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. - PowerPoint PPT Presentation
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.
Transcript
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 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.
I:1 I:2
II:1 II:2 II:3 II:4 II:7
III:1 III:2 III:3 III:4
II:8 II:9
III:5 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.
Dominant Pedigrees
Mechanisms of Dominant Diseases1. Dominant NegativeAbnormal protein disrupts function of other
proteinsOccurs when a gene forms part of a complexCommon with disorders affecting CTE.G. Marfan syndrome - FBN1 15q21
Collagen disorders - EDS, OI
Mechanisms of Dominant Diseases
2. First Hit MutationsA somatic second hit required to develop the
conditionAffected individuals inherit a predispositionMechanism of many familial cancer syndromesE.G. Retinoblastoma, MEN, HNPCC, LFM
Carcinogenesis
First Mutation Second Mutation Third Mutation …
Initiating events:Tumour Suppresser Genes
Accelerating events: Oncogenes
Familial Bowel Cancer Hereditary Non-Polyposis Colorectal Cancer HNPCC: 2-5% of familial CRC
Modified Amsterdam CriteriaAt 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 HNPCCAmsterdam CriteriaPredominantly Right Sided, Early onset Mucinous and Poorly differentiated typesSynchronous and Metachronous tumours
Gene Mutations in HNPCC
0
10
20
30
40
50
60
Mutations
MSH2MLH1PMS 1 & 2MSH6
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
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.
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 Diseases3. HaploinsufficiencyProduct of both alleles required for
normal functionOccurs in systems with minimal
functional reserveCan also have effects from over
expression. Can you think of an example?
I:1 I:2
II:1 II:2 II:3 II:4 II:5 II:6 II:7 II:8
III:1 III:2 III:3 III:4 III:5 III:6 III:7
IV:1 IV:2 IV:3
•Horizontal Transmission
•Carrier state exists
•Are new mutations common?
•Recurrence risk for siblings = ?%
consanguinity.
Recessive Pedigrees
II:1Parent
II:2
I:1Grandparent
I:2
II:3Uncle (Aunt)
II:4
III:1Sibling
III:2 III:3
III:4 III:5Affected
III:6
II:5
IV:1Nephew (Niece)
IV:2
V:1
750
III:7First Cousin
Carrier Risk in Extended CF Pedigree
II:1Parent100%
II:2
100%
I:1Grandparent
1/2
I:2
1/2
II:3Uncle (Aunt)
1/2
II:4
1/2
III:1Sibling 2/3
III:2
1/25
III:3
2/3
III:4
1/25
III:5Affected
100%
III:6
1/25
II:5
1/25
IV:1Nephew (Niece)
1/150
IV:2
1/25
V:1
1/3750
III:7First Cousin
1/200
Carrier Risk in Extended CF Pedigree
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.
Question 2002 2 35
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
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.
I:1 I:2
II:1 II:2 II:3 II:4 II:5 II:6
III:1 III:2III:3
IV:1 IV:2
?III:4
?III:5
?III:6III:7
IV:3 IV:4
Risk in a Haemophilia Pedigree
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%. D. 66%. E. 10%.
a,ba
a,b
a,c
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 Diseases
Non-Coding TNRsFragile X CGGMD CTGFriedreich Ataxia GAASCA8 CTGSCA12 CAGFragile E GCC
Unstable CGG repeat in exon1 of FMR Phenotype in males with full mutation
but also… Full mut. Females MR PreMut Females POF??PreMut Females personalityPremutation males FRAXTAS
II. Interpreting Fragile X Tests
Full Mutations>200 CGG repeatsassociated with abnormal methylation
Pre-mutation60 to 200 CGG repeats (?55 - 60)unstable in transmission
Intermediate alleles40 to 55 CGG repeatsexpansions are infrequent and small
Interpreting Fragile X Tests
The risk of expansion to full mutation is a function of the repeat size:
In Maternal transmissionRepeat 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:177/29
II:3?
II:4II:5
?III:1
male
III:2 III:3>200
III:4250
III:552
II:6
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
DMPK
…CTGCTGCTGCT…
50 – 2000+
<38NORMAL
Chromosome 19q13
polyA
DMAHPDMWD
5’ 3’
Interpretation of MD tests.
(45)0 – 101000* – 2000+
Hypotonia, MR, resp.
Congenital
48-5510 – 30100 – 1500
Muscle, heartEye, face
Classical
60-N20-7050-150CataractsMyotonia
Mild
38-49NilPremutation
DeathOnsetRpts.SymptPhenotype
*may be as low as 750
Question 2002 2 52
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.
Treatment No specific treatment. Orthotics and other physical therapies Cardiac: baseline and annual ECGs Myotonia: phenytoin, carbamazepine Cramps: clonazepam, quinine Surveillance: opthalmalogical, glucose,
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 SynHypotonia Coarse featuresPoor feeding… Ataxia…food obssesion Jerky movementgrowth retarded seizuresMR MR Both are due to mutations in chromosome
15q11-q13
Frequency of genetic cause PWS AS Rec risk
Del 15q 70 70 <1a
UPD 25-28(mat) 3-5(pat) <1a
IC 2-5 2-5 50b
Transl <1 <1 5-50b
Gene 0? 10-15 50 Unknown 0? ~10 ?
a= not yet reported b=depends on parent of origin
Mitochondrial Diseases
Mitochondria: energy producing organellesGenome - 16569 bp, circular, intron-less1000s copies heteroplasmy13 proteins + tRNAsDependent on genomic proteins
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?
