Umm AL Qura University
SINGLE GENE
DISORDERS
Dr Neda M Bogari
•
To date over 10 000 single gene traits and
disorders have been identified.
•
Most of these are individually rare but together
they affect between 1% and 2% of the general
population.
•
The management of these disorders in affected
individuals and in their extended families presents a
major challenge for clinical genetics.
HUNTINGTON'S
DISEASE (HD)
natural history of HD is characterised by
slowly progressive cell death in the
central nervous system, for which there is no
effective treatment or cure.
• The
• The
prevalence in most parts of the world is
approximately 1 in 15,000.
HD is a disorder of middle to late
adult life.
• Generally
In adult HD disease is characterized by:
a slowly progressive movement
disorder.

impairment of intellectual function with
psychiatric disturbance and eventual dementia.

The
average age of onset is around 40 years and
the mean duration of the illness is approximately
15 years.
• In
approximately 10% of cases the disorder presents
before the age of 20 years, and so called Juvenile
HD disease.
average duration of the illness is around
10-15 years
• The
• HD
is an autosomal dominant disease, close to
complete penetrance.
• It
has been noted that the disorder often shows
anticipation.
gene was located on the short arm of
chromosome 4.
• HD
• Mapping
the gene providing the first means of predictive
testing for HD
•
This work also revealed that HD homozygote are no
more severely affected than heterozygote
HD gene contain a highly polymorphic CAG
repeat sequence located in the 5' region.
• the
messenger RNA codes for a protein of
approximately 350 kDa, known as huntingtin.
• The
is expressed in many different cells
throughout the central nervous system
although its function remains unclear. One
proposed role is that it is involved in apoptosis, i.e. cell
death.
• Huntingtin
• Normal
alleles: containing 26 or fewer CAG
repeats.
• Allele
sizes of 27 to 35 CAG repeats do
not cause disease but show meiotic
instability with a potential increase or
decrease in size.
• These
'mutable' alleles, constitute a reservoir
for new mutations.
• Reduced
penetrance alleles are the third
category, these alleles containing 36 to 39
CAG repeats.
• These
are associated with either late onset
disease or complete absence of disease
expression, i.e. non-penetrance.
• Disease
alleles contain 40 or more
CAG repeats.
• There
is a direct relationship between length
of repeat and disease expression:
• the
average age of onset for repeat sizes of
40, 45 and 50 being 57, 37 and 26 years
respectively.
• Most
affected adults have repeat sizes of
between 36 and 55, whereas juvenile cases
often have an expansion greater than 60
repeats.
SUMARY
• HD
is an Autosomal dominant inheritance with
an offspring risk of 1 in 2 regardless of whether
the affected parent is male or female.
• For
reasons which are not understood, meiotic
instability appears to be much greater in
spermatogenesis than in oogenesis.
• The
discovery of the HD gene has meant that accurate
predictive testing is possible, although this should only be
offered as part of a well-planned and carefully monitored
counselling package.
• Experience
to date indicates that more women than men
take up the offer of predictive testing, with the degree of
psychological disturbance in those given positive results being
less than was expected.
• Prenatal
diagnosis is also possible for those couples who feel
that this is acceptable.
•
Obviously there are considerable emotional and ethical issues
associated with termination of pregnancy on the grounds that a
child could go on to develop a neurodegenerative disease in middle
age. This will be particularly important when effective therapeutic
strategies have been devised.
•
One appealing approach is based on the observation that large
CAG repeats result in intracellular accumulation of huntingtin
'aggregates', which are cleaved by a protease known as caspase to
form a toxic product which causes cell death (apoptosis). Caspase
inhibitors have been shown to have a beneficial effect in an HD
mouse model.
•
Other suggested therapeutic approaches include injection of fetal
stem cells into affected regions of the brain.
MYOTONIC DYSTROPHY
• Multisystem
disorder.
with cataracts, myotonia,
weakness, frontal baldness and
mental retardation.
• associated
on chromosome 19q13.3
called myotonin.
• Gene
• Its
an autosomal dominant disease and
it becomes more severe as it is
passed from generation to
generation.
MYOTONIC DYSTROPHY
increasing severity of the disease through
generation called ‘anticipation’.
• The
is expansion of a CTG repeat in the 3’
untranslated region of protein kinase gene
(Dystrophia myotonica protein kinase (DMPK).
• mutation
expansion increases as it goes from one
generation to the next via a female affected.
• the
• In
unaffected: CTG repeats 37 times.
• Affected: CTG
repeats 50 time or greater.
• Close
correlation between the number of CTG
expansion and the severity of the disease.
• Larger
CTG expansion increase the
severity of the disease.
Summary
NEUROFIBROMATOSIS
There are two main types of
neurofibromatosis, NF1 and NF2.

