Week 4 - Sickle Cell

Question 1

What is the HbS mutation?

Answer 1

• chromosome 11p
• missense mutation on 6th AA
• GAG to GTG
• glutamic acid (-ve charge) to valine (uncharged, hydrophobic)
• stiff rod polymer forms
• stretches and elongates cell
• most likely to occur when deoxygenated

Question 2

What are the physiological effects of HbS mutation?

Answer 2

• repeated polymerisation
• RBC membrane damaged
• lifespan reduced from 120 to 10-20 days
• vaso-occlusion in narrow vessels
• blockage from cell getting stuck

Question 3

What is sickle cell trait?

Answer 3

• carriers
• HbAS
• no symptoms
• situations (dehydration, low O₂) can cause sickling
• advantage of protection against malaria symptoms

Question 4

What is HbC?

Answer 4

• variant in the same locus
• example of compound heterozygosity if with sickle cell
• because it affects same gene
• missense mutation
• from glutamic acid to lysine (AAG)
• HbSC has milder SCD symptoms

Question 5

What are the possible gene therapies for SCD?

Answer 5

• improve abnormal β globin
• or restore γ globin expressions
• by gene addition
• or gene editing
• to correct/modify β globin
• or up-regulate γ globin
• target Haematopoietic Stem Cells (HSCs)

Question 6

How is ex-vivo gene therapy done?

Answer 6

• HSC (Haematopoietic Stem Cells) release stimulated
• collected by apheresis
• gene modified in culture
• reinfusion by myeloablative chemotherapy
• high dose required
• can kill off remaining bone marrow and ovaries
• risk of cancer and infertility

Question 7

What are the strategies for gene addition?

Answer 7

• gene of interest in lentiviral vector
• β^A-T87Q globin changes 87th AA from T to Q
• βAS3 globin has anti-sickling properties to inhibit remaining sickling
• γ goblin

Question 8

How can γ- globin be upregulated?

Answer 8

• BCL11A shuts off its production after fetal development
• CRISPR-Cas9 can disrupt it to restore γ globin production