Mitochondrial Genomes and Mutations

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Mitochondrial Genomes and Mutations

• MUPGRET Workshop
• June 21, 2005
• K. Newton presenter

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Organelle Genetics

• Terms are not necessarily equivalent!
• Maternal inheritance
• Non-Mendelian inheritance
• Ratios do not fit those proposed by Mendel
• Cytoplasmic inheritance
• Nonchromosomal inheritance
• Plasmon genetic elements of the cytoplasm
• e.g. homoplasmic, heteroplasmic

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• Examples of non-Mendelian inheritance
• Variegated-shoot phenotypes in four oclocks
  (Correns, 1908)

Mixed chloroplasts White/green
Mutant chloroplast White non-photosynthetic
Normal chloroplast Green photosynthetic
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Maternal inheritance
The female phenotype in a cross is always
expressed in its offspring

• Experiments were performed by Correns on the four
  o'clock plant
• Green, variegated (white and green) or white
  leaves
• Normal flowers develop at different locations
  on the plant
• Crosses were made among the flowers associated
  with
• Female Male Phenotype Progeny Phenotype
  Phenotype
• Green Green, variegated or white Green
• Variegated Green, variegated or white
  Variegated
• White Green, variegated or white White

The progeny cross always exhibited the color of
the leaf of the female Trait expresses maternal
inheritance Animals and most plants,
including corn, Arabidopsis, wheat and tomato
show strict maternal inheritance of organellar
(chloroplast and mitochondrial) DNA.
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In plants, sometimes observe biparental
inheritance

• Inheritance of variegation suggesting both
  parents contribute, but ratios are non-Mendelian.
• Male (pollen) transmission of chloroplasts
  plastids found for several plant species
• e.g. Pelargonium (geranium), Oenothera, Medicago,
  Phaseolus, Acacia
• Some plants have regular paternal transmission
  of plastids through pollen.

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Origins of mitochondria chloroplasts

• Both mitochondria and chloroplasts are believed
  to be derived from endosymbiotic bacteria.
• Endosymbiotic bacteria free-living prokaryotes
  that invaded ancestral eukaryotic cells and
  established a mutually beneficial relationship.
• Mitochondria - believed to be derived from a
  photosynthetic purple bacterium that entered a
  eukaryotic cell gt billion years ago.
• Chloroplasts - believed to be derived from a
  photosynthetic cyanobacterium.
• Many required mitochondria and chloroplast
  proteins also are coded by nuclear genes.
• numtDNA nuclear mtDNA (mtDNA transposed to the
  nucleus)

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mtDNAs --- Overview

• mtDNAs occur in (almost) all aerobic eukaryotic
  cells generate energy for cells by oxidative
  phosphorylation (producing ATP).
• Most mtDNA genomes are circular and supercoiled
  (linear mtDNAs occur in some protozoa and some
  fungi).
• mtDNAs lack histone-like proteins.
• Copy number is high, multiple genomes per
  mitochondria and many mitochondria per cell (can
  easily PCR).
• Sizes of mtDNA varies widely.
• Humans and other vertebrates 17 kb
• (all of mtDNA codes gene products)
• Yeast 80 kb
• Plants 100 kb to 2 Mb
• (lots of non-coding mtDNA)

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Plant mitochondrial genomes are large variable
in size
Photo D. Stern and K. Newton
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37 genes
Griffiths et al. Genetic Analysis 7th edition
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Maize mitochondrial genome
58 identified genes 33 known proteins 21 tRNAs
(for 14 diff aa) 3 rRNAs A tRNA is carried
on a 2 kb linear plasmid
570 kb
From Clifton et al. 2004, Plant Physiology
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What is in the NB maize mitochondrial genome ?
Based on genome complexity (one copy of large
repeats removed) 520 kb for maize NB 359 kb for
rice
From Clifton et al. 2004, Plant Physiology
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Comparison of sequenced plant mtDNAs

• Liverwort 186 kb, 66 genes
• Arabidopsis 367 kb, 58 genes
• Rapeseed 222 kb, 54 genes
• Sugar beet 369 kb, 59 genes
• Rice 491 kb, 60 genes
• Maize 570 kb, 58 genes
  
