Genetics 4 - Mitochondria Flashcards
learning outcomes
what is a mitochondrion
functions
genome
how did it originate
double membrane-bound organelle found in most eukaryotes
powerhouse of the cell - generate ATP
also involved in cell signalling
growth
death
cell cycle regulation
has independent (of nuclear DNA) circular genome
thought to have originated as free living bacteria which were taken up by eukaryotes to carry out ox phos (endosymbiotic theory)
no of mito. in nucleated cells
500-2000
cone cell photoreceptor of the eye - proportion of mito.
also muscles of eye
heart
mito. make up 80% of IC volume
60% - lateral rectus and other extra-ocular muscles
40% - heart
high proportion in energy dependent cells e.g. CNS, brain, eye, heart
human vs mitochondrial genome
- carriers of DNA
- sequences responsible for
- no of copies per cell
NUCLEAR GENOME
c. 23000 protein coding genes, 3200 Mb - million bps
23 de linear chromosomes (50-260 Mb each)
1.5% is protein coding sequences
non-functional gene related sequences (pseudogenes and gene fragments)
MITOCHONDRIAL GENOME
37 genes, 16.6 Mb
circular dsDNA (linear forms exist)
16569 bp (93% coding mRNA or tRNA or rRNA) - efficient
dozens of copies per mito. and 1000s of this genome/cell
mtDNA sequence
Cambridge sequence
no of genes of mito. genome
proportion that code for non mRNA
37 genes total
24 encode for non mRNA
22 mitochondrial tRNA (white)
1 mitochondrial 23S rRNA (blue)
1 mitochondrial 16S rRNA (blue)
no of genes transcribed and translated to proteins on mt ribosomes
what are they related to
13
all related to ox phos
7 NADH dehydrogenase subunits (ND - yellow) I
3 cytochrome C oxidase (COX - orange) IV
2 ATPase (ATP - red) V
1 cytochrome B (CYTB - peach) III
I
NADH dehydrogenase subunits
7 present
IV
cytochrome C oxidase
3 present
V
ATPase
2 present
III
cytochrome B
1 present
electron transport chain and ox phos
5 protein complexes
Pass protons along to create proton conc gradient
Protons pumped into space between inner and outer membranes
ATP produced
ROS generation and ATP synthase
ROS are generated during ATP synthesis
rate of O2 consumption has to correspond to rate of ATP synthesis so that ROS are neutralised
uncoupling of generation and consumption leads to accumulation of ROS - O2 ions and peroxides which are toxic
ROS uncoupling and accumulation
may occur in some mito. diseases especially if person has a fever
ROS accumulation ⇒ oxidative damage (including to mtDNA)
may trigger apoptosis
CNS may be particularly vulnerable to this
infection may trigger neurological problems in patients with mt disease
no of gene products needed to make a mitochondrion
where are mito. proteins encoded
3000 gene products to make a mito.
most mito. proteins are encoded on nuclear genome (transferred from mito. genome over evolutionary time) and synthesised in cytoplasm
therefore most inherited disorders of mito. are related to changes in nuclear DNA rather than mtDNA
no of mito. gene products needed to make ATP
13 - mtDNA
77 - nuclear genome
what remaining mito. gene products do
signalling molecules involved in regulation of:
MP
cell cycle control
development
apoptosis
cellular metabolism
how is mitochondrial DNA inherited
almost exclusively along maternal line
fertilised oocyte degrades mtDNA carried by sperm
mothers transmit mtDNA to sons and daughters
only daughters can transmit their mtDNA to the next generation
sons can inherit mtDNA disease but can not transmit
mtDNA sequence variation
mtDNA of any individual shows variation from Cambridge Consensus Sequence
most variation is silent polymorphisms
region on mtDNA - useful for forensic purposes
Control Region
highly polymorphic
mtDNA copies per cell
replication and cell cycle
100s-1000s of mtDNA copies per cell
variable - replication not coordinated with cell cycle
mtDNA vs genomic DNA - variation
reduced stringency of proofreading and replication error correction with mtDNA
no mtDNA repair mechanism
many fold higher sequence variation than genomic DNA
homoplasmy
all mtDNA sequences are the same
heteroplasmy
variation in mtDNA sequences (common)
intercellular heteroplasmy
variation between cells
intracellular heteroplasmy
variation within cells
what represents the only source of mtDNA genetic diversity
mutation
as mitochondria do not undergo recombination during cell division
