Transcript No Slide Title

Topics
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Interpretation of pedigrees
• Autosomal dominant (including trinucleotide repeats),
recessive, X linked, mitochondrial, imprinting (risk
assessment questions).
• Knowledge about genetics of common adult monogenic
disorders.
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When is testing appropriate.
• presymptomatic testing (HD, children etc)
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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
Dominant
 Recessive
 X linked
 Trinucleotide repeat diseases
 Mitochondrial Inheritance
 imprinting
 Complex Disease

Dominant Conditions
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
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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
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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
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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
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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
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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
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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
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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%
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80% Genetic & other
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5 % breast cancer
due to known
genetic factor
80% BRCA 1 & 2
Others
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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
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Family History
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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
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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.
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There are a number of questions
addressing the Knudson two hit
hypothesis.
Question 2002 1 58
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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
Nephew (Niece)
1/150
IV:2
V:1
1/3750
1/25
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
First Cousin
1/200
Question 2001 1 64
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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%.
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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
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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?
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A) Heterozygous Cys282Tyr.
B) Heterozygous His63Asp.
C) Double-heterozygote for Cys282Tyr and
His63Asp.
D) Homozygous Cys282Tyr.
E) Homozygous His63Asp.
Question 2004 70 c
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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
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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
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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.
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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
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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%.
a
C.
81%.
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.
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Trinucleotide Repeat
Diseases
Non-Coding TNRs
Fragile X
CGG
MD
CTG
Friedreich Ataxia GAA
SCA8
CTG
SCA12
CAG
Fragile E
GCC
Translated TNRs
HD
CAG
SCA1
CAG
SCA2
CAG
SCA3
CAG
SCA6
CAG
SCA7
CAG
SBMA
CAG
DRPLA
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
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II. Interpreting Fragile X
Tests
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Full Mutations
>200 CGG repeats
 associated with abnormal methylation
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Pre-mutation
60 to 200 CGG repeats (?55 - 60)
 unstable in transmission
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Intermediate alleles
40 to 55 CGG repeats
 expansions are infrequent and small
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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
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…CTGCTGCTGCT…
50 – 2000+
NORMAL
5’
<38
3’
polyA
DMWD
DMPK
Chromosome 19q13
DMAHP
Interpretation of MD tests.
Phenotype
Sympt
Rpts.
Premutation Nil
38-49
Mild
Cataracts
Myotonia
50-150
Classical
Muscle, heart 100 –
1500
Eye, face
Congenital
Hypotonia,
MR, resp.
*may be as low as 750
Onset
Death
20-70
60-N
10 – 30
48-55
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%
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J Med Genet 1995 32:
105-8
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
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Huntington Disease
 Movement,
 Clinical:
cognitive, psychiatric disorder
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.
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Question
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2000.P1.Question 25 (Clinical Genetics)
Which one of the following most accurately reflects
the clinical value of DNA testing for Huntington’s
disease?
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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
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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
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Frequency of genetic cause
PWS
AS
Del 15q
70
UPD
25-28(mat)
IC
2-5
Transl
<1
Gene
0?
Unknown
0?
a= not yet reported
Rec risk
70
3-5(pat)
2-5
<1
10-15
~10
<1a
<1a
50b
5-50b
50
?
b=depends on parent of origin
Mitochondrial Diseases
Mitochondria: energy producing
organelles
Genome - 16569 bp, circular, intronless
1000s copies  heteroplasmy
13 proteins + tRNAs
Dependent on genomic proteins
 Matrilineal Inheritance
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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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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.