Course Code: Biochemistry 4194BiochemistryMolecular Genetic

Biswas, Aneel

Giarrano, Gianni

Marcum, Josh

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Genotype-phenotype discord in Cystic FibrosisGianni Giarrano, Josh Marcum, Aneel BiswasWhat is Cystic Fibrosis (CF)?• Cystic Fibrosis is a genetic disease caused by mutation(s) in the cystic fibrosistransmembrane regulator (CFTR) gene.• The CFTR gene contains the blueprint for creating the CFTR protein. When there is amutation in the CFTR gene, the blueprint is altered and so is the CFTR protein thusresulting in a change in its function.• The CFTR protein is found in the cells of many important organs including the lungs,pancreas, and sweat glands• There are over 1000 gene mutations associated with cystic fibrosis.Cystic Fibrosis Transmembrane Conductance Regulator Protein (CFTR)• CFTR protein acts as a channel on cell surfaces to move chloride(salt) in and out of the cell. When the CFTR gene is mutated, CFTRprotein is not present or unable to move chloride.• Dysfunction of the CFTR protein results in defective chloridesecretion and defective mucus clearance.• Chloride is required to draw in water, without it a thick andsticky mucus builds up in the lungs, pancreas, liver, and intestinesSource: https://www.yourgenome.org/facts/what-is-cystic-fibrosisHow is CF inherited?• CF is an autosomal recessive genetic disease (see figure to the right)• The frequency of CF occurrence in the UnitedStates is about 1 in 3,500 live births ofEuropean (Caucasian) descent; the frequencyis much lower in other ethnic groupsSource: https://www.cff.org/intro-cf/cf-genetics-basicsHow is CF diagnosed?• Immunoreactive Trypsinogen (IRT) enzyme testo A blood test for IRT, an enzyme that may have increased levels as a result of pancreatic blockage due to mucus accumulation• Sweat chloride level testo Using a sweat-inducing topical medication and light electricalstimulation, sweat is tested for high chloride levels• Genetic testingo DNA is analyzed for known CFTR gene mutations usingpolymerase chain reaction (PCR), a technique used to amplifysections of DNA for various types of analysisSource: https://www.lecturio.com/concepts/cystic-fibrosis/Genetic testing• Deletion of the amino acid phenylalanine at position 508 onchromosome 7 is the most common mutation causing CF; there aremany other mutations each carrying differing risks• Genetic testing can provide insight into what type of mutation anindividual has and if they are at higher risk for certain symptoms• Therapy differs significantly for each class of mutation, and CFtreatment can be tailored toward the individual for better outcomesSource: https://www.news-medical.net/health/Tests-for-Cystic-Fibrosis-During-Pregnancy.aspxCommon symptoms• Decreased pulmonary functiono Mucus in the lungs blocks airways and traps bacteria, leading to infections• Pancreatic insufficiencyo The pancreas absorbs chloride and secretes bicarbonate via the CFTR channel, maintaining the proper pH of digestive fluids and other pancreatic secretions. o Altered functions of CFTR causes improper pancreatic secretions, resulting in mucus which blocks the release of digestive enzymes and thereby improper nutrition and growtho Many CF patients regain pancreatic sufficiency in adulthood with proper treatment • Male infertility97% of men who have CF are infertile. They produce sperm but are missing the sperm canal.Source: https://www.nhlbi.nih.gov/health/cystic-fibrosis/symptomsOther common symptoms and complications• Chronic constipation and digestive issueso Possibility for newborns to have meconium ileus, a blockage of stool from leaving the small intestine.o May cause chronic constipation in adulthood • Liver diseaseo Mucus can block the bile duct, a tube that releases a fluid to help digestion, and lead to liver disease• Reduced uptake of nutrientso Mucus buildup in the pancreas and intestines cause low uptake of proteins, vitamins, and minerals essential for proper nutritiono Teens especially require more caloric intake than others their same age, around 1½ to 2 times the energy intake compared to otherso Dietary supplements may be required to ensure proper nutritiono A dietician who specializes in CF can develop plans to assist in creating a diet planCurrent state of treatment• Median life expectancy has increased tremendously over the past few decades• Changes in diet, development of new drugs, and antibiotics are contributing factorshttps://www.cff.org/community-posts/2017-11/survival-trending-upward-what-does-really-mean• Disciplined regimen including taking prescribedmedication, daily exercise, physical therapy, enzymesupplementation, and proper diet• Cystic fibrosis remains difficult to manage and cancause overwhelming stress for patients and parents.The mental health challenges cannot be ignored andshould be monitored in both children and adults.• An increasing number of patients can live productive,happy lives. Studying at college, marrying, and startingfamilies can be within reach of most CF patients.Source: https://www.medindia.net/patients/patientinfo/cystic-fibrosis.htmCurrent state of treatmentWhat is genotype-phenotype discord?