Genetic counselling in single gene disorders

Question 1

Define genetic counselling.

Answer 1

Genetic counseling is the process of helping people understand and adapt to the medical, psychological and familial implications of genetic contributions to disease.

Question 2

What does genetic counselling involve ?

Answer 2

• Interpretation of family and medical histories to assess the chance of disease occurrence or recurrence.
• Education about inheritance, testing, management, prevention, resources and research.
• Counseling to promote in formed choices and adaptation to the risk or condition.

Question 3

List single gene disorders.

Answer 3

• CF
• Spinal Muscular Atrophy
• Recurrent miscarriage
• Alzheimer’s

Question 4

Describe the genetics of CF.

Answer 4

• Autosomal Recessive inheritance

* Mutated chloride channel in epithelial cells

Question 5

Describe the epidemiology of CF.

Answer 5

• Most common fatal inherited disease in Caucasians

Question 6

Describe the screening process for CF.

Answer 6

• Heel prick test at 5 days, tests for immunoreactive trysinogen (IRT) (Stressed pancreas)
• NOT a specific test (since stressed pancreas is a sign of CF but may be due to other causes)

• Raised IRT- test using CF mutation kit (initially looks for 4 mutations. If one mutation found then look for others (about 30 others))
• CF suspected if IRT raised and one pathogenic
mutation found
• CF confirmed if 2 pathogenic mutations found

Question 7

Identify some of the genes responsible for CF.

Answer 7

ΔF508 , G551D, G542X and 621+1G>T

Question 8

Explain what it means to be compound heterozygote in CF. Give an example of compound heterozygote genotype in CF.

Answer 8

Different mutations on each chromosome

• G551D mutation (known to be pathological, CFTR protein gets to the right place in the cell but channel insufficiently ‘open’ to allow Chloride transport)
• R117H mutation (Mild mutation. Some patients have it but not the disease. Effect of R117H varies according to Intron 8 splice site efficiency)

If G551D mutation + R117H mutation with 5T, then risk of developing symptoms of CF during childhood

Question 9

Do the majority of R117H compound heterozygotes present with CF in childhood ?

Answer 9

No

Question 10

Identify any pathologies associated with R117H besides CF.

Answer 10

Risk of infertility

Question 11

Explain the role of R117H mutation in CF.

Answer 11

♦ Amount of normal CF protein (CFTR) produced depends on sequences in intron 8.
♦ 7T most common in population.
♦ 5T, associated with poor splicing resulting in exon 9 skipping.
♦ R177H alone not-enough to cause CF, but effect compounded if also 5T ie not producing very much full length protein.

Question 12

If a patient has CF with G551D mutation and R117H mutation, how can we figure out whether the latter is actually pathogenic (i.e. patient will have CF) ?

Answer 12

1) Figure out who John inherited his mutations from
- i.e. which mutation from father which from mother

2) Then look at number of T residues (of parents)
-If patient inherited R117H mutation from father, and it turns out father has 5T residues on same chromosome as R117H mutation, then the patient must also have 5T on the same chromosome as R117H.
(and vice versa if mother)

Question 13

What are possible management options for a CF patient with G551D mutation and R117H mutation ?

Answer 13

• Ivacaftor (Kalydeco) therapy (only for CF patients with G551D mutation since it causes G551D- protein gets to right place in cell but channel not sufficiently ‘open, this drug artificially opens channel)
• Follow up through CF clinic

Question 14

Identify issues for the rest of the family when a member is found to have inherited CF, with G551D mutation and R117H mutation.

Answer 14

-Risk to brother (of infertility, due to R117H mutation) (if currently well, should be okay)
-Cascade testing of relatives (don’t pursue this unless family particularly anxious)
-Options for any future pregnancy
1) Prenatal Diagnosis (PND)
• Chorionic villus sampling (CVS) (earlier diagnosis but greater risk of miscarriage)
• Amniocentesis
2) Pre-implantation Genetic Diagnosis (PGD)

Question 15

Why may parents of a current CF patient want to avoid birth of another affected individual ?

