Transcript No Slide Title

FRACP teaching
• Ian Hayes
• Clinical Geneticist
• NRGS.
Topics
• Interpretation of pedigrees
• Autosomal dominant (including trinucleotide repeats), recessive, X
linked, mitochondrial, imprinting (risk assessment questions).
• Knowledge about genetics of common adult monogenic disorders.
• When is testing appropriate.
• presymptomatic testing (HD, children etc)
• Cancer genetics: breast, bowel cancer, rare genetic tumour
syndromes.
• How genetic tests are performed and their interpretation.
• Basic science knowledge questions.
Mechanisms of Genetic
Diseases
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•
Dominant
Recessive
X linked
Trinucleotide repeat diseases
Mitochondrial Inheritance
imprinting
Complex Disease
Dominant Conditions
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•
Neurofibromatosis (NF2).
Adult polycystic kidneys (ADPKD)
BRCA, HNPCC, MEN
Von Hippel Lindau.
Tuberous Sclerosis.
Marfan syndrome, EDS, OI
HMSN (CMT 1a, HNPP).
Hypertrophic Cardiomyopathy.
Dominant Pedigrees
I:1
II:1
III:1
II:2
III:2
I:2
II:3
II:4
II:8
III:3
III:4
III:5
II:9
II:7
III:6
•Vertical Transmission
•Are all offspring affected?
•Does father to son transmission occur
•Are new mutations common?
• Interpretation of pedigrees complicated by
somatic mosaicism, reduced penetrance and
variable expressivity.
Mechanisms of Dominant
Diseases
1. Dominant Negative
Abnormal protein disrupts function of
other proteins
Occurs when a gene forms part of a
complex
Common with disorders affecting CT
E.G. Marfan syndrome - FBN1 15q21
Collagen disorders - EDS, OI
Mechanisms of Dominant
Diseases
2. First Hit Mutations
A somatic second hit required to develop
the condition
Affected individuals inherit a predisposition
Mechanism of many familial cancer
syndromes
E.G. Retinoblastoma, MEN, HNPCC, LFM
Carcinogenesis
Initiating events:
Tumour Suppresser
Genes
Accelerating
events:
Oncogenes
First Mutation  Second Mutation  Third Mutation  …
Familial Bowel Cancer
• Hereditary Non-Polyposis Colorectal Cancer
HNPCC: 2-5% of familial CRC
Modified Amsterdam Criteria
At least 3 relatives with cancer: Colorectal, Endometrial, Small
bowel, Ureter / Renal pelvis. Histology Confirmed.
One the 1° relative of the other 2 (2 generations with CRC)
One or more cancers diagnosed before age 50
• Familial Adenomatous Polyposis: 2%
• Other Rare Syndromes: Peutz-Jeagar, Hyperplastic
Polyposisis, Cowden syn.
Bowel Cancer in HNPCC
Amsterdam Criteria
Predominantly Right Sided, Early onset
 Mucinous and Poorly differentiated types
Synchronous and Metachronous tumours
Gene Mutations in HNPCC
60
50
40
MSH2
MLH1
PMS 1 & 2
MSH6
30
20
10
0
Mutations
Question 2000 1 3
• A 45-year-old man develops stage C cancer of the caecum.
There is a strong family history of bowel cancer in the
absence of polyps and there is no history of colitis.
• The most likely underlying inherited genetic abnormality is
in:
• A) the ras gene.
• B) the DNA mismatch repair (MMR) gene.
• C) the p-glycoprotein (MDRI) gene.
• D) the adenomatous polyposis coli (APC) gene.
• E) the deleted in colon cancer (DCC) gene.
Question 1999 1 13
• A 35-year-old man presents with rectal bleeding. He describes the
blood as being mixed with the stool. He is otherwise well. His brother
was diagnosed with bowel cancer at the age of 38.
• At colonoscopy he was found to have two exophytic tumours, one at
the hepatic flexure, the other in the distal transverse colon. The
remainder of the colon was normal. Biopsies of both areas showed
adenocarcinoma.
• The most likely genetic disorder in this family is:
• A) familial adenomatous polyposis coli (FAP).
• B) hereditary non-polyposis colorectal cancer (HNPCC).
• C) Gardner's syndrome.
• D) Peutz-Jeghers syndrome.
• E) a p53 gene mutation.