Autosomal dominant

Prevalence 1 in 3000

diagnostic criteria - need 2+
for diagnosis

café au lait spots (CAL)
- 6 or more: it’s a small lightbrown pigmented lesions.

neurofibromas - 2 or more:
it’s a benign tumours which
arise most commonly in the skin.
• axillary
freckling
• Lisch nodules (specks in iris): These are
small harm­less raised pigmented
hamartomata of the iris.
• optic
glioma
• thinning
of long bone cortex
• family
history

Macrocephaly ( head is abnormal large)

Short stature

Dysmorphic features (difference of body structure)

Learning difficulties

Epilepsy

Scoliosis

Pseudoarthrosis of the tibia

Raised blood pressure

Neoplasia ( abnormal mass of tissue )

CNS (optic gliomas), endocrine

Autosomal dominant

Variable expression
 inter-familial

Gene identified - 17q
 tumour

suppressor gene
Mutations different in different families
 therefore

and intra-familial
no simple diagnostic test
50% due to new mutations
 usually
paternal in origin
Neurofibromas
Neurofibromas: These many be frequent and very
prominent as in the individual in (A). They also may be
more subtle and isolated as in (B).
analysis of the NF1 gene has shown that it
encodes a protein, known as neurofibromin.
• Sequence
•
plays an important role in signal transduction by down
regulating RAS activity.
of the NF1 gene has provided a means of
offering both presymptomatic and prenatal
diagnosis using either linkage or direct
mutation analysis.
• Mapping
 The
main features of NF2 are:
 CNS
a
and spinal tumours
few CAL spots
 NF2
gene is on Chromosome 22q
Sumary
Summary
CYSTIC FIBROSIS IN
CHILDHOOD
commonest inherited life-shortening
disorder amongst Northern European
populations
• The
• Inherited
• CF
as an autosomal recessive trait
incidence = 1 in 2000
The abnormality in CFTR explains the pathology of
cystic fibrosis.
•
High sodium sweat: Primary secretion of sweat duct is
normal, but CFTR does not absorb chloride ions,
which remain in the lumen and prevent sodium
absorption.
•
Pancreatic insufficiency: Production of pancreatic
enzymes is normal but defects in ion transport
produce relative dehydration of pancreatic secretions
causing their stagnation in the pancreatic ducts.
•
Biliary disease: Defective ion transfer across bile duct
causes reduced movement of water in the
lumen so that bile becomes concentrated causing
plugging and local damage.
•
GI disease: Low volume secretions of increased viscosity,
changes in fluid movement across both small and large
intestine, dehydrated biliary and pancreatic secretions cause
intra luminal water deficiency.
•
Respiratory disease: Dehydration of the airway
surfaces reduces mucociliary clearance and favors bacterial
colonization, local bacterial defenses are impaired by local
salt concentrations and bacterial adherence is increased
by changes in cell surface glycoproteins.
•
Increased bacterial colonization and reduced
clearance produces inflammatory lung damage due to an
exuberant neutrophilic response involving mediators such
as IL8 and neutrophil elastase.
• Cloned
• Lies
in 1989
on the long arm of chromosome 7
for a protein known as the cystic fibrosis
transmembrane conductance regulator (CFTR)
• Codes
• CFTR
•
acts as a chloride transport channel
The net effect is to reduce the level of intracellular
sodium chloride, which improves the quality of
cellular mucus secretions
The Cystic Fibrosis Gene
• CF
•
shows autosomal recessive inheritance
ΔF508 mutation accounts for the majority of
mutations
•
a deletion of three adjacent base pairs which results in the loss of
a phenylalanine residue
• Over
1300 other mutations in CFTR have been
identified.
•
These include missense, frameshift, nonsense and
deletion mutations. Most of these are extremely uncommon,
although a few can account for a small but significant proportion
of mutations in a particular population.
Cystic fibrosis (CF) Gene
The remarkable spectrum of mutations (and polymorphisms) in the
CFTR gene. The density of the lower line emphasizes the broad
spectrum of changes and the difficulties that can develop attempting
testing on the basis of suspected mutation(s).
• Mutations
in CFTR can influence the
function of the protein product by:
1. causing a complete or partial reduction
in its synthesis, e.g. G542X and IVS8-6(5T)
2. preventing it from reaching the epithelial
membrane, e.g. AF508
3. causing it to function incorrectly when it
reaches its final location, e.g. G551D and
R117H.
• The
net effect of all these mutations is to reduce
the normal functional activity of the CFTR protein.
• Acts
•
as a chloride transport channel
Expressed in epithelial cells
• Pancreas  cystic fibrosis (reduced
secretion)
• Bronchial
• Sweat
enzyme
tree  recurrent infections
glands  useful diagnostic test
• Rectum  can result in rectal prolapse
• Cervix  female underfertility
• Epididymis  male infertility
• Upper
respiratory tract  sinusitis and polyposis
Summary
•
•
Pancreatic insufficiency
•
Maldigestion
•
Abnormal stools
•
Failure to thrive
Recurrent bronchopulmonary
infection
•
Pneumonitis
•
Bronchiectasis
•
Scarring
•
Abscesses
• Reduced
or absent secretions
•
Enzymes (lipase, amylase, trypsin)
•
Bicarbonate
• Malabsorpition
•
Fat, protein, complex carbohydrates
•
Not simple carbohydrates
•
Fat soluble vitamins: A, D & E
• Early
years
•
Staph aureus
•
Homophiles influenza
• Later
•
years
Pseudomonas aeruginosa
• Prone
to have antibiotic resistant organisms
• Improved
antibiotic therapy probably main reason
for enhanced survival
• Choice
of antibiotic best left to specialist clinic

