Lesson 3 - Haemoglobinopathies and mutation 23-24.pdf

Haemoglobinopathies and mutation MD210 – GGE – Genetics Lesson 3 1 Essential Learning Outcomes By the end of this lesson you should be able to: • Describe the common types of mutation and categorise their different functional consequences with reference to Muller’s Morphs • Demonstrate an underst...

Haemoglobinopathies and mutation MD210 – GGE – Genetics Lesson 3 1 Essential Learning Outcomes By the end of this lesson you should be able to: • Describe the common types of mutation and categorise their different functional consequences with reference to Muller’s Morphs • Demonstrate an understanding of the concept of balanced genetic polymorphism with reference to sickle cell trait and sickle cell anaemia • Understand how complex gene regulation is and describe how mutations distant from structural genes can cause disease, with reference to thalassaemia 2 Menti question • Which of the following blood types are least likely for the parents of a girl with blood type O? • If girl is blood group O, there is only 1 possible genotype: • OO homozygous • So she must have got O allele from both parents upstream downstream 4 The central dogma of biology DNA Transcription Pre mRNA Splicing mRNA Translation peptide Post –Translation Folding Protein Then multimer assembly…. https://www.youtube.com/watch?v=TfYf_rPWUdY 5 Mutations Mutation: • A permanent heritable change in the nucleotide sequence of a gene or chromosome • • • • Mutation may be described in terms of change in: genomic DNA (prefix g.) complimentary (coding) DNA (prefix c.) protein ( prefix p.) • G > A means G change to A • Guanine is replaced by Adenine 6 Classification of Mutations 1. 2. 3. 4. 5. Deletions Insertions Substitutions (missense, nonsense, splice site) Frameshifts (may arise from 1 or 2) Dynamic mutation (tandem repeats e.g. HD) See notes for more details 7 3 Functional consequences of mutations • Loss of function (inactivating) – protein has less/no function – often recessive • Gain-of-function (activating) mutations – increase in normal gene function (e.g. increased gene expression) or different and abnormal function – usually dominant • Silent mutations (most mutations are clinically silent) – why (Menti)? 11 Silent Mutations 1. No change in aa sequence (synonymous nucleotide change) 2. Or result in a change in aa (non-synonymous nucleotide change) that results in no change of function 12 Muller’s Morphs – Mutation Effect Category Effect on normal function Heterozygote pattern Example Amorph (null) Complete loss Recessive, but dominant if haploinsufficient* O Blood type allele Hypomorph (reduced) Partial loss Recessive, but dominant if haploinsufficient* CFTR mutations in Cystic Fibrosis Hypermorph Increased Dominant EGFR oncogene in cancer Antimorph (dominant negative) Antagonistic Dominant FBN1 mutations in Marfan Syndrome Neomorph Different/new Dominant BCR-ABL fusion protein in CML Isomorph (silent) None N/A Loss of function Gain of function *Haploinsufficient = a single copy of wild-type allele is not sufficient for normal phenotype 13 Questions – breakout groups • What category of mutation best describes the Glutamic acid > Valine mutation that converts HbA to HbS, with respect to O2 carrying function? • What category of mutation best describes the PML-RARα fusion protein in APML? Sickle Cell Anaemia- Case Description Presentation/history A 6 year old girl with sickle cell anemia Dry cough, nasal discharge, blocked nose, temperature For 2 Days Drinking less fluids Has been given paracetamol She is taking daily penicillin (amoxicillin) Examination T37.7, P 100, R 30, BP 98/52 Alert and interacting No rashes or skin lesions Clear Nasal Discharge No Pharyngeal Erythema/Tonsillar Swelling No cervical lymphadenopathy What Complication Has She Got? 15 Sickle Cell Anaemia- Case Description A Viral Respiratory Tract Infection? The Risk is Sickle Cell Crisis: Acute Chest Syndrome (ACS) Chronic pain Organ Damage Swelling in hands & feet Bacterial infections Autosplenectomy by mid-childhood– Require immunisation against common pathogens and prophylactic ABs Credit: FDA Voice blog 16 Moral and Ethical Issues Related to Prenatal Diagnosis There are important moral and ethical issues related to prenatal diagnosis, in particular when performed with a view to termination of pregnancy if the foetus is affected In this course discussion of prenatal diagnosis is intended to help you understand what is technically /scientifically possible. It is not intended to promote a particular view on the moral and ethical issues Dr Campbell’s lessons will cover important ethical issues including PGD 17 Sickle Cell Prenatal Diagnosis How Would You Do Prenatal Diagnosis ? 18 Sickle Cell Prenatal Diagnosis Chorionic Villous Sampling (9-10 weeks) PCR Amplify Fragment of β-globin Gene Oligonucleotide Probe Hybridization/Sequencing Can also do amniocentesis – (amniotic fluid, 15-18 weeks) – but need to grow cells in lab first to do PCR 19 Sickle Cell Screening • Newborn screening available in the US and UK (1 in 2000) – blood spot test • Is it available in Ireland? • Certain “at-risk” mothers are screened for the sickle cell gene in some hospitals Courtesy CDC Sickle Cell -Genetics Allele Frequency 16% in Parts of Africa 4% in US African Americans Incidence of Sickle Cell Disease 1/100 in Parts of Africa 1/500 in African Americans Why is incidence lower in African Americans? 