if an individual has a mtDNA mutation associated with a dysfunctional allele product, what does it mean for their daughter cells and somatic cells
proportion of mtDNA sequences in each cell/tissue that carries the dysfunctional allele may vary
during cell division random segregation of mito. (and mtDNA) between 2 daughter cells occurs
however mtDNA mutations in somatic cells are not heritable
mitochondrial heteroplasmy and threshold effect
phentoypically affected daughter cells are the ones that end up with more mito. with mutated DNA
Incomplete penetrance
Variable expressivity
Pliotrophy - 1 gene can influence 2 or more seemingly unrelated phenotypic traits
pliotrophy
1 gene can influence 2 or more seemingly unrelated phenotypic traits
how can oocytes vary
different oocytes from one woman may vary in the extent of heteroplasmy
some may get a high proportion of pathogenic mtDNA - genetic bottleneck
predicting phenotype from genotype and genetic counselling
what does phenotype depend on
proportion of pathogenic mtDNA in tissues of each family member
highly variable
hallmark of disease associated with mtDNA
identical mtDNA mutations may be associated with different manifestations
can also happen that very similar manifestations arise from different mutations
what is mitochondrial disease often associated with
systems involved
multi-organ degenerative disease
especially high energy organ systems
CNS
muscle (skeletal and heart)
liver
kidney
things to remember
learning outcomes
retina of a patient with LHON
Leber Hereditary Optic Neuropathy
bilateral, painless, subacute visual failure that develops during young adult life
males are 4x more likely to be affected than females
symptoms of LHON
entirely asymptomatic until onset in 1 eye
similar symtpoms appear in other eye an average of 8 weeks later
in 25% of cases, visual loss is bilateral at onset
small % of cases, central vision gradually improves but recovery is incomplete
for most central vision loss is profound and permanent
some individuals with LHON, usually women, may also have MS like illness - LHON plus
type of mutation - LHON
point mutation
LHON severe phenotype, with little chance of recovery mutation
m.3460G>A in MT-ND1
most common (Asians/Caucasians) LHON mutation
m.11778G>A in MT-ND4
mutation associated with LHON in French Canadians
mild, with best long-term prognosis
m.14484T>C in MT-ND6
FOUNDER EFFECT = Small population that interbreed ⇒ Higher incidence of particular allele
LHON heteroplasmy
(difference in mito. sequence in DNA =) heteroplasmy in polymorphonuclear leukocytes occurs in circa 10-15% of those with a pathogenic LHON causing mtDNA sequence
individuals with a m.11778G>A pathogenic variant load of <75% in their leukocytes may be unaffected
heteroplasmy in cone cells may influence risk of developing visual impairment
in general heteroplasmy of < 75% pathogenic variant load is rare
majority of LHON carriers are homoplasmic
nature of mtDNA - majority of LHON carriers
homoplasmic
LHON management
avoid smoking
limit alcohol use
avoid other environmental toxins
diet
LHON genetic counselling
gender and age dependent penetrance - 50 (av age of onset)
mother of a proband usually has the mtDNA mutation and may or may not have symptoms
up to 40% of cases = SIMPLEX - occur in a single individual in a family
transmission of LHON - males vs females
a male (affected or unaffected) with a primary LHON-causing mtDAN mutation cannot transmit the mutation to any of his offspring
a female (affected or unaffected) with a primary LHON-causing mtDNA mutation transmits the mutation to all of her offspring but to varying degrees (heteroplasmy) -
Depends on proportion of mutated mtDNA that ends up in the ova that make the child - Also environmental factors - THRESHOLD EFFECT
prenatal diagnosis of LHON
mtDNA mutational load in amniocytes and chorionic villi is unlikely to correspond to that of other foetal or adult tissues
presence of mtDNA mutation does not reliably predict the occurence, age of onset, severity, or rate of disease progression
penetrance of LHON
50% of males, 10% of females who harbour a primary LHON causing mtDNA mutation become affected
a compensatory mechanism of activated mitochondrial biogenesis and increased mtDNA copy number may explain why some carriers are unaffected
promoted by oestrogens in females, which amy partially explain the gender bias
true
