By Eric Mallack, MD, MBE, and Barry E. Kosofsky, MD, PhD

Dr. Mallack is Fellow, Division of Pediatric Neurology, Weill Cornell Medical College; and Dr. Kosofsky is Chief, Division of Pediatric Neurology; Goldsmith Professor of Pediatrics, Neurology and Neuroscience, and Radiology, Weill Cornell Medical College.

Dr. Mallack and Dr. Kosofsky report no financial relationships relevant to this field of study.

SYNOPSIS: Leigh syndrome is a genetically heterogeneous neurodevelopmental disorder. The application of next-generation sequencing has enabled a deeper understanding of the diverse nature of the genetic and molecular etiologies that give rise to the shared clinical phenotype of Leigh syndrome.

SOURCE: Lake NJ, Compton AG, Rahman S, et al. Leigh syndrome: One disorder, more than 75 monogenic causes. Ann Neurol 2016;79:190-203.

Leigh syndrome (LS) is the most common disease of mitochondrial energy production in children. Symptoms often present after illness or infection following a brief period of normal development. Patients undergo developmental delay, often followed by regression, associated with hypotonia, dystonia, ataxia, and progressive ophthalmological disease. Radiographically, lesions develop in the bilateral basal ganglia and brainstem, with evidence of elevated cerebrospinal fluid (CSF) lactate detected by magnetic resonance spectroscopy. At the biochemical level, LS results from impairment in any of the five multiprotein complexes in the oxidative phosphorylation pathway, in the electron carrier coenzyme Q10 (CoQ10), or in the pyruvate dehydrogenase complex (PDHc). Death usually occurs in infancy or early childhood.

The majority of known mutations disrupt nuclear-encoded proteins and are of autosomal recessive (or X-linked) inheritance. A minority of mutations are found in maternally inherited mitochondrial DNA (mtDNA), for which heteroplasmy becomes a large determinant in phenotypic severity, with higher mtDNA mutation burden resulting in more significant disease expression. The application of next-generation sequencing (NGS), specifically massively parallel sequencing (MPS), has allowed for identification of 30 new genetic causes of LS in the past 5 years. Advanced sequencing has provided a refined understanding of the genotype-phenotype correlation in LS, and has also uncovered novel mechanisms by which pathogenic mutations give rise to LS, summarized in this review as follows.

Nuclear-Encoded Genes

Complex I deficiencies are the most common cause of LS, and are predominantly characterized by mutations in NADH dehydrogenase (ubiquinone) Fe-sulfur protein 4 (NDUFS4), NDUFV1, and NDUFS1, all giving rise to an early-onset, severe form of disease. However, mutations in 15 other nuclear-encoded genes and five additional mitochondrial encoded genes comprising Complex I subunits give rise to less severe and more variable phenotypes.

Mutations in the Complex II subunit SDHA are responsible for two separate LS phenotypes: one a severe infantile presentation with early death, the other a milder course, with preservation of cognitive ability and survival into childhood.

Complex III and CoQ10 mutations account for a small percentage of LS. Individuals with Complex III assembly factor TTC19 mutations develop neuroimaging abnormalities consistent with LS. Patients with CoQ10-deficient LS have variable response to supplementation, with some surviving into adulthood, while others with a PDSS2 mutation do not respond.

Mutations in SURF1 impair Complex IV assembly and are the single largest genetic cause of LS, with more than 200 cases reported in the literature. Although mutations in this gene cause a clinically and biochemically homogenous phenotype, a subset of patients exhibit atypical neuroradiological features more consistent with leukodystrophy or cerebral atrophy.

Multiple disease genes that cause LS, including SCO2, LRPPRC, ETHE1, SERAC1, and AIFM1, additionally exert their effects via secondary mechanisms at the cellular level. For example, LRPPRC mutations not only affect Complex IV, but also have been found recently to have post-transcriptional and translational affects, which in turn affect ATP synthase. They disrupt mitochondrial energy function at the nucleic acid level and via more global dysregulation of oxidative phosphorylation. ETHE1 gives rise to the LS phenotype by causing secondary dysfunction to Complex IV via sulfur accumulation. Similar mechanisms hold for patients with ECHS1 and HIBCH mutations. SERAC1 and apoptosis-inducing factor AIFM1 mutations reduce the structural stability of the mitochondrial membrane, thus leading to mitochondrial energy failure.

Mitochondrial-Encoded Genes

Further genetic heterogeneity is introduced by mutations in mtDNA, including SUCLA2, SUCLG1, MTND3, MTND5, and MTATP6, genes encoding structural components of Complexes I, III, IV, and V. Such mtDNA mutations will lead to a combined oxidative phosphorylation deficiency and, subsequently, the LS phenotype.

Despite being different clinical entities, shared mutations in MTTL1 and MTTK exist between LS and two distinct syndromes: mitochondrial encephalopathy lactic acidosis and strokelike episodes (MELAS) and myoclonic epilepsy with ragged red fibers (MERRF). Conversely, there are Leigh-like syndromes that phenotypically overlap with MERRF and MELAS resulting from mutations in MTND5 and MTND3.

A new class of mutations affecting a nuclear-encoded mt-tRNA modifying enzyme, mitochondrial methionyl-tRNA formyltransferase required for mitochondrial translation, provides a novel mechanism for nuclear-mitochondrial interaction via translational regulation, which gives rise to LS.

COMMENTARY

The authors’ detailed review categorizing more than 75 mutations known to cause LS provides a 21st century view of a disease first described in 1951. By applying the power of NGS sequencing, our knowledge of the genetic and molecular heterogeneity contributing to a single clinical, radiographic, and pathologic disorder has been richly elucidated. Conversely, despite providing a unified biochemical understanding of LS as a disorder of oxidative phosphorylation, the authors highlight the variability in the phenotypes attributable to particular genotypes evident for a number of specific mutations. The application of NGS will undoubtedly lead to the discovery of additional genetic causes of LS, underscoring the complex array of proteins and cellular processes vulnerable to genetic disruption.

The authors additionally demonstrate that a mutation in a particular gene can result in both variable severity of disease, as well as different disease phenotypes altogether. Mutations of SURF1, resulting in Complex IV deficiency, also give rise to clinical and neuroradiological findings highly atypical of LS. Similarly, mutations in MTND5 are responsible for both LS and MELAS, two distinctly different diseases. In terms of disease severity, SDHA-encoded mutations in Complex II-associated LS are responsible for both mild and severe courses of the disease. As a result, clinicians increasingly will be required to look more closely for clinical, and when available preclinical, evidence of the functional significance of the individual mutations identified in their patients. Likewise, such detailed information may be of therapeutic significance, as some LS-causing mutations can respond to treatment with vitamins and cofactors.

This review also expands our understanding of molecular pathogenesis of LS from the primary affected pathway — in this case the electron transport chain and PDHc — to include novel mechanisms at the broader cellular level. For example, the transcriptional dysregulation induced by LRPPRC mutations impact cellular function more globally, likewise broadening the phenotype of LS. Similar logic applies for secondary mechanisms that disrupt the primary pathway, in this case sulfur or toxin accumulation seen in ETHE1, ECHS1, and HIBCH mutation-induced LS, which affect cell viability via dysregulation of factors responsible for the apoptotic pathway.

The authors have provided a higher resolution insight into the genetic, molecular, biochemical, and cellular basis of LS. By doing so, they have illustrated how powerful advances in next-generation sequencing will exponentially enhance our ability to understand both the molecular reductionism, as well as phenotypic complexity of mitochondrial energy disorders, specifically, and genetic disease, in general.