1 / 38

Principles of Molecular Disease : Lessons from the Hemoglobinopathies

Principles of Molecular Disease : Lessons from the Hemoglobinopathies. The effect of mutations on protein function. mutations resulting in a loss of function of the protein mutations resulting in a gain of function of the protein

lanza
Télécharger la présentation

Principles of Molecular Disease : Lessons from the Hemoglobinopathies

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Principles of MolecularDisease: Lessonsfrom the Hemoglobinopathies PostgraduatecourseHumanGenetics 09/12/11 Bert Callewaert, MD, PhD Center forMedicalGenetics GhentUniversity Hospital

  2. The effect of mutationsonproteinfunction • mutationsresulting in a loss of function of the protein • mutationsresulting in a gain of function of the protein • mutationsresulting in a novelpropertyby the protein • mutationsresulting in gene expression at the wrong time or place PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  3. Loss-of-functionmutations • deletion of the entire gene (and eventuallyalsocontiguousgenes) • examples: microdeletionsyndromes, monosomies (Turner), a-thalassemias • chromosomalrearrangements • premature stop codon (nonsenseorframeshift mutations) • missense mutationsmayabolishproteinfunction • e.g. Catshl syndrome: loss- of – function FGFR3 FGFR3 p.R621H Severity of disease ~ amount of function lost PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  4. Gain-of-functionmutations = mutationsthatenhanceoneor more of the normalfunctions of the protein • mutationsthatenhanceonenormalfunction of the protein • f.e.: the G380R mutation in • FGFR3causingachondroplasia • mutationsthatincrease the production of a normalprotein • in itsnormal environment • f.e.: trisomy 21 (Down syndrome) note: Alzheimer • duplication of PMP22 in Charcot-Marie-Toothdisease type 1A • chromosomalduplications in cancer PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  5. Novelpropertymutations = (missense) mutationsnovelproperty of the protein +/- normalfunction infrequent (most AA substitutionseitherneutralordetrimental) e.g. sicklecelldisease PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  6. Mutationsassociatedwith heterochronicorectopic gene expression • = mutationsthat alter the regulatoryregions of a gene • Examples: • oncogenemutations in cancer • hereditarypersistence of HbF (continuedexpression of the g-globin gene) PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  7. Hemoglobinopathies • most commonsingle-gene disorders in humans • more than 5% of the world’spopulation is carrier of anabnormalglobin gene PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  8. Hemoglobin • 4 subunits: 2 a (like) and 2 b (like) - chains • each subunit is composed of : • - a polypeptidechain (globin) • - a prostheticgroup (heme): iron-containing pigment that combines with O2 • highlyconservedstructure • Hb A (adulthemoglobin): a2 b2 PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  9. PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  10. identical chromosome 16 differonly in 10/146 AA chromosome 11 In adultlife: >97% HbA 2% HbA2 <1% HbF commonancestral gene PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  11. Globinswitching PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  12. 6 kb Chr. 11 LCR e Gg Ag yb d b • expression of b-globin gene controlledbynearbypromoterand LCR • locuscontrolregion (LCR): requiredfor the expression of all the genes • in the b-globin cluster • deletions of LCR results in egdb- thalassemia PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  13. Groups of mutations Mutationsthat alter the structure of the globinprotein Reducedavailabilityoneor more globinchains mutationsthatimpair the globindevelopmentalswitching PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  14. 1. StructuralVariants • usuallydue to point mutationsin one of the globingenes • more than 400 abnormalhemoglobinvariants have been described • onlyabout 50% are clinically significant • three classes: • - mutantsthatcausehemolyticanemia • - mutantsthat alter oxygen transport • - mutantsthatreduce the abundance of the globinchain (thalassemias) PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  15. StructuralVariantsthatcauseHemolyticAnemia • the mutant makes the Hbtetramerunstable •  loss-of-function • e.g.: HbHammersmith (b-chain Phe42Ser mutation) • the mutant gives the globinchainanunusualrigidstructure • novelpropertymutations • f.e.: sicklecellglobin; HbC PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  16. SickleCellDisease • HbS: firstabnormalHbdetected (Glu6Val mutation in b-chain) • severeARcondition • common in equatorialAfrica; 1/600 AfricanAmericans is bornwith the disease • sicklecelltraitrefers to the heterozygous state • about 8% of AfricanAmericans are heterozygous • heterozygotes are protectedagainstmalaria PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  17. SickleCellDisease

