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