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Motor Neurone Disease Exam/Essay Summary
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Part 1: Disease IntroductionDescription: human neurodegenerative disease causing atrophy and skeletal muscle changes
Often lower motor neuron degeneration + upper motor neuron degeneration = loss of corticospinal
neuronal soma in primary motor cortex
Loss of corresponding axons in lateral corticospinal tract of SC
Disease Phenotype of Motor Neuron Disease
Group of diseases affecting motor neurons
1. Upper motor neurons (in the brain)
a. Upper motor neurons send signals to the lower motor neurons which send signals to
muscles
b. Send axons down through the brain, brain, brainstem, and spinal cord to form a
glutamatergic synapse on the lower motor neuron
2. Lower motor neurons (in the brainstem and spinal cord)
a. Controls muscles in the face, mouth, throat, and tongue
b. Lower motor neurons in the spinal cord control all the voluntary muscles of the body, in
limbs, trunk, head and neck and the muscles used for breathing
a. Motor neurons selectively degenerate in ALS but not ALL motor neurons die
b. There is a differential vulnerability – motor neurons under voluntary control most vulnerably
c. More resistant motor neurons include
i. Those controlling urethral and anal sphincters located in the Onuf’s nucleus
ii. Those controlling eye muscles in the oculomotor nucleus
d. Motor neurons innervating fast twitch muscle fibres are also more vulnerable than those
innervating slow twitch muscle fibres – some muscle groups are more affected than others
e. Does MN vulnerability depend on activity/metabolic load?
Pathological findings
Widespread muscle atrophy + few gross pathological changes in brain and spinal cord
Thinned appearance of motor nerve roots when viewed in comparison with sensory roots
Focal atrophy of motor cortex occasionally observed
More pronounced atrophy of frontal and temporal lobes can be seen in frontotemporal lobar
dementia-motor neuron disease
Loss of white matter in ALS patients but NO diminished weight of the brain or gross changes to the
centrum semiovale or internal capsule
Different types of motor neuron disease
Can be significant overlap between different forms, and one can develop into another over time – specific
diagnosis not always possible
1. Amyotrophic lateral sclerosis (ALS)
2. Progressive bulbar palsy (PBP)
3. Progressive muscular atrophy (PMA)
4. Primary lateral sclerosis (PLS)
5. Pseudobulbar palsy
a. Similar to progressive bulbar palsy
b. Affects motor neurons that control the ability to talk, chew, and swallow
6. Spinal muscular atrophy (SMA)
a. Four types depending on age of first symptoms
b. Affects lower neurons
c. SMN1 gene
7. Spinal bulbar muscular atrophy or Kennedy’s Disease
a. Lower motor neurons
b. Affects males only
c. AR gene
MND statistics
Incidence: 2 cases per 100,000 people per year
Prevalence: due to rapid progression of MND, the prevalence is 7 per 100,000
MND affects around 5000 people in UK at any one time
,Motor Neurone Disease (MND)
Can affect any adult at any age but the highest incidence is between 50-70 years of age
Men are affected approx. twice more often
Key feature is muscle weakness/paralysis in more than one muscle group
Often considered to be a rare disease
MND kills 5 people in the UK every day
Comparison with neurological diseases
Proportion of total deaths
o In 2007, 1 in 300 people who died in England, Wales and Northern Ireland had MND
o Ranking order to number of deaths from each disease, MND is third, below AD and PD
Symptoms of ALS
Most patients present with combo of upper and lower motor neuron signs
Weakness in legs and/or arms – problems with walking and manual tasks in 75%
Difficulty with keeping head upright
Difficulty with speech and swallowing
Problems with breathing
Emotional lability
What is not affected
Senses of touch, taste, sight, smell and hearing
Does not directly affect bowel and bladder functions
In most cases, sexual function is unaffected until the later stages of the disease
Eye muscles are generally not affected
Heart muscles not affected
Cognitive Changes and Dementia in MND
Up to 50% of patients undergo some cognitive changes or personality change – can be very mild
and go un-noticed, in others these changes can be quite marked
Mild difficulties may improve
Memory, learning, language and concentration – problems may be subtle and difficult to separate
from normal ageing process
5% cases have frank frontal dementia – due to degeneration in the frontal lobe
FTD can precede or follow onset of MND symptoms
o Establishes association between MND and FTD – disorders may be two extremes of a
spectrum of neurodegenerative disorders
Part 2: Genetics of ALS
Sporadic ALS (sALS): 90% of all cases no family history
Familial ALS (fALS): 10% of cases family history
Little difference in the symptoms and course of fALS and sALS
Susceptibility genes: some ALS-causing genes have been identified – play a role in sALS but do not
cause the disease on their own
Introduction