Question 2003 2 18
• Hereditary non-polyposis colon cancer (HNPCC or Lynch
syndrome) is associated with a number of extracolonic
malignancies.
• Which one of the following extra-colonic malignancies is
most strongly associated with this diagnosis?
• A) Melanoma.
• B) Sarcoma.
• C) Leukaemia.
• D) Endometrial cancer.
• E) Renal cell cancer.
Question 2001 2 12
• A 40-year-old man had profuse colonic polyposis diagnosed 15 years
ago. A clinical diagnosis of familial adenomatous polyposis (FAP) had
been made. He had a total colectomy. There is no family history of
polyposis or colorectal cancer.
• Mutation studies fail to identify a pathogenic mutation in the
adenomatous polyposis coli (APC) gene in a blood sample.
• The normal DNA result is best explained by:
• A) the correct clinical diagnosis being juvenile polyposis.
• B) the mutation occurring in a non-coding region of the APC gene.
• C) gonadal mosaicism of the APC gene mutation.
• D) the causative mutation being in another gene.
• E) the APC gene mutation occurring only in cells derived from
adenomatous polyps.
Question 1999 2 57
• Which one of the following is least likely to be
associated with familial colon cancer?
• A)Mutations in the adenomatous polyposis coli
gene.
• B)Mutations in DNA repair genes.
• C)Chromosomal translocations.
• D)Microsatellite instability.
• E)Loss of heterozygosity for tumour suppressor
genes.
Overview of Familial Contribution
to Breast Cancer
BRCA1 & 2
16%
80% Genetic & other
• 5 % breast cancer due
to known genetic
factor
• 80% BRCA 1 & 2
• Others
–
–
–
–
ATM
E-cadherin1
TP53
CHEK2
Age of Onset
Am J Hum Genet. May 2003; 72(5): 1117–1130.
Non-genetic Risk Factors
risk in general
population
• Menarche before 12 yrs
• Menopause after 55 yrs
• First live birth after
30yrs
• Nulliparity
• Obesity
• Alcohol use
• Previous biopsies
• BRCA carriers
• Breast feeding for one
year 
• OC for 3yrs after 30 
BRCA1
• OC prolonged starting
before 25 
• Parity in BRCA2 
• HRT postoophorectomy
incidence of breast ca
Indicators of a Genetic Cause
• Family History
– 3 generations, maternal and paternal sides
– Age of diagnosis: pre-menopausal, particularly under
40 yrs
– Male breast cancer
– Bilateral breast cancer
– Ovarian and breast cancer in the same individual or
family line
• Ethnicity
Question 2004 2 88
A 35-year-old mother with breast cancer reports that her aunt had developed breast cancer at 48 years of
age. She is keen to clarify the risk of her young daughter developing breast cancer. A blood sample
from the mother is submitted for mutation analysis of the BRCA1 and BRCA2 genes, but no mutation
is found.
What impact does the mutation analysis have on the estimate of the daughter’s risk of developing breast
cancer?
A. The maternal studies place the daughter at low risk of developing breast cancer.
B. The maternal studies do not clarify the daughter’s risk.
C. The maternal studies place the daughter at high risk of developing breast cancer.
D. The daughter’s risk cannot be clarified without studies of her father’s BRCA1 and BRCA2 genes.
E. The daughter’s risk cannot be clarified without studies of her own BRCA1 and BRCA2 genes.
Question 2001 2 28
• A 35-year-old woman seeks your advice about her
risk of developing breast cancer. Which one of the
following would place her at greatest risk of
developing breast cancer?
• A) Menarche less than 12 years.
• B) Birth of first child after the age of 25.
• C) Oral contraceptive use for more than 10 years.
• D) Sister and aunt diagnosed with breast cancer.
• E) Excision of a benign breast lump.
• There are a number of questions addressing
the Knudson two hit hypothesis.
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•
Question 2002 1 58
A man is admitted to hospital for surgical
treatment of bilateral vestibular schwannomas
(i.e. acoustic neuromas). He has a family
history of the disorder and a familial mutation
in the NF2 gene has been identified. At
operation, samples of blood and tumour tissue
are collected for DNA studies. A Southern blot
of these samples (with a normal control
sample for comparison) is probed with a DNA
fragment derived from the NF2 gene. The
result is shown in the figure below.
What is the most likely interpretation of this
result?
