# Lecture 23: Mitochondrial Inheritance

## Genetics

---

## Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the structure and gene content of the mitochondrial genome
2. Explain maternal inheritance and why mitochondrial DNA is transmitted exclusively through the maternal line
3. Define heteroplasmy and homoplasmy and explain their significance for disease expression
4. Describe the threshold effect and its role in the variable expressivity of mitochondrial diseases
5. Identify the major categories and clinical features of mitochondrial diseases
6. Discuss the diagnostic approaches and emerging therapies for mitochondrial disorders

---

## Lecture Content

### I. The Mitochondrial Genome

Each human cell contains hundreds to thousands of mitochondria, and each mitochondrion harbors 2-10 copies of mitochondrial DNA (mtDNA). The **mtDNA** molecule is a circular, double-stranded DNA of 16,569 base pairs, vastly smaller than the approximately 3.2 billion base pairs of the nuclear genome. Its two strands are designated the heavy (H) and light (L) strands based on their buoyant density in cesium chloride gradients. The mitochondrial genome is extremely compact, with very few non-coding sequences and no introns. The single major non-coding region, the **D-loop (displacement loop)**, contains the origin of H-strand replication and promoters for transcription.

The 37 genes encode all essential RNA components for mitochondrial protein synthesis and a subset of the oxidative phosphorylation machinery: 13 protein-coding genes (7 subunits of Complex I, 1 subunit of Complex III, 3 subunits of Complex IV, and 2 subunits of Complex V; notably, Complex II is entirely nuclear-encoded), 22 transfer RNA genes, and 2 ribosomal RNA genes (12S and 16S). The mitochondrial genetic code differs from the universal code in several ways: UGA codes for tryptophan rather than stop, and AGA and AGG code for stop rather than arginine. Simplified codon-anticodon pairing rules mean that only 22 tRNAs are needed.

Critically, approximately 1,500 mitochondrial proteins are actually nuclear-encoded, synthesized in the cytoplasm, and imported into mitochondria. These include DNA polymerase gamma (POLG), RNA polymerase, ribosomal proteins, import machinery, and most OXPHOS subunits.

<image>Panel A: Map of the human mitochondrial genome — a circular diagram showing the 37 genes arranged around the 16,569 bp circle: the 13 protein-coding genes (color-coded by OXPHOS complex), 22 tRNA genes (single-letter amino acid abbreviations), 2 rRNA genes, the D-loop control region, and the origins of replication (OH and OL); the H-strand and L-strand are labeled. Panel B: Diagram of the mitochondrial oxidative phosphorylation system embedded in the inner mitochondrial membrane — Complexes I through V shown with subunits color-coded to distinguish those encoded by mtDNA (13 subunits highlighted) from those encoded by nuclear DNA; the electron transport chain and ATP production are illustrated. Panel C: Comparison table of the nuclear genome vs. the mitochondrial genome — listing size, number of genes, structure (linear vs. circular), introns (present vs. absent), inheritance pattern (biparental vs. maternal), genetic code differences, and copy number per cell.</image>

### II. Maternal Inheritance

Mitochondrial DNA is inherited exclusively from the mother, a pattern known as **maternal (matrilineal) inheritance**. The oocyte contributes approximately 100,000-200,000 copies of mtDNA to the zygote, while sperm mitochondria (carrying roughly 100 copies) are either excluded from the egg, diluted out, or actively destroyed through ubiquitin-mediated proteasomal degradation. Rare exceptions involving paternal leakage have been reported but remain extremely uncommon and controversial.

The pedigree characteristics of maternal inheritance are distinctive. An affected mother passes the trait to all of her children, both sons and daughters. An affected father does not transmit the trait to any of his children. The trait can appear in every generation through the maternal line, and there is no male-to-offspring transmission, which is the key feature distinguishing mitochondrial inheritance from autosomal or X-linked patterns. **Mitochondrial Eve** refers to the observation that all living humans trace their mtDNA lineage to a single female ancestor who lived approximately 150,000-200,000 years ago in Africa. The properties of mtDNA, including maternal inheritance, high copy number, and high mutation rate, make it widely used in population genetics, forensic identification, and evolutionary studies.

