Genetic testing for mitochondrial disease: the United Kingdom best practice guidelines

Abstract

Primary mitochondrial disease describes a diverse group of neuro-metabolic disorders characterised by impaired oxidative phosphorylation. Diagnosis is challenging; >350 genes, both nuclear and mitochondrial DNA (mtDNA) encoded, are known to cause mitochondrial disease, leading to all possible inheritance patterns and further complicated by heteroplasmy of the multicopy mitochondrial genome. Technological advances, particularly next-generation sequencing, have driven a shift in diagnostic practice from 'biopsy first' to genome-wide analyses of blood and/or urine DNA. This has led to the need for a reference framework for laboratories involved in mitochondrial genetic testing to facilitate a consistent high-quality service. In the United Kingdom, consensus guidelines have been prepared by a working group of Clinical Scientists from the NHS Highly Specialised Service followed by national laboratory consultation. These guidelines summarise current recommended technologies and methodologies for the analysis of mtDNA and nuclear-encoded genes in patients with suspected mitochondrial disease. Genetic testing strategies for diagnosis, family testing and reproductive options including prenatal diagnosis are outlined. Importantly, recommendations for the minimum levels of mtDNA testing for the most common referral reasons are included, as well as guidance on appropriate referrals and information on the minimal appropriate gene content of panels when analysing nuclear mitochondrial genes. Finally, variant interpretation and recommendations for reporting of results are discussed, focussing particularly on the challenges of interpreting and reporting mtDNA variants.

Fig. 1. Overview of genetic testing strategies for mitochondrial disease patients.

Upper panel: blood sample received. Strategy 1, a more targeted approach, may be appropriate for routine referrals or where there are barriers to parallel testing. Strategy 2 permits a more rapid and thorough, but more expensive, testing strategy and is amenable to both routine and clinically urgent cases, such as critically ill paediatric patients. When adopting Strategy 2, simultaneous testing of both mtDNA and nuclear DNA is recommended if resources allow; if sequential testing is required, mtDNA testing can be prioritised due to reduced cost and turnaround time. Lower panel: muscle biopsy received. As muscle biopsy is no longer widely used in first-line diagnostics, this pathway will typically apply if genetic analysis of blood DNA does not identify a cause or is inconclusive.