Mainly older children and adults

Symptoms may be severe due to underlying
lung disease

Strong tendency to recur

Surgical management with pleurodesis
 best
for the pneumothorax
 may
inhibit later transplantation

Symptoms

Wheeze

Cough

Shortness of breath

Rust-colored sputum (occasionally)

Focal infiltrates on chest X-ray

Aspergillus & anti-aspergillus Antibodies

Treatment with corticosteroids
• Following
neonatal
screening
• Neonatal
meconium
ileus
•
Intestinal obstruction by
inspissated undigested
intestinal contents, often
with perforation and
peritonitis
• Neonatal
• Failure
meconium ileus
to thrive
• Neonatal
hepatitis syndrome
• Recurrent
• Heat
stroke due to salt loss
• Delayed
• Upper
• Male
rectal prolapse
growth and pubertal development
respiratory tract involvement
infertility
• Affects
about 10% eventually
• Impaired
glucose tolerance even
commoner
• Not
necessarily easily managed
• Anorexia
• Malabsorpition
• Refined
• Adverse
• Liver
CHO easily absorbed
effects of infection
disease
• Malabsorpition
• Abdominal
pain
• Diarrhoea
• Flatus
• Recurrent
• Liver
infection
disease
• Inactivity
• Avoidance
of enzymes
CF - Complications

Diabetes

Anorexia

CF Hepatopathy
Picture shows ascites in a child with
abdominal scars from surgery for
meconium ileus
• Sedentary
indoors existence
• Hypoxia
• Chronic
infection and inflammation
• Bone
density correlates with C-reactive
protein
• Liver
disease
• Malabsorpition
• Pubertal
delays
of fat soluble vitamins
• Malabsorption, esp
• Diabetes
• Hepatopathy
• Anorexia
• Lack
of exercise
fat malabsorption
CF - Complications