Answer 15

Cross contamination between affected individuals + CF sufferers are not encouraged to meet + can new child infect existing child.

(Difficult to discuss with CF support staff disloyalty to existing child)

Question 16

What is the most common indication for Pre-implantation Genetic Diagnosis (PGD) ?

Answer 16

CF

Question 17

Explain the genotype-phenotype correlation in CF.

Answer 17

♦ Different mutations can cause different phenotypes.
♦ Modifying genes can mean that different patients with the same mutations can have different phenotypes.
e.g. CF syndrome – eg absence of the vas deferens, obstructive azoospermia, and severe oligozoospermia, pancreatitis, respiratory infections, sinusitis.

Question 18

Describe the main features of spinal muscular atrophy.

Answer 18

Progressive muscle weakness from degeneration of anterior horn cells

Respiratory difficulties, difficulty sucking and swallowing, ‘floppy’, unable to sit (usually do this 4-7 months)

Question 19

Describe the genetic basis of spinal muscular atrophy.

Answer 19

• Autosomal recessive
• 95% cases due to deletion of SMN1 (survival
motor neurone 1)

Question 20

What are the main concerns for the family of a patient affected by SMA.

Answer 20

• Treatment for child: none presently
• Risk of similar problem in a future pregnancy
• Options for further pregnancy
– Prenatal diagnosis
– Pre-implantation genetic diagnosis

Question 21

Why may some families be opposed to prenatal diagnosis ?

Answer 21

Because opposed to abortion

Question 22

Describe the process of PGD.

Answer 22

IVF performed
Remove 1 cell day 5.
Whole genome amplification.

Question 23

How does performing PGD help the parents determine their options for future pregnancies in SMA ?

Answer 23

Need to establish exactly which ch child inherited (two alleles involves since recessive)

Haplotyping performed (look at inheritance of chromosome markers around site of disease associated mutation. Want to determine which chromosomes carry the mutations and from this which chromosomes have been inherited by the embryo (ask which embryos are NOT carrying the mutation, and compare with embryologists view as to which embryos are healthy). The closer the markers are together the more likely they are to be inherited together)

Question 24

Identify problems which makes haplotyping more difficult.

Answer 24

Rearrangements can happen in meiosis.

If homozygous at a locus cannot distinguish chromosomes, uninformative.

Question 25

In haplotyping, we not look directly for mutation?

Answer 25

• Single cell – very low copy number DNA
• Problem with contamination (esp from mom)
– False positive result (potential residual DNA from mum or dad)
• Allele drop out
– False negative result from failure to amplify mutation (i.e. since we would be looking for a deletion, is it not there because it is deleted or because the reaction didn’t work)

Question 26

What are the requirements for PGD ? Is this funded in Scotland ?

Answer 26

No living unaffected children.
Hormone level good for IVF.
BMI<30.
Not smoke.

Funded for 2 cycles in Scotland.

Question 27

Identify conditions for which pre-natal diagnosis and PGD is allowed.

Answer 27

• CF

- Spinal muscular atrophy

Question 28

How are miscarriages and pregnancies represented on a genetic tree ?

Answer 28

Miscarriages: triangle
Pregnancy: losange

Question 29

Which screening tests are women with recurrent miscarriages offered ?

Answer 29

Ultrasound earlier than normal

Question 30

Identify potential causes for the following:

Polycystic kidneys
Encephalocoele

in a fetus at 13 weeks (seen on an ultrasound)

Answer 30

1) Meckel Gruber syndrome

2) Trisomy 13 (Patau syndrome)

Question 31

Describe the main genetic features of Meckel Gruber syndrome.

Answer 31

• variable phenotype
• autosomal recessive
• at least 6 genes

Question 32

If findings of Polycystic kidneys + Encephalocoele was done in an fetus upon US, what are potential questions asked by the parents ? What are the answers to some of these ?

