MCBG Session 17 - Genetics in Medicine

Question 1

Outline the clinical basis for Sickle Cell Anaemia.

Answer 1

• First recognised by Western medicine in 1910.
• Symptoms are anaemia, fever, severe pain, often accompanied by sudden death.
• Treatment usually aimed to limit symptoms and complications.
• The first disease to be understood molecularly – by chemist Linus Pauling, 1949. He showed that haemoglobin isolated from SCA patients had different physical properties to normal haemoglobin.

Question 2

Outline the molecular basis for Sickle Cell Anaemia.

Answer 2

• Haemoglobin a tetramer comprised of two molecules of alpha globin and two molecules of beta globin.
• SCA is caused by a single DNA mutation, an A to a T in the beta-globin gene.
• Causes a change in the beta-globin protein at position 6 in the amino acid chain from glutamate to valine.
• Both copies of beta-globin need to be mutated in an individual to cause the disease: autosomal recessive inheritance.
• Individuals heterozygous for the mutation have a mild form called sickle cell trait.

Question 3

Discuss the geographical variation with sickle cell anaemia.

Answer 3

• SCA is very rare in non-African populations. But common in African populations, and populations with recent African ancestry.
• This is shown by distribution of the mutation that causes sickle cell anaemia.
• Similarity in distribution across populations with malaria endemicity.
• Individuals who are heterozygous for the mutation (sickle cell trait) are resistant to severe malaria.
• Particularly strong effect in infants between 2 months and two years old, against the most lethal form of malaria caused by the parasite Plasmodium falciparum
• About a million children die a year from malaria

Question 4

Outline the epigenetics therapy of SCA

Answer 4

• All cells have the same genome and the same genes. However, cells differ between each other and across time.
• Genes are turned on and off by other proteins. These proteins are made by genes that are turned on and off by other proteins. Hence, there is a complex web of gene regulation, also called epigenetics.
• Foetal (gamma) globin turned off after birth, adult (beta) globin turned on. Can the gamma globin be kept switched on?
• Protein BCL11A is a repressor, it binds to the DNA and switches off the gamma globin gene.
• In mice with the Bcl11a gene deleted, gamma globin is kept switched on after birth. Could BCL11A be targeted by drugs in humans?
• Persistence of foetal haemoglobin is a rare genetic condition in humans, but is not harmful, suggesting that keeping gamma globin switched on could work.

Question 5

Outline the gene editing therapy for SCA.

Answer 5

• HSPCs taken from SCA patients
• Either mock-treated or treated with a correct SNP donor molecule using CRISPR/Cas.
• HbS = sickle cell mutant beta-globin transcript
• HbA = normal beta-globin transcript

Question 6

Outline Huntington’s disease in light of its nature, symptoms, cause, and inheritance.

Answer 6

• Inherited neurodegenerative disease.
• Seizures, abnormal gait, personality change, dementia.
• First disease gene mapped to a chromosome (1983)
• Genetic cause identified in 1993.
• Mutation is a short tandem repeat (STR) – which in disease
• Autosomal dominant.
• DNA test detection: use a polymerase chain reaction (PCR) to amplify the CAG repeat.

Question 7

Explain what is meant by the term anticipation.

Answer 7

• Age of onset commonly 40-50, but varies between childhood and very old age.
• Important for patients and treating the disease. From generation to generation, age of onset becomes earlier and symptoms become more severe. This is called “Anticipation”
• Age of onset is affected by CAG repeat length
• However, there is still a lot of variation.
• The influence of the age of onset is multifactorial. Other genetic factors are involved - one on chromosome 8 and two on chromosome 15. Yet to identify the actual genes.

Question 8

Outline the treatment of Huntington’s disease.

Answer 8

• We all have the HTT gene and it is turned on in all cells. However, we do not understand what the protein does. With genetics, this doesn’t matter.
• One approach is to use a suppressor screen – delete every gene in turn and identify genes that, when deleted, prevent HD. T
• hese genes encode proteins that could be a drug target. This can be done using yeast:

I. Deleted genes in the kynurenine biochemical pathway “rescued” yeast from “Huntington’s disease”

• Chemical inhibition of kynurenine biochemical pathway also prevents it in yeast, mice and fruitflies.

Question 9

Provide the clinical basis for developmental disorders.

Answer 9

• Individually rare, but collectively common (2-5% of children born with a major congenital malformation or show neurodevelopmental disorder in childhood).
• Can be caused by environment (infection, maternal alcohol) or new mutation in a gene.

Question 10

Provide a molecular basis fof developmental disorders.

Answer 10

• Sequencing the genome, but only the exons which code for proteins: the exome.
• Sequence exome in child and parents, search for new mutation in a gene – bingo!
• 4293 families with a child with a developmental disorder sequenced.
• Looked at candidate pathogenic mutations – average of two per child.
• Many genes had 2 or more de novo mutations in unrelated children – good evidence for these being causative.

Question 11

Outline the molecular basis for rare disorders.

Answer 11

• What about the next step – rare diseases where parents are not available or the disease is inherited.Just sequence the exome and find the mutation, right? NO.
• Not necessarily de novo mutation / and healthy people have variants too!
• 60,000 exomes from healthy people
• Everybody has an average of 35 homozygous DNA variants that truncate a protein
• So it is important that a mutation that is thought to cause a rare disease is not a variant that is observed in healthy people.