Diabetes
Anorexia
Hepatopathy
Osteoporosis

Arthritis



CF - Complications

Diabetes
Anorexia
Hepatopathy
Osteoporosis
Arthritis

Psychological disturbances




Almost invariable
 Involve family as much as patient
 Requires professional input

 Clinical
psychologist
 Social worker
CF - Other GI Complications
Neonatal jaundice
 Distal intestinal obstruction syndrome
 Gastro-oesophageal reflux
 Rectal prolapse
 Pancreatitis
 Gall stones


Salt depletion

Acrodermatitis enteropathica

Hypoprothrombinaemia

Rickets & other fat-sol vitamin deficiencies
• Constantly
• Median
improving
age at death projected around 30 yrs
CF - Prognosis

Constantly improving

Median age at death projected around
30 yrs

Main causes of death
 Respiratory failure
 Hepatopathy
80
73
67
60
60
40
59
52
3737
31
29
21
20
0
66
62
Male
Female
3-D Column 3
0-<5
29
21
15
5-<10 10-<15 15-<20 20-<25 25-<30 30-<35
18
35+
• May
thrive surprisingly well on breast milk
• May
not have obviously malabsorptive stools, but
classically stools are
•
Pale or orange
•
Very offensive
•
Greasy or oily
• Importance
• May
of record-keeping, charts,etc
have deficiencies of fat soluble vitamins
CF - Antenatal Screening
Establish CF gene distribution in local
population
 Couple screening vs sequential screening
 Ethical issues re improving prognosis
 Economic issues re increasing costs of CF
care
 May be combined with post-natal
screening


Raised immunoreactive trypsin (IRT) on heelprick blood (performed with Guthrie, thyroid and
other neonatal screening tests)

High false positive rate
 So
combined with gene mutation analysis
 But
will miss up to 30% by including gene
testing

Probably improves prognosis to be identified
early

Does permit rational decisions re future
reproduction - including extended family

Does avoid unnecessary investigation, confusion of
wrong diagnoses (and hence wrong treatment), etc
• Now
parents of an affected child can almost
always be offered prenatal diagnosis, either by
direct mutational analysis of DNA from
chorionic villi, or by linkage analysis using
polymorphic markers if one or both of the
mutations in the affected child cannot be
identified.
Cystic Fibrosis - Gene Therapy

Introduce with adenovirus
Inflammation
 Immunogenic


Introduce with liposomes

Physically difficult to generate particles
in respirable range (2-6 microns) for
deposition in airway
DUCHENNE MUSCULAR
DYSTROPHY
•
1/3500 live born males
affected
•
affected boys may appear
normal until 4/5 yrs of age
when evidence of awkward gait,
inability to run and difficulty
climbing stairs is noticed
•
progressive weakness and
muscle wasting leads to
wheelchair by about 14yrs
•
Weakness is predominately proximal
•
Associated with calf muscle hypertrophy
• Death
due to respiratory failure by 20 yrs
•
Affected boys have grossly elevated
levels of creatine kinase between 1000 20,000 iu (normal to 50)
•
Muscle biopsy shows variation in fiber
size, necrosis with replacement of muscle
fiber with fat and connective tissue
•
Two thirds of carrier females have and
elevated creatine kinase
X-Linked Recessive Inheritance
Carrier female
Affected male
Normal male