### III. Heteroplasmy, Homoplasmy, and the Threshold Effect

**Homoplasmy** exists when all mtDNA copies in a cell or individual are identical, either all wild-type or all mutant. **Heteroplasmy** exists when a mixture of wild-type and mutant mtDNA molecules coexists within a cell or individual. Most pathogenic mtDNA mutations are heteroplasmic, and the proportion of mutant mtDNA can vary between cells, tissues, and organs within the same individual.

The **threshold effect** explains the relationship between mutant load and disease. A minimum proportion of mutant mtDNA must be present before cellular dysfunction occurs, typically 60-90% depending on the specific mutation and tissue. Below this threshold, sufficient wild-type mtDNA maintains normal OXPHOS function. Above it, OXPHOS capacity falls below the minimum required for cellular function, and disease manifests. Tissues with the highest energy demands, including brain, heart, skeletal muscle, retina, and kidney, have the lowest tolerance and are most commonly affected.

The **mitochondrial genetic bottleneck** during oogenesis has profound clinical implications. The number of mtDNA molecules is dramatically reduced in primordial germ cells to approximately 200 copies before being amplified in mature oocytes. This bottleneck causes random sampling that can shift the proportion of mutant mtDNA dramatically between a mother and her offspring, explaining the variable severity of mitochondrial disease among siblings born to the same heteroplasmic mother. **Replicative segregation** further contributes to variability: during cell division, mitochondria and their mtDNA are distributed randomly to daughter cells, so heteroplasmy levels can drift over time within tissues.

<image>Panel A: Diagram illustrating homoplasmy vs. heteroplasmy — a cell with all blue mitochondria (wild-type homoplasmy), a cell with all red mitochondria (mutant homoplasmy), and a cell with a mixture of blue and red mitochondria (heteroplasmy), with percentages indicated. Panel B: The threshold effect shown as a graph — the x-axis represents the percentage of mutant mtDNA (0% to 100%), the y-axis represents OXPHOS capacity; a horizontal dashed line marks the minimum OXPHOS needed for normal function; as mutant load increases past the threshold (~60-90%), OXPHOS drops below the minimum, leading to disease; different tissues (brain, muscle, liver) are marked at their respective threshold levels. Panel C: The mitochondrial genetic bottleneck during oogenesis — a diagram showing a heteroplasmic mother with moderate mutant load, the reduction of mtDNA copies in primordial germ cells (bottleneck), random sampling, and subsequent amplification in mature oocytes, resulting in offspring with varying mutant loads (one child low, one moderate, one high), explaining variable disease severity among siblings.</image>

### IV. Mitochondrial Diseases

Mitochondrial diseases can result from mutations in mtDNA or in nuclear genes encoding mitochondrial proteins. Among **mtDNA point mutations**, **MELAS** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) is most commonly caused by the m.3243A>G mutation in the MT-TL1 gene (tRNA-Leu), presenting with stroke-like episodes before age 40, seizures, dementia, lactic acidosis, and ragged-red fibers on muscle biopsy. **MERRF** (Myoclonic Epilepsy with Ragged-Red Fibers) is most commonly caused by m.8344A>G in MT-TK (tRNA-Lys), presenting with myoclonus, epilepsy, ataxia, and myopathy. **NARP** (Neuropathy, Ataxia, Retinitis Pigmentosa) results from m.8993T>G/C in the MT-ATP6 gene; at mutant loads exceeding 90%, it causes Leigh syndrome, a severe infantile necrotizing encephalopathy. **LHON** (Leber Hereditary Optic Neuropathy) typically involves homoplasmic mutations in Complex I genes (m.11778G>A in ND4, m.3460G>A in ND1, or m.14484T>C in ND6), causing acute or subacute bilateral vision loss predominantly in young males with incomplete penetrance.

**mtDNA deletions and rearrangements** cause several syndromes. **Kearns-Sayre syndrome** involves a large-scale single deletion presenting before age 20 with progressive external ophthalmoplegia, pigmentary retinopathy, and cardiac conduction defects. **Pearson syndrome** involves a large-scale single deletion causing sideroblastic anemia and pancreatic dysfunction, often fatal in infancy; survivors may develop Kearns-Sayre syndrome. **Chronic progressive external ophthalmoplegia (CPEO)** involves single or multiple deletions causing ptosis and ophthalmoplegia.