SOD1 was the first genetic cause of ALS to be discovered in 1993 hinged upon existence of
large families for linkage analysis, positional cloning to identify genes in linkage interval, and use of
Sanger method for gene sequencing
Today, more than 3 dozen genes are linked to ALS
o Fuelled by automation of Sanger sequencing, publication of the human reference genome,
and utilisation of next-generation technologies (whole-exome and whole-genome
sequencing)
Familial LS genetic explanation can be achievedin 38% to 73% of pedigrees in white populations
8% to 14% of individuals with sporadic ALS carry moderate or high-penetrance mutations in known
genes
Argument: The genetic factors of ALS are due to a wide range of altered functioning highly genetically
heterogeneous disorder
,C9orf72
RNA Binding Protein mutation is an expansion of the hexanucleotide repeat causes
Role of WT - C9orf72 gene encodes an open reading frame on chromosome 9 the gene
harbors an expansion of a GGGGCC intronic repeat sequence (hexanucleotide
expansion)
- Hexanucleotide expansion occurs in a non-coding region (does not encode a
protein) so it generates a lengthy RNA transcript from the expanded DNA
- Expansions of hexanucleotide repeat in first intron of C9ORF72 are most
common and identifiable cause of Als
- Unaffected individuals have low numbers of GGGGCC repeats in first intron of
C9ORF72
Mutant - The mutation is a hexanucleotide repeat expansion (HRE) in a noncoding region
of C9ORF72, a gene of unknown function- is ths most common cause of ALS
- The normal gene has between 2 and 20 GGGGCC repeats, while the mutant
version has several hundred or more.
- Pathology linked to RNA - linking it to TDP-43 and FUS, but not to SOD1
(previously the most common gene for ALS)
- This mutation is thought to account for ~ 40% FALS cases of European descent,
30% fALS, 8-10% of sALS and 0.5% of controls
Those with ALS show expansion of the repeat into thousands of copies
- Explains 15% of the disease (50% familial, 5-10% sporadic in white populations)
- Minimum number of repeats needed to develop disease is uknown
- 20-30 may predispose to neurodegeneration
, - Those with the repeat have similar or earlier ages of onset and are more likely to
present with bulbar symptoms and have disease course complicated
Mechanism Gitler and Tsujii, 2016
of Disease Harms et al., 2013
Haeusler et al., 2014
Gain of Function
The G rich nature of the repeats makes the transcript susceptible to the formation of
highly stable G-quadruplex secondary structures that trigger abnormal reactions with a
range of proteins (Fratta et al., 2012)
1. RNA misprocessing
a. Processing of the expanded transcript is altered (Sareen et al., 2013)
i. C9ORF72 transcript containing two alternatively used first exons
(1a and 1b) with repeat expansion residing in the intron between
them
ii. Isoforms produced from the transcript contain one or both fo the
exons with presence of repeat expansion favouring transcription
from exon 1
iii. Leads to an increase in proportion of transcript produced that
contain the repeat expansion
b. Portion of repeat-containing transcripts are subject to repeat-associated
non-ATG (RAN) translation
i. Results in the production of abnormal dipeptide repeat proteins
that form neuronal inclusion in CNS
ii. GGGGCC repeats are translated into highly repetitive dipeptide
proteins using RNAT from both the sense and antisence strands
1. The dipeptide proteins form intracellular protein aggregates
(identification is pathognomonic for the disease)
2. Some dipeptide proteins are detected in the CSF of
symptomatic and presynaptic C9 expansion carriers 0
biomarker?
3. Belzil et a., 2013
4. Ciura et al., 2013
5. Zhang et al., 2014
iii. fALS patients with C9orf72 mutation generally develop TDP-43
aggregate pathology neurons in hippocampus and cerebellum
contain star-shaped or small dot-like inclusion bodies that are
TDP-43 negative
1. These structures contain proteins including dipeptides
corresponding to 3 possible open reading frames of the
intronic GGGGCC expansion (Gly-Ala, Gly-Pro, and Gly-
Arg)
2. Translation of the non-coding sequence attributed to
repeat associated non-ATG-initiated (RAN) translation
3. Relevance and causative role of RAN-translated dipeptides
in ALS are currently unexplored
c. Expanded repeats cause downstream effects on RNA processing
i. RNA transcripts containing repeat expansion form RNA foci
(Dejesus-Hernandez et al., 2011) that are found in a majority of
the C9ORF72-ALS patients (Schipper et al., 2016)
ii. RNA foci are able to aberrantly interact and sequester RNA-
binding proteins dyregulated expression and splicing of other
RNAs
iii. Impairs function and leading to more general effects on RNA
expression and splicing
iv. C9ORF72 RNA transcripts containing hexanucleotide repeats
accumulate as frequent nuclear and occasional cytoplasmic foci of
affected neurons
d. May involve an unusual form of translation called repeat-associated non-
ATG translation (RAN translation) in the expanded repeat sequence,
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