affected person is more likely to homoplasmic than a non-affected person
males with mtDNA mutation 4x more likely to manifest LHON than females with the same mtDNA mutation
haplogroups
describes individual branches, or closely related groups of branches, on the genetic family tree of all humans
all members of a haplogroup can trace their ancestry back to a single individual
haplogroups arose through mutation and migration
mtDNA haplogroups
mtDNA sequence polymorphism variations that have occurred over more than 150,000 years and correlate with the geographic origins of populations traced through maternal lineages
daughters of cinderella
what mutation is possible for LHON, and is in fact common with mtDNA
de novo mutation
how to group individuals that appear to have a more/less recent common ancestor
by aligning mtDNA sequences
members of a haplogroup have a MRCA
subclades within haplogroups with an even MRCA
members of an mtDNA haplogroup - mtDNA
members of an mtDNA haplogroup have more similar total mtDNA to other members of the same haplogroup
have similar mutations/polymorphisms in other mtDNA genes
as most mtDNA codes for proteins, mito. polymorphisms can influence energy metabolism and contribute to the expression of the overall clinical phenotype
mitochondrial haplogroups and LHON - higher risk of LHON with what haplogroups
Parkinson’s - associated haplogroups
higher incidence among mtDNA haplogroups H but lower for J and K
protection from sepsis
H haplogroup
increased longevity
I
J
T
uquivalent haplogroup for male line
Y chromosome
EXCEPT FOR pseudoautosomal region
is mtDNA inheritance exclusively matrilineal
the central dogma of maternal inheritance of mtDNA remains valid
there are some exceptional cases where paternal mtDNA could be passed to the offspring
Leigh Syndrome
mortality
progressive neurometabolic disorder
50% mortality per year from diagnosis
characterisations of Leigh Syndrome
mito. dysfunction caused by hereditary genetic defect
MRI necrotising lesions in the midbrain and brainstem
bilateral symmetrical degeneration of the brainstem, cerebellum and basal ganglia
typical onset in infancy or childhood, often after a viral infection (metabolic challenge)
may begin later
1 in 40,000 births
clinical manifestations of Leigh Syndrome
elevated lactic acid (blood and CSF)
bilateral lesions in the thalami, posterior parietal and temporal lobes
positive on metabolic screening
cough, cold, fever, difficulty breathing
seizures
slow to obtain developmental milestones
dysarthria
dysphagia
hypertonic and involuntary movements
palpable liver of 2cm below costal margin
Leigh-like Syndrome
criteria for LS only partially met despite overall clinical picture indicative of LS
often peripheral nervous system involvement, including polyneuropathy or myopathy
often non-neurological abnormalities - dysmorphic features, cardiac, endocrine and GI
resp impairment due to brainstem involvement usually absent
genetic heterogeneity of Leigh Syndrome
mutations in at least 75 genes
mito. disorder with broadest genetic (and clinical) heterogeneity
pyruvate dehydrogenase (PDHC) deficiency (citric acid cycle)
resp chain enzyme defects - complexes I, II, IV and V (ox phos)
NARP mutation
MERRF mutation
4 different patterns of inheritance for Leigh Syndrome
matrilineal (mitochondrial) 20%
de novo mutation
X linked recessive
autosomal recessive
so many patterns because so many genes are involved
present in mtDNA, nuclear DNA, X chromosomes
where are most mito. proteins encoded
on nuclear genome
synthesised in cytoplasm and transported to mitochondria
most inherited disorders of mito. are related to changes in nuclear DNA rather than mtDNA
these mutations are inherited according to the classic Mendelian rules, not mitochondrial (matrilineal) inheritance
mutations where cause the majority of PDH complex deficiencies
in the X linked pyruvate dehydrogenase (E1) and α subunit (PDHA1) [nuclear DNA]
inheritance pedigree may appear autosoma lrecessive because parents are not usually affected
in females (XX), a mutation would have to occur in both copies ofthe gene to cause the disorder
unlikely, so males are affected by X linked recessive disorders much more frequently than females
how to prevent passing on your hereditary mitochondrial disease
mito. transplantation/donation
can be done before or after fertilisation
3 parent embryo
things to remember
