Gene - Function - Clinic

Question 1

What % of cancers are the BRCA1 and BRCA2 genes responsible for?

Answer 1

. 16% of familial cancers

this accounts for 5-10% of all breast cancers

Question 2

How does possessing a BRCA gene increase risk of breast cancer?

Answer 2

average woman has 12% risk of developing breast cancer by 90

BRCA1 or BRCA2 increase the risk of breast cancer to 85% by 70

Question 3

How do the BRCA1 and BRCA2 genes affect risk of ovarian cancer?

Answer 3

normal risk is < 2%

With BRCA1, risk is elevated to 55%

With BRCA2, risk is elevated to 25%

Question 4

What are the 4 main advantages to testing for BRCA mutations?

Answer 4

1. women may feel relieved knowing whether or not they are at a higher risk for breast cancer
2. women with breast cancer may have better responses to treatments specifically designed for BRCA positive patients
3. women may take preventative measures to reduce their risk of breast cancer (e.g. diet, tamoxifen)
4. other family members may want to be tested based on the results

Question 5

What are 2 treatments for breast cancer specifically designed for BRCA positive patients?

Answer 5

cisplatin and PARP

Question 6

What are the 4 main disadvantages to testing for BRCA mutations?

Answer 6

1. women may become worried, panicked or stressed if they discover they have a higher than average risk
2. women who test positive may have difficulty telling family members, who may also have the mutation
3. women who test negative may falsely believe they will never get breast cancer
4. women who test positive may have to deal with comlications with health or life insurance

Question 7

What are the options for prophylactic surgery in women who test positive for BRCA mutations?

Answer 7

removing the ovaries after having a family reduces risk by 85%

having a preventative mastectomy reduces risk by 90%

Question 8

What is the problem with screening for some late onset disorders?

Answer 8

they can be predicted but not prevented

e.g. Huntingdon disease, retinitis pigementosa (late onset blindness)

Question 9

What is the role of diagnostics and counselling in clinical practice?

Answer 9

1. can confirm clinical diagnosis
2. can identify carriers of recessive mutations
3. can identify cases of non-penetrance

Question 10

What is meant by non-penetrance/reduced penetrance?

Answer 10

Non-penetrance means that someone carries the gene but do not develop the phenotype

Question 11

What is Treacher Collins syndrome?

Answer 11

an autosomal dominant disorder with variable penetrance

it is characterised by deformities of the ears, eyes, cheekbones and chin

Question 12

What complications are most common in Treacher Collins syndrome?

Answer 12

1. breathing problems
2. problems with sight
3. cleft palate
4. hearing loss

Question 13

What is significant about the genetics of the family members of someone with Treacher Collins syndrome?

Answer 13

quite often the parents and grandparents may carry the same mutation as the affected child, but not express the phenotype

Question 14

What are the main 2 invasive methods of prenatal diagnosis?

Answer 14

Amniocentesis at 17 weeks

Chorionic villus sampling at 11 weeks

Question 15

What is NIPD? How does it work and what is it used for?

Answer 15

Non invasive prenatal diagnosis - conducted at 10 weeks

It screens the mother’s blood to identify the foetal DNA

It is used for sex determination testing and testing for triploidy and single gene disorders

Question 16

Why is NIPD termed “cell-free DNA screening”?

Answer 16

Maternal serum contains placental DNA which matches the foetus genome

NGS genome sequencing is performed in the cell-free DNA and the results can identify trisomies

Question 17

Why is NIPD more likely to be used in the future?

Answer 17

The results are far more accurate than in amniocentesis and have a lower false positive rate

It is also non-invasive, so women are more likely to take it

Question 18

What is the main drawback of prenatal screening?

Answer 18

Women are having prenatal tests early on in their pregnancy and choosing to terminate them e.g. Down’s syndrome

This means that in Iceland, there are very few Down’s syndrome children

Question 19

What is meant by preimplantation diagnosis?

Answer 19

It is a form of in utero fertilisation where the embryos are tested first to see if they are positive for a certain known mutation

Only the unaffected embryos are implanted in the uterus

Question 20

What are the 2 main benefits of preimplantation diagnosis?