A)The mutant NF2 allele has been lost from
the patient’s blood.
B)The mutant NF2 allele has been lost from
the patient’s tumour.
C)The mutant NF2 allele has been duplicated
in the patient’s tumour.
D)The normal NF2 allele has been lost from
the patient’s blood.
E)The normal NF2 allele has been lost from
the patient’s tumour.
Mechanisms of Dominant
Diseases
3. Haploinsufficiency
Product of both alleles required for normal
function
Occurs in systems with minimal functional
reserve
Can also have effects from over
expression. Can you think of an
example?
Recessive Pedigrees
I:1
II:1
II:2
III:1
III:2
II:3
I:2
II:4
III:3
II:5
III:4
II:6
II:7
II:8
III:5
III:6
III:7
IV:1
•Horizontal Transmission
•Carrier state exists
•Are new mutations common?
•Recurrence risk for siblings = ?%
• consanguinity.
IV:2
IV:3
Carrier Risk in Extended CF Pedigree
I:1
Grandparent
II:1
Parent
III:1
Sibling
III:2
II:2
III:3
III:4
IV:1
Nephew (Niece)
IV:2
V:1
750
I:2
II:3
Uncle (Aunt)
III:5
Affected
II:4
III:6
II:5
III:7
First Cousin
Carrier Risk in Extended CF Pedigree
I:1
Grandparent
1/2
II:1
Parent
100%
III:1
Sibling
2/3
II:2
100%
III:2
III:3
III:4
1/25
2/3
1/25
IV:1
IV:2
1/25
V:1
I:2
1/2
II:3
Uncle (Aunt)
1/2
III:5
Affected
100%
II:4
II:5
1/2
1/25
III:6
1/25
III:7
Question 2001 1 64
•
In the pedigree shown below, the man
indicated by an arrow has been shown to have
an autosomal recessive biochemical disorder
with complete penetrance. The causative gene
has not been identified. His parents are
obligate carriers and do not exhibit any
biochemical abnormalities. His sister also has
normal biochemical studies. The carrier
frequency in this population is 10%.
•
In the absence of consanguinity, what is the
risk of the sister having a child with the
biochemical abnormality?
A) 1 in 40.
B) 1 in 60.
C) 1 in 80.
D) 1 in 100.
E) 1 in 120.
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Question 2002 2 35
•
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Approximately 10% of the Caucasian
population has a mutation in the
haemochromatosis gene (HFE). Three
men in a family (shown below) have
been diagnosed with
haemochromatosis.
What is the risk that the woman
(indicated by the arrow in the pedigree
below) has inherited the genetic
predisposition to develop this disorder?
A)<1%.
B)5%.
C)10%.
D)25%.
E)50%.
Question 2000 2 51
• The following mutations (Cys282Tyr and His63Asp) are
associated with hereditary haemochromatosis. Which one
of the following genotypes provides the greatest risk for
the development of clinical disease?
•
• A) Heterozygous Cys282Tyr.
• B) Heterozygous His63Asp.
• C) Double-heterozygote for Cys282Tyr and His63Asp.
• D) Homozygous Cys282Tyr.
• E) Homozygous His63Asp.
Question 2004 70 c
• In the pedigree shown above, the affected male has a rare
autosomal recessive disorder. His niece and nephew have
a newborn son (indicated by the arrow).
• What is the chance that the baby will have the same
disorder?
• A. 1 in 18.
• B. 1 in 32.
• C. 1 in 36.
• D. 1 in 64.
• E. 1 in 128.
X linked disorders
• Male to male transmission
always/sometimes/never occurs
• All daughters of an affected male receive the
abnormal gene.
• Unaffected males never transmit the disease to
their offspring (of either sex).
• The risks to sons of women who are definite
carriers is …..
• 0/.5/all the daughters of carrier women will be
carriers themselves.
Risk in a Haemophilia Pedigree
I:1
II:1
III:1
III:3
IV:1
IV:2
II:2
III:2
I:2
II:3
II:4
II:5
?
?
III:4
III:5
III:7
IV:3
IV:4
II:6
?
III:6
Question 2003 1 2
•
A woman (indicated by the arrow in the
Figure) seeks your advice about the risk
of her unborn child having haemophilia
A. Her family history is summarised in
the pedigree shown below. The two
affected males had presented during the
second year of life and had died in their
teens. The woman is married to her first
cousin.
