An overview of neurological and neuromuscular signs in mitochondrial diseases

Clinicogenetical Variants of Progressive External Ophthalmoplegia - An Especial Review of Non-ophthalmic Manifestations

Progressive external ophthalmoplegia (PEO) is a slowly progressive myopathy characterized by extraocular muscles involvement, leading to frozen eyes without diplopia. The pattern of inheritance may be mitochondrial, autosomal dominant or, rarely, autosomal recessive. Sporadic forms were also reported. Muscular involvement other than extraocular muscles may occur with varying degrees of weakness, but this mostly happens many years after the disease begins. There are also scattered data about systemic signs besides ophthalmoplegia. This article aims to review non-ophthalmic findings of PEO from a clinicogenetical point of view.

How the most common mitochondrial DNA mutation (m.3243A>G) vanishes from leukocytes: a mathematical model

Mitochondrial diseases may be caused by alterations of the mitochondrial genome. The pathogenic variant m.3243A>G is one of the most frequent causes of mitochondrial disease and the most common mitochondrial DNA mutation. Patients with a variant in mitochondrial DNA can have a mixture of mutated and wild-type genomes (heteroplasmy). In the case of the pathogenic variant m.3243A>G, the degree of heteroplasmy (H) correlates to some extent with the severity of the disease. Several longitudinal studies, where H is measured at two different time-points, have shown an annual decline in leukocyte H values. Thus far, only an exponential decay of H with time has been noted but a mechanistic model is lacking. Here, I describe a deterministic mathematical model that accounts for the decline of H in leukocytes based on selective mechanisms acting at the stem cell level. The 'inverted-sigmoid' model provides estimates of at-birth H levels closer to those observed in post-mitotic tissues, such as skeletal muscle, than the estimates provided by an exponential decay. The new model never leads to predictions of H > 100% and provides a stronger correlation between at-birth H values in leukocytes and the scores of the Newcastle Mitochondrial Disease Scale for Adults, which can be of practical utility. This model could be extended to other mitochondrial DNA disease-causing variants.

Neuromuscular Manifestations in Mitochondrial Diseases in Children

Mitochondrial diseases exhibit significant clinical and genetic heterogeneity. Mitochondria are highly dynamic organelles that are the major contributor of adenosine triphosphate, through oxidative phosphorylation. These disorders may be developed at any age, with isolated or multiple system involvement, and in any pattern of inheritance. Defects in the mitochondrial respiratory chain impair energy production and almost invariably involve skeletal muscle and peripheral nerves, causing exercise intolerance, cramps, recurrent myoglobinuria, or fixed weakness, which often affects extraocular muscles and results in droopy eyelids (ptosis), progressive external ophthalmoplegia, peripheral ataxia, and peripheral polyneuropathy. This review describes the main neuromuscular symptomatology through different syndromes reported in the literature and from our experience. We want to highlight the importance of searching for the "clue clinical signs" associated with inheritance pattern as key elements to guide the complex diagnosis process and genetic studies in mitochondrial diseases.

The investigation and management of metabolic myopathies

Metabolic myopathies (MM) are rare inherited primary muscle disorders that are mainly due to abnormalities of muscle energy metabolism resulting in skeletal muscle dysfunction. These diseases include disorders of fatty acid oxidation, glyco(geno)lytic muscle disorders and mitochondrial respiratory chain (MRC) disease. Clinically these disorders present with a range of symptoms including infantile hypotonia, myalgia/exercise tolerance, chronic or acute muscle weakness, cramps/spasms/stiffness or episodic acute rhabdomyolysis. The precipitant may be fasting, infection, general anaesthesia, heat/cold or most commonly, exercise. However, the differential diagnosis includes a wide range of both acquired and inherited conditions and these include exposure to drugs/toxins, inflammatory myopathies, dystrophies and channelopathies. Streamlining of existing diagnostic protocols has now become a realistic prospect given the availability of second-generation sequencing. A diagnostic pathway using a ‘rhabdomyolysis’ gene panel at an early stage of the diagnostic process is proposed. Following detailed clinical evaluation and first-line investigations, some patients will be identified as candidates for McArdle disease/glycogen storage disease type V or MRC disease and these will be referred directly to the specialised services. However, for the majority of patients, second-line investigation is best undertaken through next-generation sequencing using a ‘rhabdomyolysis’ gene panel. Following molecular analysis and careful evaluation of the findings, some patients will receive a clear diagnosis. Further functional or specific targeted testing may be required in other patients to evaluate the significance of uncertain/equivocal findings. For patients with no clear diagnosis, further investigations will be required through a specialist centre.