Answer 20

1. HLA typing can be conducted to match cord blood stem cells for siblings
2. Non-disclosure testing can be performed for conditions such as Huntingdon’s disease

Question 21

What is meant by non-disclosure testing?

Answer 21

Testing that is done without the knowledge of the disease status of the parent

Question 22

How does non-disclosure testing work?

Answer 22

Instead of screening for the actual mutation, you look for genetic markers either side of the mutation

These are highly polymorphic microsatellite repeats

Question 23

In this example, how would non-disclosure testing be performed?

Answer 23

1. affected grandma has an a allele and a c allele flanking their HD gene - one of these must be affected
2. mother has a c allele and a d allele flanking their HD disease
3. we do not know whether it is an a or a c allele that carries the HD mutation
4. we only implant the embryos that have inherited the d allele from the mother as these have come from the grandad who does not have HD

Question 24

What is gene therapy? How is classic gene therapy conducted?

Answer 24

It is the use of genetic material - DNA or RNA - as a medicine

Classic gene therapy involves the introduction of functional genes, in the form of DNA, to replace mutated genes

Question 25

What are the benefits of gene therapy?

Answer 25

1. it uses gene technology to repair mutated genes
2. it can silence overactive genes
3. it can provide immune cells with the tools they need to recognise and kill cancer cells and infections

Question 26

What is Leber’s congenital amaurosis (LCA)?

Answer 26

it is a rare inherited eye disorder with a recessive inheritance pattern

it involves blindness at birth or in early infancy

Question 27

Why is the eye a good target for gene therapy?

Answer 27

1. the eye is immune privileged
2. the eye is easily accessible for subretinal injection
3. trials in dogs show improvement in vision

Question 28

What mutation was identified in LCA families?

Answer 28

RPE65 mutations

RPE65 is an enzyme that is present in the retina

Question 29

What is the gene therapy treatment for LCA?

Answer 29

a sub-retinal injection of RPE65 near the photoreceptors at the back of the eye

this leads to an improvement in vision

Question 30

What are the stages involved with CRISPR/Cas9 gene editing?

Answer 30

1. a cell is transfected with an enzyme complex containing a guide molecule, healthy DNA copy and DNA-cutting enzyme
2. the synthetic guide molecule finds the target DNA strand
3. an enzyme cuts off the target DNA strand
4. the defective DNA strand is replaced with a healthy copy

Question 31

What is meant by pharmacogenetics? Why is it significant?

Answer 31

the correlation between the effect of drugs and the genetic constitution of patients

it is significant as 5-7% of hospital admissions are due to adverse drug responses

Question 32

What are the roles of cytochrome P450 oxidases?

Answer 32

they are a family enzymes found in the liver that are responsible for the metabolic elimination of most drugs currently used

they are also important for converting pro-drugs to their active forms

Question 33

What must happen to codeine before it can exert its therapeutic effect?

Answer 33

codeine is metabolised by to the analgesic morphine by CYP2D6 enzyme

the desired analgesic effect is not achieved in CYP2D6 poor metabolisers

Question 34

What is CYP2D6?

Answer 34

it is a highly polymorphic cytochrome 450 family member that metabolises 25% of known drugs

Question 35

How do Caucasians vary in the CYP2D6 that they contain?

Answer 35

6-10% are non-metabolisers as they have no active CYP2D6, meaning they cannot metabolise drugs

7% are ultra-rapid metabolisers as they have multiple copies of CYP2D6

Question 36

What is the link between tamoxifen use and the CYP2D6 present in a patient?

Answer 36

CYP2D6 is the rate-limiting step in converting tamoxifen into its active metabolite - endoxifen

poor metabolisers due to CYP2D6 polymorphisms are associated with worse survival in breast cancer

Question 37

What is meant by personalised medicine?

Answer 37

Information about a persons genotype or gene expression profile is used to tailor their medical care

GPs would be able to prescribe drugs based on the results (e.g. CYP2D6)

Patients would be able to change their lifestyle in response to the results