  18. 1. StructuralVariants • usuallydue to point mutations in one of the globingenes • more than 400 abnormalhemoglobinvariants have been described • onlyabout 50% are clinically significant • three classes: • - mutantsthatcausehemolyticanemia • - mutantsthat alter oxygen transport • - mutantsthatreduce the abundance of the globinchain (thalassemias) PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  19. HemoglobinStructuralVariantsthat alter oxygentransport • HbHyde Park (β-chain His92Tyr) • ~ normalhemoglobinstability • ironresistant to the enzymemethemoglobin reductase. • accumulation of methemoglobin → cyanosis (usuallyasymptomatic) • homozygousstate presumably lethal. • HbHammersmith (βchainPhe 42 Ser) •  instable Hb, lower O2 affinity • mutationsin α:β interface (HbKempsey) • prevent oxigenrelatedmovement • locked in high O2 affinity state •  Polycythemia PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  20. 1. StructuralVariants • usuallydue to point mutations in one of the globingenes • more than 400 abnormalhemoglobinvariants have been described • onlyabout 50% are clinically significant • three classes: • - mutantsthatcausehemolyticanemia • - mutantsthat alter oxygen transport • - mutantsthatreduce the abundance of the globinchain (thalassemias) PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  21. 2. HemoglobinSynthesis Disorders (Thalassemias) • collectively the most commonhumansingle-gene disorders! • carriers: protectiveadvantageagainst malaria • > qalassa(sea): firstdiscovered in Mediterraneanarea • imbalance in a : b chainratio • - ↓synthesis • - instability • ↑normalchains: damage the RBCs (hemolyticanemia) • ↓Hbsynthesis hypochromic, microcyticanemia • DdIrondeficiency PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  22. α - globin production The a - Thalassemias Normal anemia Hydropsfoetalis HomotetramericHb: ineffectiveoxygencarriers  Hydropsfetalis • affect the formation of bothfetal and adultHb • in the absence of a-globins: • - HbBart’s: g4 • - Hb H: b4 PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  23. The a - Thalassemias • most commonlydue to deletion of the a-globingenes Misalignmentwith PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  24. The a - Thalassemias clinicalconditionnumber of functionala-globin gene a-chain a-genesgenotypeproduction Normal 4 aa/aa 100% Silent carrier 3 aa/a- 75% a-thalassemiatrait 2 a-/a- or 50% (mild anemia, microcytosis)aa/- - * Hb H (b4) disease 1 a-/- - 25% (moderatelyseverehemolyticanemia) Hydropsfetalis (HbBart’sg4) 0 - -/- - 0% * Carriers frequent in SoutheastAsia PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  25. The a - Thalassemias • Rare forms: • - formdue to the ZF deletion(namedafterindividual ZF) • - the ATR-X syndrome PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  26. ZF deletion transcribedfrom the opposite strand a2-gene is silenceddue to the generation of antisenseRNAsfrom the truncated LUC7L gene wild-typeantisensetranscripts do alsoexist and play a role in regulation of gene expression (f.e. X inactivation)! PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  27. The ATR-X syndrome • MR and a-thalassemia • due to mutations in the X-linked ATRX gene • encodes a chromatin remodelingprotein • activatesexpression in trans • partialloss-of-functionmutationsresult in modest reduction of a-globinsynthesis • somatic (more severe) mutations in ATRX cause the • a-thalassemiamyelodysplasiasyndrome(ifgermline: hydropsfetalis!) PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  28. The ATR-Xsyndrome • Profound MR (X-L) • MC • Short stature • Genital Δ • (Mild) anemia Erythrocytesafterincubation in briljant cresyl blue. HbH inclusions :‘golf ball’ From Gibbons R. Orphanet Journal of Rare Diseases 2006;1:15 PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  29. The b - Thalassemias • ↓b-globinproduction • twob-thalassemiaalleles: usuallythalassemiamajor (severeanemia) • oneb-thalassemiaallele: thalassemiaminor (mild anemia, noclinic) • postnatal • precipitation of excessa-chains hemolysis • low β-chain production  hypochromic, microcyticanemia • ↑HbA2 (a2d2) and ↑ HbF (a2g2) • >> single-base pair substitutions (ratherthandeletions) • >> compoundheterozygous • simple versus complex b-thalassemia PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  30. of the 3’ end of the gene >> PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  31. Point mutationsthatcauseb-thalassemia are distributedthroughout the gene. They affect virtuallyeveryprocessrequiredfor the production of normalb-globin. PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  32. Posttranscriptionalmodifications of mRNA PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  33. RNA splicingmutations in b – Thalassemias (1) PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  34. RNA splicingmutations in b – Thalassemias (2) PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  35. RNA splicingmutations in b – Thalassemias (3) Very frequent in SoutheastAsia Hb E: example of a single nucleotidesubstitutionthataffects both RNA splicing and the coding sequence PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  36. Complex b – Thalassemias (egdb)° thalassemia (illustratesimportance of LCR) (db)° thalassemia (Agdb)° thalassemia Hereditarypersistence of fetalhemoglobin PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

  37. 3. GlobinDevelopmentalSwitching Disorders • hereditarypersistence of fetalhemoglobin • group of clinicallybenignconditions • production of higherlevels of Hb F than is seen in (db)° thalassemia • theyimpair the perinatal switch fromg-globin to b-globinsynthesis PostgraduatecourseHumanGenetics – 09/12/11 Bert Callewaert, MD, PhD– Center forMedicalGenetics – Ghent University Hospital

More Related