•
•
•
•
•
•
•
What is the risk of her unborn child
having haemophilia A?
A) <1%.
B) 6.25%.
C) 12.5%.
D) 25%.
E) 33%.
Question 2004 1 24
•
•
•
•
•
•
•
•
In the pedigree shown below, the affected man has an X-linked recessive
disorder. A polymorphic DNA marker has been identified close to the
mutant gene responsible for this disorder. The genotypes at this marker
are given below each symbol in the pedigree. The recombination fraction
between the gene and the DNA marker is 10%.
What is the best estimate of the risk of the woman indicated by the arrow
being a carrier of her grandfather’s disorder?
A.
100%.
B.
90%.
C.
81%.
a
a,b
D.
66%.
E.
10%.
a,b
c
a,c
Trinucleotide Repeat
Diseases
• Result from instability of repeated DNA
sequences of three nucleotides
• Instability proportional to length, and in
some cases the sex of the transmitting
parent.
• The repeat number changes from generation
to generation - hence ‘dynamic mutation.’
• Anticipation.
Trinucleotide Repeat
DiseasesTranslated TNRs
Non-Coding TNRs
Fragile X
CGG
MD
CTG
Friedreich Ataxia GAA
SCA8
CTG
SCA12
CAG
Fragile E
GCC
HD
SCA1
SCA2
SCA3
SCA6
SCA7
SBMA
DRPLA
CAG
CAG
CAG
CAG
CAG
CAG
CAG
CAG
Fragile X
• The commonest monogenetic cause of MR
• Unstable CGG repeat in exon1 of FMR
• Phenotype in males with full mutation
but also… Full mut. Females
MR
PreMut Females
POF
??PreMut Females
personality
Premutation males FRAXTAS
II. Interpreting Fragile X Tests
• Full Mutations
– >200 CGG repeats
– associated with abnormal methylation
• Pre-mutation
– 60 to 200 CGG repeats (?55 - 60)
– unstable in transmission
• Intermediate alleles
– 40 to 55 CGG repeats
– expansions are infrequent and small
Interpreting Fragile X Tests
The risk of expansion to full mutation is a
function of the repeat size:
In Maternal transmission
Repeat no.
Pre : Full
61-70
>3 : 1
71-80
1.25 : 1
81-90
1: 2
91-100
1: 4
>100
Always
I:1
I:2
?
II:5
?
III:1
II:1
77/29
II:6
II:3
male
III:2
III:3
>200
III:4
250
III:5
52
II:4
Myotonic Dystrophy
• Commonest AD muscular dystrophy.
• Wide variation in features, severity and age
of presentation within families.
• Due to TNR expansion involving a CTG
repeat in a non-coding region of the DMPK
gene 19q13… but ?other genes.
• New mutations rare.
• Unstable in maternal transmission.
• A cause of MR – 50-60% of CDM
…CTGCTGCTGCT…
50 – 2000+
NORMAL
5’
<38
3’
polyA
DMWD
DMPK
Chromosome 19q13
DMAHP
Interpretation of MD tests.
Phenotype
Sympt
Rpts.
Onset
Death
Premutation
Nil
38-49
Mild
Cataracts
Myotonia
Muscle, heart
Eye, face
Hypotonia,
MR, resp.
50-150
20-70
60-N
100 –
1500
10 – 30
48-55
Classical
Congenital
*may be as low as 750
1000* – 0 – 10
2000+
(45)
Question 2002 2 52
•
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•
•
•
•
•
•
A 10-year-old boy (indicated by the
arrow in the pedigree below) has been
diagnosed with myotonic dystrophy.
The diagnosis is confirmed by DNA
testing. His mother’s cousin has
myotonic dystrophy, but the other
surviving relatives have no history
suggestive of a myopathy.
What is the most likely explanation for
this pedigree?
A) Consanguinity.
B) Imprinting.
C )Non-paternity.
D) Incomplete penetrance.
E) Mitochondrial inheritance.
Risk of Congenital DM
• Mothers with Mild phenotype have only
small risk
• Empirical recurrence risk 20 – 40%
• Maternal repeats <300 10%
>300 59%
J Med Genet 1995 32: 105-8
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Treatment
No specific treatment.
