Mol. Bio

Exam 2

Mitochondrial genetics relevance in pharmacy

• mitochondria are organelles responsible for energy production via ATP

• play a key role in metabolic processes, essential for understanding drug metabolism

• mitochondrial dysfunction can affect drug response and energy demanding tissues (muscles, fibers)

mitochondria

include several copies of circular DNA molecule

mitochondrial proteins

mitochondrial DNA (mtDNA) encodes 5% of

electron transport chain (ETC) or oxidative phosphorylation

all mitochondrial proteins are involved either in

mitochondrial DNA

nuclear DNA (nDNA) encodes 95% of

mitochondria genome

Circular DNA; 16,500 bp, 2 strands (heavy and light strand)

37 genes

the mitochondrial genome encodes

37 genes encoded in genome of mitochondria 

22 tRNA, 2 rRNA, 13 protein encoding genes 

buoyant density in a cesium chloride gradient

individual strands of double stranded mtDNA molecules are distinguished by their

guanine rich (purine rich)

the H strand is

guanine poor

the L strand is

replication origin site

the H strand D-loop region has

D-loop

displacement loop =

D-loop contains

1. 2 transcription initiation sites (2 heavy strand promoters-HSP 1 and 2)

2. LSP, light strand promoter

3. O_H : replication origin of heavy strand

replication origin site 

the L strand has its own 

11 kb away from O_H

the replication origin site of L-strand (O_L) is located approximately

H strand is synthesized

the L strand is synthesized in the direction opposite to the direction in which

asymmetric

mitochondrial genome replication is

asymmetric replication of mitochondrial genome 

initiated at 2 different times from 2 different origins of replication 

first

the H strand replication is initiated

completion of 2/3 rd of the H strand replication

the L strand replication is initiated only after

L strand replication is initiated

teh H strand remains single stranded until

reactive oxygen species (ROS) 

mitochondria are the major source of 

superoxide dismutase

SOD

glutathione peroxidase 1

GPX

superoxide (O2-) at complex 1 and 3

ROS are produced from leakage of e- to form

mitochondrial electron transport 

ROS are produced as byproducts during the 

ROS

reactive molecules and free radicals derived from molecular oxygen

ROS

oxygen, superoxide anion, hydrogen peroxide, hydroxyl radical, peroxide, hydroxyl ion

damage

ROS causes DNA

damage

ROS also causes mitochondrial DNA

DNA damage due to ROS causes

DNA fragmentation, mitochondrial DNA damage, telomere attrition, Y chromosome microdeletions, epigenetic abnormalities

nuclear DNA

mitochondrial NDA shows higher mutation rate than

mitochondrial DNA

have close proximity to ETC, thus higher chance of ROS-induced mitochondrial DNA damage

no structural proteins and lack of DNA repair machinery and close proximity to ETC

mitochondrial DNA has higher mutation rate than nuclear DNA because 

mitochondrial disorder

can affect every organ in the body

mother

children receive mitochondrial DNA from

maternal inheritance

mitochondrial disorder always follows

do not have disease 

if father has mutant mitochondrial gene the children 

all children have disease

if the mother has mutant mitochondrial genome then

very similar and can be matched

the mitochondrial DNA sequence of maternally related individuals such as a grandmother and her grandson or granddaughter are

dilution model and active degradation model

2 models for causes of maternal inheritance

dilution model 

sperms have very low number of mitochondrial DNA (100 copies while egg has 100000 copies) 

active degradation model

sperms mitochondrial DNA is degraded, either before or after fertilization

individual

all mitochondrial DNA may or may not be identical in an

homoplasmic wild type, homoplasmic mutants, heteroplasmic

3 types of genome based on mitochondrial DNA

homoplasmic wild type 

identical and wild type 

homoplasmic mutants

identical but mutant

heteroplasmic

existence of wild type mutant mitochondrial DNA (variability in drug response may occur due to differing levels of functional mitochondria)

same cell

mutated and normal mitochondrial DNA coexist in

critical threshold 

the mutated mtDNA will only cause symptoms when the number of copies exceeds a 

mitochondrial threshold

can vary from mutation to mutation, organ to organ, and between different family members

normal child

a heteroplastic mother may give birth to

highly symptomatic child

if mutation load > threshold =

moderately symptomatic 

if mutation load = threshold then child will be 

asymptomatic or normal child

if mutation load is < threshold =

mitochondrial function

certain drugs (antiretrovirals, statins, chemotherapy agents) can impair

drug induced mitochondrial toxicity

statin induced myopathy, nucleoside analog induced mitochondrial toxicity

particularly vulnerable to drug induced mitochondrial toxicity 

tissues with high energy demand (brain or muscle) are

mitochondrial disorder

Leber Optic Hereditary Neuropathy (LOHN)

LOHN

inherited form of vision loss, vision loss results form the death of cells in optic nerve

unknown

the prevalence of LOHN in most population is

northeast england and Finland 

LOHN affects 1 in 30000 -50000 people in 

optic nerve

transfers visual information from retina to brain

retinal ganglion cells (RGCs)

process visual information

axons

RGCs transmit visual information to the brain via

axons of retinal ganglion cells 

make up the optic nerve 

optic nerve

composed of RGCs axons and glial cells

LOHN pathogenesis

mutations in MT-ND1, -ND4, -ND4L, -ND6 gene to complex 1 deficiency - release of apoptotic factors ( decrease ATP, increase ROS) , RGCs death- optic nerve degeneration- LOHN

LOHN

has no proven treatment

Raxone

contains active substance idebenone is the only treatment option available at present for LOHN symptoms 

treatments for LOHN

administration of quinone analong and vitamin B12 and C supplementation

polyploid

nDNA is diploid and mtDNA is

mtDNA

nDNA has more genes than

93%

nDNA has 2% of coding sequence and mtDNA coding sequence is 

nDNA not mtDNA

introns are present in

nDNA not mtDNA

telomeres are present in

universal (AUG: methionine)

nDNA codon usage is

AUA: methionine 

mtDNA codon usage 

long polycistronic transcripts

mitochondrial genome is transcribed as

monocistronic mRNA

a mRNA that encodes only one protein (eukaryotes)

polycistronic mRNA

a mRNA that encodes several proteins (bacteria, mitochondria)