**Nuclear gene mutations** affecting mitochondrial function include POLG mutations (causing Alpers syndrome, PEO, and mitochondrial depletion), SURF1 mutations (a Complex IV assembly factor causing Leigh syndrome), and frataxin (FXN) mutations in Friedreich ataxia, a GAA trinucleotide repeat expansion causing iron-sulfur cluster assembly defects.

### V. Diagnosis of Mitochondrial Diseases

Clinical suspicion for mitochondrial disease arises when a patient presents with multisystem disease affecting high-energy tissues (brain, muscle, heart, eye), elevated lactate, and a maternal inheritance pattern. **Muscle biopsy** provides several informative stains: Gomori trichrome reveals ragged-red fibers (subsarcolemmal accumulation of abnormal mitochondria), succinate dehydrogenase staining shows ragged-blue fibers, cytochrome c oxidase staining reveals COX-negative fibers in a mosaic pattern reflecting heteroplasmy, and electron microscopy may show paracrystalline inclusions. **Biochemical testing** measures OXPHOS enzyme activity in muscle tissue or fibroblasts.

**Genetic testing** has been transformed by next-generation sequencing of the full mitochondrial genome, nuclear gene panels for mitochondrial disease genes, and whole exome/genome sequencing for undiagnosed cases. An important caveat is that heteroplasmy levels may differ between blood and affected tissues, so muscle biopsy may be needed for accurate assessment. Blood lactate and pyruvate levels are elevated in OXPHOS deficiency, and neuroimaging may show bilateral basal ganglia lesions (Leigh syndrome) or stroke-like lesions not conforming to vascular territories (MELAS).

<image>Panel A: Clinical features collage of major mitochondrial syndromes — MELAS (brain MRI showing stroke-like lesion), MERRF (muscle biopsy with ragged-red fibers on Gomori trichrome stain), LHON (fundoscopy showing optic atrophy), and Kearns-Sayre syndrome (patient with ptosis and external ophthalmoplegia); each image is labeled with the syndrome name and characteristic finding. Panel B: Diagnostic algorithm flowchart for suspected mitochondrial disease — starting from clinical suspicion (multisystem disease, maternal inheritance, elevated lactate), branching to genetic testing (mtDNA sequencing, nuclear gene panel) and muscle biopsy (histochemistry, OXPHOS enzymes), leading to confirmed diagnosis. Panel C: Pedigree demonstrating maternal inheritance — an affected mother transmitting the mitochondrial disease to all children (sons and daughters); the affected sons do not transmit to their children, while affected daughters transmit to all of their children; variable severity among affected siblings is indicated by different shading levels representing different heteroplasmy levels.</image>

### VI. Emerging Therapies and Prevention

Current management of mitochondrial diseases is largely supportive, with no curative therapies available for most conditions. Coenzyme Q10 and idebenone supplementation provide antioxidant and electron carrier support. Arginine and citrulline are used for MELAS to improve nitric oxide production and vascular function. Patients must avoid mitochondrial toxins such as valproic acid, aminoglycosides, and in some cases statins. Specific organ complications require targeted management including cardiac pacing, diabetes treatment, and seizure control.

**Mitochondrial replacement therapy (MRT)** offers a preventive approach. In **maternal spindle transfer**, the nuclear DNA from the mother's egg is transferred to an enucleated donor egg with healthy mitochondria before fertilization. In **pronuclear transfer**, the pronuclei from the affected zygote are transferred to an enucleated donor zygote with healthy mitochondria after fertilization. Both approaches result in a "three-parent baby" carrying nuclear DNA from both parents and mtDNA from the donor. MRT was legalized in the UK in 2015, and the first baby was born in 2016 (the procedure was performed in Mexico). Ethical considerations include germline modification, identity issues, and donor anonymity.

**Preimplantation genetic testing** allows women with known heteroplasmic mtDNA mutations to undergo IVF with selection of embryos carrying low mutant mtDNA loads. Experimental gene therapy approaches include allotopic expression (re-engineering mtDNA genes with nuclear-compatible codons and mitochondrial targeting sequences) and mitochondrially targeted restriction enzymes or CRISPR to selectively eliminate mutant mtDNA and shift heteroplasmy toward the wild-type.

---