Mitochondrial Genetics

Each mitochondrion contains how many copies of mtDNA? How about each cell?

2-10 copies Several thousand copies

____ tRNAs coded by the mtDNA are sufficient to decode the mRNAs

22

mtDNAs differ by about ______ nucleotides, and about 10 of these result in _________ changes

50 Amino acid

What does the *threshold effect* state?

A certain level of mutation may be required to phenotypically express a defect and this level may *vary among different tissues depending on their respiratory requirements*

Which mutation is MELAS usually associated with?

A3243G point mutation in mtDNA encoded tRBA Multiple respiratory chain complexes affected

The mother transmits her haplotype to ______ her children, but only the ______ will pass it down

ALl Daughters

Many mtDNA mutations (*large scale deletions, mutations in tRNA genes*) affect biosynthesis in which respiratory chain complexes? What is the ultimate effect?

All complexes It is not possible to increase respiratory chain capacity

There are no reliable treatments for mitochondrial respiratory chain diseases, but metabolic therapies have had limited success, such as:

Coenzyme Q10, menadione, succinate, ascorbate, carnitine, thiamine, riboflavin

A polymorphism or a mutation in mtDNA may result in the _______ in the same cell of more than one _______ of mtDNA

Coexistence Population

Which of the ETC complexes is completely nuclear-encoded?

Complex II

Mitochondrial genetics contributes to the clinical _______ of mitochondrial diseases

Complexity

The signals that work the nucleus can emanate from _______ pathways responding to ________ signals from outside of the cell, or from the _______ themselves

Cytosolic Neurohormonal Mitochondria

Mitochondrial diseases are defined by ______ of the mitochondrial respiratory chain

Deficits

The threshold level is relative to which factors:

Dependent upon the type of mtDNA mutation Dependent upon the tissue and its requirements for oxidative metabolism May vary in a tissue with respect to time or functional demands

What is Leigh Syndrome and how does it present?

Devastating encephalomyopathy of infancy or childhood Psychomotor regression, seizures, ataxia, optic atrophy

How is lactic acidosis treated?

Dichloroacetate, which stimulates pyruvate dehydrogenase complex activity and lowers cerebral lactate concentrations

In heteroplasmic cells, daughter cells can receive ________ amounts of the two mtDNA popuations (wild-type and mutated mtDNA)

Differing

Clinical manifestations of mitochondrial diseases (do/do not) always agree with genetic classifications

Do not

Dominant and recessive (do/do not) apply to mitochondrial DNA

Do not

The mitochondrial genome is a ________-stranded DNA circle

Double

Exposure to numerous toxins and ______ can affect the mitochondrial respiratory chain function and result in mitochondrial disease

Drugs

A defect in the respiratory chain may be the result of mutations in which type of DNA?

Either nuclear DNA-encoded or mtDNA-encoded

Mitochondria are present in nearly all _______ cells

Eukaryotic

It's a rare exception that the ______ will also contribute to the inheritance of mtDNA, thus making it _______

Father Biparental

Differentiation of mitochondria for oxidative phosphorylation requires coordinate expression of...

Genes in both mitochondrial and nuclear DNAs

What are some common clinical features of mitochondrial diseases?

Hearing loss Diabetes mellitus Hepatopathy Optic neuropathy Ophthalmoplegia Retinopathy Cardiomyopathy Weakness, myopathy, neuropathy, fatigue Seizures, myoclonus, ataxia, stroke, dementia, migraine

There is extensive sequence (homo/hetero)geneity of mtDNA among humans

Heterogeneity

Mitochondrial disease can often be of the _______ group of disorders, and can be either genetically or _______ heterogeneous

Heterogenous Clinically

Causes of the discordance between clinical and genetic manifestations include:

Heteroplasmy Mutation levels *may change over time* Nuclear genetic factors may influence expression of mtDNA mutations Environmental factors may contribute to the expression of an mtDNA mutation

mtDNA sequence analyses are often used for human _______, and can be used forensically for what types of cases?

Idenfitication Missing persons, war casualties, mass disasters, criminal cases

In differentiated cells, various external stimuli that impose a cellular stress lead to ________ transcription of nuclear genes and ________. What do these nuclear genes encode?

Increased mtDNA Mitochondrial proteins

How do antibiotics affect function?

Inhibit bacterial protein synthesis, along with mitochondrial protein synthesis Result in *lactic acidosis, neuropathies, myelosuppression*

NRTIs do what? WHat does it result in?

