletion syndromes from disorders caused by large-scale deletions of mtDNA or by multiple mtDNA deletions, although all fall under the broader category of mtDNA maintenance diseases. Genetic counseling is important for affected families because of the autosomal recessive inheritance.

Research continues into mechanisms that regulate mitochondrial DNA copy number and into therapies that might restore mtDNA or compensate for respiratory chain failure. Early diagnosis, multidisciplinary care and involvement in clinical trials when available can improve management of this challenging group of pediatric and adult mitochondrial diseases.