Answer 32

• What do these findings mean?
– Possible diagnosis 
○ Trisomy 13
○ Meckel Gruber syndrome
 – Severe phenotype, lethal in infancy

• Could you have made a mistake?

• What do we do now?
-Some couples decide to carry on and see abnormalities themselves- faciliates mourning process.

• What would you do?
Dangerous question frequently asked. Re- emphasise options- illustrate with knowledge of what other people did in those situations.

Question 33

Identify some questions addressed as part of post-termination genetic counselling in the case of a termination of pregnancy following abnormal findings.

Answer 33

• Was the diagnosis confirmed?
-Post Mortem encouraged to clarify diagnosis

• Why did it occur?
-Some parents may blame themselves even though totally genetic condition

• Can we ever have a normal baby?
-If recessive, 3/4 chances of normal

• What tests are available in a future pregnancy?

• Did we make the right decision?
-Remind that it’s a lethal condition

Question 34

• What tests are available in a future pregnancy for parents who terminated pregnancy with Meckel Gruber syndrome child ?

Answer 34

Unable to be able to test directly for mutation because of shear number of genes and mutations (no PGD). Plenty of scans to reassure as much as possible.

Question 35

True or False: parents chosing the time and place of death is important to the couple after terminating a pregnancy.

Answer 35

True

Question 36

What tests can be performed to confirm a diagnosis of Alzheimer’s ?

Answer 36

Blood sent to research lab in Belgium looking for mutations

Question 37

Identify any genetic mutations associated with Alzheimer’s.

Answer 37

• Substitution mutation in exon 7 of PSEN-1 gene changing codon 214 from CAC to TAC
• Replaces histidine with tyrosine in the PSEN-1 protein
• missense

Question 38

State a reason why someone with Alzheimer’s may have had neither parent affected by it.

Answer 38

Early death or not fully penetrant

Question 39

Define variant of unknown significance. Give an example.

Answer 39

Allele, which has been identified through genetic testing, but whose significance to the function or health of an organism is not known (e.g. could be pathogenic or a normal variation).

Question 40

How may we determine if a mutation the cause of a pathology.

Answer 40

• Look at the literature - has the mutation been described in other families with early onset dementia?
• Look at an unaffected population cohort from the same ethnic group as the patient to see if the mutation is a rare variant
• Run computer programme to look for sequence conservation across species
——
If needed:
• Look at other affected family members and see if they carry the same mutation

Question 41

What is the significance of sequence conservation across species ?

Answer 41

Evolutionarily conserved parts of proteins tend to be the parts which are most essential for their function. Thus if a change happens in these regions more likely to suspect that it changes function- can provide useful hints even if function of protein unclear.

Question 42

Identify possible problems in the way of obtaining genetic information from family members.

Answer 42

1) Families sometimes fractured and different parts don’t talk to each other.
2) Non-paternity.
3) (in Alzheimer’s) Beware- neurodegeneration affects ability to consent. Who gives permission? Affects whole family.
4) May live a long way away

Question 43

If sufficient evidence is obtained suggesting a certain mutation does underlie a disease (e.g. Alzheimer’s) what is the next step for the patient ?

Answer 43

Pre-symptomatic testing offered to relatives

Question 44

Why may relatives of Alzheimer’s chose to take, or not take a pre-symptomatic testing (given that sufficient evidence is obtained suggesting a certain mutation does underlie a disease) ?

Answer 44

CON
-No medical advantage in knowing.
PRO
-Opportunity for facilitated decision making

Question 45

Is there any preventive therapy available for Alzheimer’s ?

Answer 45

No preventative treatment available

Question 46

Describe the pre-symptomatic testing protocol for Alzheimer’s.

Answer 46

1) Full information on test and limitations given (meet 2-3x before)
- Non-directive

2) Help patient consider:
– Insurance (insurance company cannot use results from genetic counselling to increase premium)
– Employment
– Relationships (who do you tell ?)

Question 47

Upon taking a family history, how many generations should be investigated ?

Answer 47

At least 3 generations