21 Revision - Evolution • Natural Selection • Individuals who inherit adaptations will survive in greater proportion and pass on their genes. “Survival of the fittest”. Over a long time these genes accumulate in the population • Evolution • In population genetics, evolution is defined as the change in the frequency of an allele in a population over time • Adaptation • a heritable trait that aids the survival and reproduction of an organism in its current environment Polymorphism • poly = multiple • morph = form/type • polymorphism = multiple forms • Polymorphism means that two or more discontinuous (i.e. clearly different) forms occur in a single population in the same place at the same time • Single (panmictic) population means random (unrestricted) mating within the group • Is increased height a polymorphism? Blood Group A, B, AB and O represents polymorphism (cryptic) /discrete phenotypes 23 Balanced Genetic Polymorphism • The simultaneous occurrence in the same population of two or more “discontinuous” genetic forms in “such proportions” that the frequency of occurrence of the rarest of them cannot be explained just by recurrent mutation or immigration • “Such proportions” = with a frequency of at least 1% of alleles • The Implication is that something in the environment is acting to select for maintenance of the equilibrium (balance) between the different forms in the population • i.e. Natural selection 24 How does that Relate to Sickle Cell? •Sickle Cell Allele is particularly associated with populations that originated from areas with high incidence of malaria •Here natural selection favours HbAS sickle cell trait •Sickle Cell Disease (HbSS) is an inevitable consequence of selection pressure for the maintenance of the heterozygous state •If the heterozygous state was not advantageous you might expect extinction of HbSS by negative selection HbAA HbAS HbSS https://www.newscientist.com/artic le/dn20450-how-sickle-cellcarriers-fend-off-malaria/ 25 Sickle Cell Anaemia vs Sickle Cell Trait • • Sickle Cell Anaemia (HbSS) is A • Character/Trait (Clinically Manifest Phenotype) • • Pattern of Inheritance is Recessive • Usually Parents Do Not Have Sickle Cell Anaemia Sickle Cell Trait (HbAS) is a different Character/Trait “Cryptic” Phenotype • Pattern of inheritance is Dominant • Parent Almost Always Has HbAS 26 Inherited Disorders of Balanced Haemoglobin Biosynthesis (Thalassaemias) Important by itself An Example of Complexity of Regulation of Gene Expression 27 Thalassaemias A Good Example of the Complexity of “Genetic Context” A specific β globin allele associated with different phenotype Depending on co-inherited modifying factors Level of expression of foetal globin Level of expression of α globin 28 β-Globin Gene Mutations Associated with β-Thalassaemia 29 β-thalassaemias HBB gene promoter (where transcription machinery binds) position -34 is normally A Mutation A-G = no binding and no transcription Common in black people with thalassaemia Also occurs in Chinese people with thalassaemia Disease Associated with the Point Mutation Differs in Different Ethnic Groups Differences in Ability to Compensate by Synthesis of HbF in response to erythroid stress β α α β Hb A (α2βA2) γ α α γ Hb F (α2γ2) 30 β-thalassaemias In some cases globin genes may be fine BUT Regulation of gene expression is critical to function LCR Deletions β-globin gene is structurally normal DNA sequence is normal for 500 bp 5’ and 3’ Large 5’ deletion Far less β-globin produced (no enhancer) 31 Question – breakout groups • How would you do prenatal diagnosis of β-thalassaemia? • Blood of baby won’t work – not expressing β-globin to sufficient levels for clear delineation of hetero/homozygous • First check blood of both parents by Hb electrophoresis • If both have β-thalassaemia trait then can do more invasive tests (CVS/amniocentesis) and PCR/sequencing β-thalassaemias Why is the face an abnormal shape? Many of the Features of the Syndrome related to physiological response that represents an effort to compensate for the physiological deficit associated with the inherited mutation Hypoxia > high EPO> bone marrow hyperplasia Increased haematopoiesis distorts bones 33 Things to Remember 1. A mutation – a heritable genetic change (generally DNA nucleotide sequence change) 2. Inherited disorders of Hb may be haemoglobinopathies - production of abnormal globin chains or thalassaemias – imbalance in production of structurally normal globin chains 3. Sickle cell anaemia is an important condition with an autosomal recessive pattern of disease mainly affecting people of African descent 4. Balanced polymorphism – refers to the persistence of a number of alleles in a stable equilibrium in a population and is generally discussed in the context of one allele that is disadvantageous in homozygous state 34