biology

Mitochondrial complex II deficiency

Mitochondrial complex II deficiency is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mitochondrial complex II deficiency rather than just read about it. In short: Mitochondrial complex II deficiency, also called CII deficiency, is a rare mitochondrial disease caused by deficiency of mitochondrial complex II, also known as Succinate dehydrogenase (SDH). SDH plays a key role in metabolism; the catalytic end, made up of SDHA and SDHB oxidizes succinate to fumarate in the tricarboxylic acid (TCA) cycle.

Key takeaways

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Reference excerpt

Mitochondrial complex II deficiency, also called CII deficiency, is a rare mitochondrial disease caused by deficiency of mitochondrial complex II, also known as Succinate dehydrogenase (SDH). SDH plays a key role in metabolism; the catalytic end, made up of SDHA and SDHB oxidizes succinate to fumarate in the tricarboxylic acid (TCA) cycle. The electrons from this reaction then reduce FAD to FADH2, which ultimately reduces ubiquinone to ubiquinol in the mitochondrial electron transport chain. As of 2020, about 61 cases have been reported with genetic studies, but there are also documented cases of CII deficiencies as determined by biochemical and histological analysis without genetic studies.

Signs and symptoms Mitochondrial complex II deficiency affects the body's mitochondria and can have a variety of presentations. In some cases, the brain, peripheral nervous system, heart, liver, kidneys, and muscles are affected, while in other cases, only a subset of these organs are affected. The condition can present differently among different individuals, even those in the same family, but symptoms can largely be separated into two categories. Those with biallelic loss of function who don't have any functional SDH have an average age of onset of 8.6 ± 7.9 months, while those with monoallelic loss of function have an average age of onset of 21.5 ± 20.3 years. In the biallelic form, there is typically a Leigh syndrome phenotype with symptoms including seizures, white matter lesions, nystagmus, developmental delay, microcephaly, leukodystrophy, hypertonia, multiorgan failure, spasticity, tetraparesis, and more. In the monallelic form, often caused by heterozygous mutations of SDHA, symptoms include exercise intolerance, rhabdomyolosis, muscle pain, dyspnea, hypertension, dilated cardiomyopathy, hypertrophic cardiomyopathy, and optic atrophy.

Causes CII deficiency is a genetic disorder caused by Nuclear DNA (as opposed to mitochondrial DNA) and has both autosomal recessive and dominant inheritance patterns. Most pathogenic mutations for CII deficiency occur on SDHA or SDHAF1, but other pathogenic mutations for CII are known for SDHB and SDHD. SDHA is the only SDHx gene reported to date in which a dominant pathogenic variant has been identified, although most affected individuals harbour either homozygous or compound heterozygous pathogenic variants. For unknown reasons, several cases of CII deficiency have been linked to Umeå, Sweden.

Diagnosis The most effective way to diagnose CII deficiency is by measuring the activity of complex II in muscle biopsy, however, there is no clear correlation between residual complex II activity and severity or clinical outcome. Other diagnostic tests include brain MRIs, which can detect symptoms characteristic of leigh syndrome, electromyography (EMG), which can detect myopathies, and blood tests for biochemical signals of mitochondrial dysfunction.

Prognosis The prognosis can vary wildly for CII deficiency: In those who present earlier, the prognosis is worse, and especially for the biallelic form, few reach the age of 4. Those presenting with Leigh syndrome or greater neurological involvement have worse outcomes. In severe cases where multiple organ systems are affected, death can occur in early life due to multisystem failure. There is no cure for CII deficiency, though some reported patients showed clinical improvement following riboflavin therapy, a vitamin essential for mitochondrial function. Vitamins, such as the other B vitamins and Coenzyme Q-10, have been used with little success, and treatments primarily focus on symptoms, such as sodium bicarbonate for managing acidosis and opthamological procedures for correcting eyesight.

Epidemiology CII deficiency is extremely rare, with less than 100 documented cases worldwide. Most cases recorded in literature are due to the biallelic form, and these are most common for those whose parents are consanguineous. There are potentially more cases of the monallelic form, but possibly due to their less severe nature or less unique presentation, they may not be reported as frequently. As human mitochondrial diseases and the effects of herbicides and fungicides on the mitochondria are a current area of research, our understanding of CII deficiency may progress significantly.

References

Illustrations

Worked examples

Example 1 — a first encounter with Mitochondrial complex II deficiency

Start with the simplest possible case. Write down what Mitochondrial complex II deficiency claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mitochondrial complex II deficiency before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mitochondrial complex II deficiency ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mitochondrial complex II deficiency

In research: Mitochondrial complex II deficiency appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mitochondrial complex II deficiency in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mitochondrial complex II deficiency is common in secondary-school and first-year university syllabi. It links to neighbouring topics Autosomal recessive disorders, Genetic diseases and disorders, Mitochondrial diseases, so understanding it makes those chapters shorter.
In everyday life: Look for Mitochondrial complex II deficiency outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Mitochondrial complex II deficiency in simple terms?

Why does Mitochondrial complex II deficiency matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

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Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

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