Orthotics and other physical therapies
Cardiac: baseline and annual ECGs
Myotonia: phenytoin, carbamazepine
Cramps: clonazepam, quinine
Surveillance: opthalmalogical, glucose,
TFTs
• Anesthetic risk
Huntington Disease
• Movement, cognitive, psychiatric disorder
• Clinical:
Early -  coordination, involuntary mvts, planning difficulties,
depression, irritability.
Later - Chorea (90%), Oculomotor disturbance (75%),
Hyperrefelxia (90%), Progressive dementia, dysarthria
Psychiatric problems: personality change (75%), affective
disorders (20-90%), schizophrenic psychosis (4-12%)
Suicide (12%), Behavioral disturbance esp. outbursts.
End stage- Severe motor disability, mute, dysphagia,
incontinent, weight loss, sleep disturbance
Question
Which one of the following is the most appropriate
way to undertake DNA genetic diagnosis of
Huntington's disease?
A) RFLP (restriction fragment length polymorphism)
analysis.
B) PCR (polymerase chain reaction) analysis.
C) Size estimation of a triplet repeat.
D) Southern hybridisation.
E) Identification of gene-specific mutations.
Question
• 2002.P2.Question 85 (Clinical Genetics)
A young woman is referred by her general practitioner with symptoms
of depression and a family history of Huntington disease (HD). Her
brother has recently developed abnormal hand and facial movements.
She is clinically depressed, but her neurological examination is normal.
• With regard to the acute management of this young woman, mutation
analysis of her HD genes is:
A) indicated only if the specific mutation in her brother’s abnormal HD
gene has been identified.
B) indicated to differentiate the early psychiatric features of HD from
depression.
C) indicated to determine the drug therapy of choice.
D) indicated to determine if she would require long-term follow-up.
E) not indicated at this stage.
Question
• 2000.P1.Question 25 (Clinical Genetics)
• Which one of the following most accurately reflects the
clinical value of DNA testing for Huntington’s disease?
•
A) It allows the age of onset to be determined.
B) It allows investigation of an individual presenting with
tremor.
C) It allows young children to be tested.
D) It allows an at risk individual to be tested before clinical
features develop.
E) It requires only a sample of blood.
Imprinting
• Certain genes retain a ‘memory’ of
which parent they were inherited from.
• Behave differently when inherited from
either mother or father
• About 200 imprinted genes
• Methylation appears to be the principle
mechanism
• Important also in some cancers
Imprinting
• Common example:
Prader-Willi Syn / Angelmans Syn
Hypotonia
Coarse features
Poor feeding…
Ataxia
…food obssesion
Jerky movement
growth retarded
seizures
MR
MR
• Both are due to mutations in
chromosome 15q11-q13
Frequency of genetic cause
PWS
Del 15q
UPD
IC
Transl
Gene
Unknown
AS
70
25-28(mat)
2-5
<1
0?
0?
a= not yet reported
70
3-5(pat)
2-5
<1
10-15
~10
Rec risk
<1a
<1a
50b
5-50b
50
?
b=depends on parent of origin
Mitochondrial Diseases
• Mitochondria: energy producing organelles
Genome - 16569 bp, circular, intron-less
1000s copies  heteroplasmy
13 proteins + tRNAs
Dependent on genomic proteins
• Matrilineal Inheritance
Matrilineal Inheritance
?
I:1
I:2
?
II:1
II:2
II:3
II:4
II:5
II:6
II:7
II:8
?
III:1
IV:4
III:9
IV:5
III:3
III:4
IV:1
III:5
III:7
IV:2
III:6
III:8
IV:3
Syndromes / Symptoms
MELAS
MERRF
Lebers HON
Kearns-Sayre
Non-synd deafness
DM
Visual Loss
PEO
Myopathy (RRF)
Encephalopthy
Epilepsy
Diabetes
Lactic acidosis
Deafness
Cytopenias
Question
What is the most likely
mode of inheritance?
Mitochondrial.
X-linked.
Autosomal dominant.
Autosomal recessive.
Polygenic.
Question 1999 52
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•
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•
•
•
•
In the pedigree shown, individuals
marked with an X have a rare
genetic disorder. Those marked
with an N are clinically normal.
Those with a slash are deceased
and the remaining family members
have not been examined clinically.
From the pedigree which one of the
following is the most likely mode
of genetic inheritance?
A)Autosomal recessive.
B)Autosomal dominant.
C)X-linked.
D)Mitochondrial.
E)Epigenetic.