• 1 carried on plasmid
  

Why study plant mitochondrial DNAs?
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Louis with inbred corn
Louis with hybrid corn
Mitochondrial DNA mutations can help in the
production of hybrid corn!
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CMS cytoplasmic male sterility is a trait
carried by mitochondrial DNA Rf Restorer of
fertility gene--from nucleus which causes an
override of mitochondrially-determined sterility
Drawn by K.T. Yamato
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Cytoplasmic Male Sterility
One type of mitochondrial mutation in
plants--chimeric genes Defect is stage specific.
maize T-urf13 (Dewey et al. Cell 44439)
rrn26 3 flank
?
rrn26
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3
88
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amino acids
Promoter is a strong promoter From the atp6 gene
The molecular basis of cytoplasmic male sterility
and fertility restoration. P.S. Schnable R.P.
Wise Trends in Genetics 1998
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Petunia CMS
Flower from CMS petunia no pollen produced
Normal petunia with abundant pollen
petunia S-pcf (Young and Hanson Cell 5041)
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Human Mitochondrial Genomes

• Human Nuclear Genome About 30,000 genes on 23
  chromosomes (3.3 billion base pairs/haploid cell)
• Mitochondrial Genome contains 37 genes
• 13 code for some of the proteins involved in
  oxidative respiration
• 22 tRNA genes
• 2 rRNA genes
• 16,569 base pairs, circular, very compact, filled
  with genes

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Maternal Inheritance of mtDNA defects
Mitochondrial Inheritance. As mitochondria are
inherited almost exclusively from the mother,
defects in mtDNA will be passed on from the
mother to her children, as illustrated in this
pedigree.
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Human Mitochondrial DNA Mutations

• Are maternally inherited only offspring of
  affected mothers are affected
• Show deficiency in mitochondrial function
• Are caused by a mutation in a mitochondrial gene
• Examples
• myoclonic epilepsy and ragged red fiber disease
  (MERRF)
• Deafness, dementia, seizures
• Point Mutation in a mitochondrial tRNA

• Lebers Hereditary Optic Neuropathy (LHON)
• Sudden bilateral blindness
• Point mutation in small subunit of Complex I of
  the ETC
• Kearns-Sayre Syndrome (KSS)
• Symptoms in eyes, muscles, heart, brain
• Deletion mutation in mtDNA

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DiMauro et al. 1998 BBA 199-210. As shown in
Griffiths et al. Genetic Analysis 7th edition
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Commonly Affected Systems in Mitochondrial
Disorders
http//www.mitoresearch.org/treatmentdisease.html
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Subunits of complexes in the mitochondrial
electron transfer chain are encoded by both
nuclear and mitochondrial DNA
Note This is for human mtDNA. Plant mtDNAs code
for a few extra subunits Mutations in the nuclear
genes coding for subunits of the mitochondrial
ETC complexes are usually inherited as Mendelian
recessive traits
Figure from Griffiths et al. Genetic Analysis,
7th edition
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If mitochondrial function is so important, why
arent all of the defective mitochondrial DNA
mutations lethal?Heteroplasmy
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Heteroplasmic Cells
Homoplasmic Cell
70 mutant mitochondria severe symptoms
30 mutant mitochondria mild symptoms
HETEROPLASMY Normal mitochondria with normal
DNA vs Mitochondria with mutant DNA "Homoplasmic
Cell. Healthy people have homoplasmic cells --
that is, each cell has normal mitochondrial DNA.
People with mitochondrial DNA mutations have
heteroplasmic cells. Each cell has a mixture of
good and bad mitochondria.
http//www.mitoresearch.org/mitodiseases.html
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Variable penetrance of mitochondrial disease
corrrelates With the amount of mutant mtDNA
http//www.mitoresearch.org/mitodiseases.html
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From the following articlePremature ageing in
mice expressing defective mitochondrial DNA
polymerase Aleksandra Trifunovic, Anna
Wredenberg, Maria Falkenberg, Johannes N.
Spelbrink, Anja T. Rovio, Carl E. Bruder,
Mohammad Bohlooly-Y, Sebastian Gidlöf, Anders
Oldfors, Rolf Wibom, Jan Törnell, Howard T.
Jacobs and Nils-Göran LarssonNature 429, 417-423
(27 May 2004)
Premature aging in mice carrying mtDNA mutations
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Heteroplasmy Mitochondrial Defects in Corn
Deletions in in essential mitochondrial genes
(Newton lab focus)
Reduced height
Striped (sectored) leaves
NCS
Sectors of aborted kernels
Normal