Genetic disease –taking samples

1
Genetic disease taking samples
Bill Newman St Marys Hospital May 2008
2
.. so what can we really test for?
3
Genetic Disease

• Chromosomal6 per 1000 LB
• Aneuploidy
• Structural abnormalities
• DNA/single gene13 per 1000 LB
• Mendelian inheritance
• Non-Mendelian inheritance
• Multifactorial e.g CL/CP,NTD,cancer
• Susceptibility genes
• Environmental influences eg drugs

4
Chromosomal disease
Can identify large chromosomal changes
gt4Mb Culture leukocytes dividing cells with
long chromosomes
5
When to do a chromosomal test

• Prenatal
• maternal agegt37yrs USS changes Family history
• Triple test increased risk
• Postnatal
• Learning developmental disability growth
  retardation
• Infertility
• Recurrent miscarriage, primary infertility

6
Prenatal Diagnosis
Chorionic villus biopsy gt10/40 Miscarriage
1 Results 1 week
Amniocentesis gt14/40 Miscarriage 0.5 Results 3
weeks
7
Aneuploidy

• Autosomal
• Trisomy 21 (Down syndrome)
• Trisomy 18 (Edward syndrome)
• Trisomy 13 (Patau syndrome)
• Sex chromosome
• 47XXY (Klinefelter syndrome)
• 45X (Turner syndrome)

8
Trisomy 21
1 in 600 livebirths Learning disability Congenital
heart defects Duodenal atresia Macroglossia Brach
ycephaly Epicanthic folds Brushfield spots
9
Trisomy 13
1 in 10,000 livebirths Holoprosencephaly, Polydact
yly Flexion of the fingers facial clefting heart
defects Scalp defects Microcephaly Polycystic
kidneys
10
Trisomy 18
1 in 3,000 livebirths Neurodevelopmental
delay Flexion of the fingers coloboma heart
defects Microcephaly Horseshoe kidney
11
Turner syndrome 45 X
12
Klinefelter syndrome 47, XXY
13
Rapid Aneuploidy Screening by FISH

• Available on amniocentesis sample
• Uncultured amniocytes
• FISH probes for X,Y, 21,18,13
• Result in 24-48 hours
• Proceed onto full karyotype (11-14 days)

14
FISH analysis
Normal two copies of chromosome 18 in male
15
New techniques

• qf (quantitative) PCR able to measure number of
  copies of a chromosome used for trisomy
  screening
• Fetal DNA at 6-8 weeks to determine sex look
  for presence of Y chromosome material

16
Structural Chromosome Abnormalities

• De novo (spontaneous) or inherited
• Deletionvisible cytogenetically eg 5p-
• Microdeletionuse FISH eg Williams Syndrome

17
Array Comparative Genomic Hybridisation (aCGH)

• New technique identifies subtle chromosome
  deletions/duplications
• Detects changes in 25 children with learning
  difficulty dysmorphism and normal chromosomes

18
Mosaicism

• Mixed population of normal/abnormal karyotype
• May affect some tissues
• Karyotype from skin biopsy
• Skin pigmentation
• Growth distortion

19
Reciprocal Translocation

• Balanced exchange of genetic material between
  chromosomes
• Can result in unbalanced gametes?
• recurrent miscarriage
• fetal abnormality
• liveborn with structural defects dysmorphism
  MR

20
Reciprocal Translocation
Balanced translocation
Normal
Normal
Balanced
Dup/del
Dup/del
21
Molecular genetic testing

• Diagnostic to confirm diagnosis
• Predictive/presymptomatic predict future
  illness
• Carrier screening reproductive risk

22
Risk estimation in genetics
Inherited disease Autosomal dominant e.g. NF1,
HD, Marfan Autosomal recessive e.g. CF X-linked
recessive e.g. Duchenne MD mitochondrial e.g.
Lebers optic atropy chromosomal e.g.
translocations, Complex disease e.g. asthma,
autoimmune disease
23
Autosomal Dominant
e.g. Huntingtons Disease
Anticipation Trinucleotide repeat expansion
?
50 - dependent upon age
24
Autosomal Dominant

• Neurofibromatosis type I
• Pigmented skin lesions café au lait spots
• Neurofibromata
• Axillary freckling
• Macrocephaly
• Learning difficulties
• Variable phenotype even within a family

25
Neurofibromatosis type 1
26
Neurofibromatosis type 1
50?? New mutation?? Examine parents No signs
New mutn a few ? Mosaic 1-2 definite NF1
50 Examine child if gt6 and not affected then ok
?
27
Autosomal Recessive
My brothers child has cystic fibrosis My
wife is pregnant Will our child have cystic
fibrosis?
28
Cystic fibrosis
Most common AR genetic disease in UK Caucasian
population Carrier frequency 1/25 Many different
mutations (gt700) Common mutation DF508 (3bp
deletion) Lung disease GI Disease (meconium
ilieus/pancreatic insufficiency) Infertility in
males Survival increasing gt35yrs Genetic
testing available Prenatal diagnosis available
29
Cystic fibrosis
1/2
1/25
1/25 X 1/2 X 1/4 1/200
30
Cystic fibrosis- genetic tests
1/2
1/25
31
Cystic fibrosis
1/25
1
- - - - - - -
1/25 X 1 X 1/4 1/100
32
Cystic fibrosis
1
1/160
- - - - - - - -
- - - - - - -
1/160 X 1 X 1/4 1/640
33
X-linked recessive

• No male to male transmission
• Males affected
• Females usually unaffected carriers (except
  skewing)
• Affected male all daughters are carriers, no
  sons affected
• Carrier female 50 risk affected son, 50 risk
  carrier daughter
• eg DMD/BMD, haemophilia

34
X Linked recessive inheritance
Is the mother a carrier for condition confirmed
if recurrence, by genetic or other tests
If the mother is not a carrier for condition
lower risk of recurrence and no risk to other
family members
35
GOOD LUCK!