Inhibit mtDNA replication Results in depletions of mtDNA

There (is/is no) paternal contribution to mtDNA

Is no

What are some possible lab findings from mitochondrial diseases?

Lactic acidosis (serum or CSF) Myoglobinuria Ragged-red fibers

What are *pure nerve disorders?*

Leber's hereditary ocular neuropathy (LHON)

This point mutation is associated with ______ clinical phenotype(s). There is a ______ inheritance pattern

Many Maternal

The mtDNA haplotype is one of _________ lineage

Maternal

Mutations in *structural RNAs (tRNAs and rRNAs)* usually exhibit _______ inheritance, and are associated with ______ respiratory chain enzyme deficiencies

Maternal Multiple

Mutations in *protein coding genes* usually display ______ inheritance, and affect _____ respiratory chain enzyme deficiencies

Maternal Specific

Other therapies include treating:

Metabolic disturbances Myoblast transfer Gene therapy (manipulation of nuclear DNA) Gene shifting (increase ratio of wild type)

Inreased signaling and ATP utilization leads to increased ATP production in the mitochondria and increased production of ________. This causes _______ signals to be sent to the nucleus which leads to an increase of what?

Metabolites Retrograde Transcription of mitochondrial proteins

Again, what is the threshold effect?

Minimum critical relative proportion of mutated vs. wild type mtDNA that is required to express a defect

mRNAs are translated within _______ on mitochondria-specific robosomes

Mitochondria

What does strong positive succinate dehydrogenase staining cells denote?

Mitochondrial biogenesis in response to cellular energy deficit caused by the deficiency in the respiratory chain function

Nuclear mutations can affect a wide variety of mitochondrial processes, such as:

Mitochondrial dynamics (fusion, fission and transport) Mitochondrial protein import mtDNA replication Mitochondrial nucleotide metabolism (replication, repair, maintenance of mtDNA) Mitochondrial translation Mitochondrial respiratory chain activity (structural subunits of respiratory chain complexes, as well as assembly factors and cofactor synthesis)

mtDNA mutations usually affect what?

Mitochondrial protein synthesis Specific respiratory chain subunits

mtDNA-encoded polypeptides combine with nucleus-encoded peptides to form what?

Mitochondrial respiratory chain peptide

What are some biochemical deficits that can occur?

Mitochondrial substrate transport Mitochondrial substrate utilization Citric acid cycle Respiratory chain enzymes

Partitioning of mtDNA to daughter cells is called what? What can this result in?

Mitotic segregation Proportions of wild-type and mutated mtDNAs *varying* among cells, tissues or different individuals in a family

What is *heteroplasmy?*

More than one haplotype of mtDNA in an individual, cell or mitochondrion

There are also _____-system disorders, such as:

Multi MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, stroke-like episodes) MERRF (myoclonic epilepsy and ragged-red fibers) KSS (Kearns-Sayre syndrome)

There are ______ forms of inheritance in Leigh syndrome. What are they?

Multiple mtDNA and nuclear DNA encoded

What are the other complexes that are partially formed from mtDNA-encoded polypeptides?

NADH dehydrogenase (complex I) Ubiquinol-cytochrome c oxidoreductase (complex III) Cytochrome C oxidase (complex IV) ATP synthetase

Mitochondrial ribosomal proteins, translation fators and tRNA synthetases are encoded in the ________ DNA

Nuclear

What are the three general types of drugs that may affect function?

Nucleoside reverse transcriptase inhibitors Antibiotics (chloramphenicol, linezolid) Pesticide exposure

Synthesis of the mitochondria is controlled and directed by which organelle?

Nucleus

The synthesis of a majority of respiratory chain subunits along with the biogenesis and other functions of the mitochondria are directed by the...

Nucleus

mtDNA is replicated by ______-encoded, mitochondria-specific DNA polymerase (POLG) and associated _______ factors

Nucleus Replication

Transcription of mtDNA is performed by what?

Nucleus-encoded, mitochondria-specific RNA polymerase and transcription factors

Decreased capabilities from aging combined with respiratory chain deficiency may make these tissues susceptible to dysfunction in _______ individuals

Older

What is *homoplasmy?*

Only one haplotype of mtDNA in an individual, cell or mitochondrion; all identical

Mammalian mtDNA is inherited via the ________ cytoplasm, and therefore has a ______ pattern of inheritance

Oocyte Maternal

Mitochondrial DNA contributes only to the synthesis of a small portion of the mitochondrial proteins needed for...