Mutant genes are on the X (sex) chromosome
Women must inherit 2 mutated copies to be affected
All men who inherit the mutation are affected
(only one X chromosome)
•
X-linked recessive
•
gene on Xp21 and called dystrophin
•
mutation in two thirds of cases is a deletion
•
1/3 cases are a new mutation
•
high new mutation rate due to the large size of
the gene (2 x 106 bp)
•
Accurate carrier detection can now be achieved
for most female relatives of affected DMD
males by direct mutation/deletion analysis or
indirectly by linkage studies using polymorphic
intragenic markers.
•
Dystrophin protein is normal
along the plasma membrane of
muscle cell
•
In DMD this protein is absent
•
Female heterozygous showed
normal and defective cells.
•
There is no known cure for Duchenne muscular
dystrophy.
•
Treatment is aimed at control of symptoms to
maximize the quality of life.
•
Gene therapy may become available in the future.
•
Activity is encouraged. Inactivity (such as bedrest)
can worsen the muscle disease.
•
Physical therapy may be helpful to maintain muscle
strength and function.
•
Orthopedic appliances (such as braces and
wheelchairs) may improve mobility and the ability for
self-care.
•
Experiments in mice have supported the idea of
treating Duchenne muscular dystrophy (DMD) by
implanting normal muscle precursor cells into
dystrophin-deficient muscles.
•
However, similar experiments on DMD patients
have had little success.
•
Gene therapy for DMD, by introducing dystrophin
constructs via retroviral or adenoviral vectors, has
been shown to be possible in the mouse, but the
efficiency and safety aspects of this technique will
have to be carefully examined before similar
experiments can be attempted in man.
•
Direct injection of dystrophin cDNA constructs into
mdx muscles has given rise to very low levels of
dystrophin and this may be a possibility for the
treatment of heart muscle.
HEMOPHILIA
•
Hemophilia is one of hereditary genetic disorders
that damage the body's ability to control blood
clotting or coagulation.
•
There is two classes of hemophilia
•
Hemophilia A, clotting factor VIII is absent.
•
In Hemophilia B, factor IX is deficient.
•
These two factor have a major role in the intrinsic
pathway activation of prothrombin to thrombin.
Which then converts fibrinogen to fibrin which form
structural framework of clotted blood
•
The incidence of Hemophilia
•
Hemophilia A is about 1 in 5,000–10,000 male births,
whileHemophilia B occurs at about 1 in about 20,000–
40,000
•
The classification of the severity of hemophilia has
been based on either clinical bleeding symptoms or on
plasma procoagulant levels, which are the most widely
used criteria.
•
Persons with less than 1% normal factor (<0.01 IU/
mL) are considered to have severe hemophilia.
•
Persons with 1-5% normal factor (0.01-0.05 IU/mL)
are considered to have moderately severe hemophilia.
•
Persons with more than 5% but less than 40% normal
factor (>0.05 to <0.40 IU/mL) are considered to have
mild hemophilia.
•
Clinical bleeding symptom criteria have been used
because patients with FVIII or FIX levels less
than 1% occasionally have little or no spontaneous
bleeding and appear to have clinically moderate or
mild hemophilia. Furthermore, the reverse is true
for patients with procoagulant activities of 1-5%,
who may present with symptoms of clinically severe
disease.
•
The genes for both FVIII (ie, hemophilia A) and
FIX (ie, hemophilia B) are located on the long arm
of chromosome X.
•
The gene for FVIII (F8C) located within the Xq28
region, is unusually large, representing 186 kb of
the X chromosome. It comprises 26 exons and 25
introns.
•
Mature FVIII contains 2332 amino acids.
Approximately 40% of cases of severe FVIII
deficiency arise from a large inversion that disrupts
the FVIII gene. Deletions, insertions, and point
mutations account for the remaining 50-60% of
hemophilia A defects.
•
Low FVIII levels may arise from defects outside
the FVIII gene, as in type IIN von Willebrand
disease, in which the molecular defect resides in the
FVIII-binding domain of von Willebrand factor.
•
The FIX gene (F9), located within the Xq27 region,
has 34 kb and composes 8 exons and 7 intervening
sequences. The mature protein is composed of 415
amino acids. Point mutations and deletions in the
FIX gene are the most common causes of
hemophilia B.
•
The hallmark of hemophilia is hemorrhage
into the joints. This bleeding is painful and
leads to long-term inflammation and
deterioration of the joint, resulting in
permanent deformities, misalignment, loss of
mobility, and extremities of unequal lengths.
•
Though there is no cure for hemophilia, it can be
controlled with regular infusions of the deficient
clotting factor, i.e. factor VIII in haemophilia A or
factor IX in hemophilia B.
•
Factor replacement can be either isolated from
human blood serum, recombinant, or a combination
of the two.
•
Some hemophiliacs develop antibodies (inhibitors)
against the replacement factors given to them, so
the amount of the factor has to be increased or nonhuman replacement products must be given, such as
porcine factor VIII or by immunosuppresion.
Thank you