Oxidative phosphorylation

The extent to which the dysfunction occurs is dependent on the extent to which a given tissue requires....

Oxidative phosphorylation ot produce energy

mtDNA encodes for 13 ________, 2 rRNAs, and 22 _______

Polypeptides tRNAs

What are some morphological features of mitochondrial diseases?

Presence of raged-red fibers in the skeletal muscle (denotes mitochondrial proliferation) Presence of morphologically abnormal mitochondria Cytochrome C oxidase-negative cells Strong *positive succinate dehydrogenase* staining cells

How does Kearns-Sayre Syndrome (KSS) present?

Progressive external ophthalmoplegia Pigmentary degeneration of retina Onset before age 20

Clinically, mitochondrial disorders are diverse. What are *simple muscle disorders?*

Progressive external ophthalmoplegia (PEO) (Paralysis of extraocular muscles)

Heteroplasmy and mitotic segregation result in different _____ of mutated and wild type mtDNA in cells or tissues

Proportions

What are some biochemical deficiencies associated with Leigh's syndrome?

Pyruvate metabolism NADH dehydrogenase Succinate dehydrogenase Cytochrome c oxidase ATP synthetase Multiple respiratory chain enzyme deficiencies

At cell division mitochondria and mtDNAs are distributed to daughter cells, which occurs at _______

Random

All mtDNA encoded proteins are _____________ subunits

Respiratory chain

What does mtDNA contribute to the synthesis of?

Respiratory chain complexes

Numerous *biochemical deficits* can result in...

Respiratory chain dysfunction

What are the signals called that originate from the mitochondria?

Retrograde signals

How do pesticides elicit effects on the respiratory chain?

Rotenone - inhibits NADH dehydrogenase Results in *Parkinson-like features*

How does myoblast transfer work?

Skeletal muscle is composed of multi-nucleated fibers formed from the fusion of many myoblasts. Wild type mtDNA could be transferred to affect muscle *via myoblasts* *Would only benefit skeletal muscle*

Large scale rearrangements of mtDNA are usually ______ or exhibit _______ inheritance patterns, and are associated with ________ respiratory chain enzyme deficiencies

Sporadic Mendelian Multiple

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) presents in what way?

Stroke-like episodes at a young age, encephalopathy, lactic acidosis, ragged-red fibers in muscle biopsy

Nuclear mutations can affect what?

Structural respiratory chain subunits Respiratory chain complex assembly factors Mitochondrial protein transport Mitochondrial membranes and motility Mitochondrial transcription and translation mtDNA maintenance and replication

The mtDNA encodes a _______ of mitochondrial *respiratory chain* units and RNAs necessary for their synthesis

Subset

Pathognomonic lesions of this disease include:

Symmetric, bilateral cell necrosis, demyelination, vascular proliferation in the basal ganglia, thalamus and brainstem

Mitochondria-specific ribosomes are prokaryote-like in that...

They are sensitive to some inhibitors of bacterial translation, such as *chloramphenicol and tetracycline*

Phenotypic presentation of a heteroplasmic mutation is dependent on the ______ effect

Threshold

Though *any tissue* may be affected, which types of tissues are more severely affected than others?

Tissues with high oxidative demands (brain and muscle)

(True/false): The contribution of both genomes is required for mitochondial respiratory chain function

True

Tissues have different metabolic _________ to mitochondrial respiratory chain dysfunction

Vulnerabilities

Analysis of mtDNA is more sensitive than that of nuclear DNA. Why is this?

mtDNA is present in thousands of copies per cell, whereas the nuclear sequence is present in only two copies per cell

KSS is associated with which type of mutation?

mtDNA large-scare deletion; multiple respiratory chain enzyme deficiencies *Sporadic* in most patients

Phenotypic expression of a respiratory chain defect can change over time. What are the reasons for this?

mtDNA mutation levels can change due to cell growth and mitotic segregation Metabolic demands of tissues can change during growth and development Metabolic capabilities of numerous tissues decline during aging

What are four etiologies of mitochondrial diseases?

mtDNA mutations Nuclear DNA mutations Nuclear DNA encoded mutations that *alter mtDNA maintenance* Acquired mitochondrial dysfunction

All required factors except what are *nuclear-encoded?*

tRNAs and 2rRNA