• A person’s genotype is their collection of genes thatwere inherited from their parents• A person’s phenotype is the set of observable traits thatarise from inherited genes and their interaction with theenvironment• In some cases, a person’s observed phenotype doesnot coincide with what is expected based on theirgenotype• Understanding what causes the discord betweengenotype and phenotype allows for better treatment ofgenetic disease Source: https://www.expii.com/t/phenotype-definition-overview-10228What affects the expression of CF Symptoms?• Proper treatment including medication, diet, and exercise has a positive effect on the severity of symptoms• There are 5 classes of mutations that correlate to the severity of some symptoms• Modifier genes are genes that are not associated with CFTR but change phenotypic expression and severity of symptoms• Still many unknowns, but progress is constantly being made in understanding CF and developing new treatment.Five classes of CFTR mutationsPhenotypic severity is generally highest with a class 1 mutation and lowest with a class 5 mutationSource: https://www.cff.org/sites/default/files/2021-12/Know-Your-CFTR-Mutations-Infographic.pdf• Mutation class may be determinedthrough genetic testing• Certain medications may only beused depending on a patient’smutation class• While mutation class may influencethe severity of symptoms, there aremany other factors that effectphenotypic expressionSource: https://www.cff.org/sites/default/files/2021-12/Know-Your-CFTR-Mutations-Infographic.pdfFive classes of CFTR mutationsVariation in phenotypic expression• Some phenotypes seem to have no dependence on genotypeo This class includes liver disease which is only found to be correlated with a gene called GSTP1o Phenotypic expression of pulmonary symptoms has no correlation to any specific genotype nor class of mutation• Some genotypes are correlated with milder phenotypeso Two genotypes known as “R117H” and “DelF508” contain a likelihood of milder symptoms including pancreatic sufficiency, lower sweat chloride levels, and older age at first diagnosiso Other genotypes known as “G551D” and “DelF508” carry a possible decreased risk of meconium ileus at birthOther genotype-phenotype correlations• Patients with pancreatic insufficiency (oftenassociated with Class I-IV mutations) haveworse pulmonary function than patients withpancreatic sufficiency• Deficiencies in Mannose Binding Lectin(MBL), a protein that acts as the first line ofdefense of the immune system, increaseslikelihood of the severe pulmonary phenotypeas bacterial infections become more common(see figure). The basis is unclear.Source: https://www.imd-berlin.de/en/subject-information/diagnostics-information/deficiency-of-mannose-binding-lectin-mbl-as-a-cause-of-impaired-pathogen-defence.htmlLumacaftor, Tezacaftor, and Ivacaftor• Three new drugs that treat CF at a molecular level have recently been approved for treatment (2012, 2015, and 2018)• Lumacaftor and tezacaftor stabilize the mutant CFTR protein and allowing it to reach the outer cell surface to perform its function • Ivacaftor is the only drug that enhances CFTR channel activity allowing the movement of chloride across the membrane• F508del homozygotes (class 2 mutation) are treated with a combination of ivacaftor and either lumacaftor or tezacaftorLumacaftorivacaftorTezacaftorAvailable resourcesCystic Fibrosis Foundation• Funds and allows access to CF research• Provides care and support options for CF patients• Provides financial assistance and help navigating insuranceReferences• Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P (2015) Molecular biology of the cell, 6th edition, Garland Science, NewYork, NY, pp.1297-1340.• Chalmers JD, Fleming GB, Hill AT, Kilpatrick DC (2011) Impact of mannose-binding lectin insufficiency on the course of cystic fibrosis: a review andmeta-analysis. Glycobiology 21(3): 271-282. https://doi.org/10.1093/glycob/cwq161• Cystic fibrosis: Diet and nutrition. (c.1995-2022) Tampa (FL). Johns Hopkins All Children’s Hospital. [Accessed May 19, 2022].https://www.hopkinsallchildrens.org/Patients-Families/Health-Library/HealthDocNew/Cystic-Fibrosis-Diet-and-Nutrition• Fiedorczuk K, Chen J (2022) Mechanism of CFTR correction by type I folding correctors. Cell 185(1): 158-168. https://doi.org/10.1016/j.cell.2021.12.009• Geborek A, Hjelte L. (2011). Association between genotype and pulmonary phenotype in cystic fibrosis patients with severe mutations. J CystFibros. 10(3): 187-192. https://doi.org/10.1016/j.jcf.2011.01.005• Gibson-Corley KN, Meyerholz DK, Engelhardt JF (2016) Pancreatic pathophysiology in cystic fibrosis. J Pathol. 238(2): 311-320. https://doi.org/10.1002/path.4634• Intro to CF. (n.d). Bethesda (MD). Cystic Fibrosis Foundation. [Accessed July 20, 2022]. https://www.cff.org/intro-cf.• Trivedi BP (2020) Breath from salt. BenBella Books, Inc., Dallas, TX• Turner MW (2003) The role of mannose-binding lectin in health and disease. Mol. Immunol. 40(7): 423-429. https://doi.org/10.1016/S0161-5890(03)00155-X• Yu YC, Sohma Y, Hwang TC (2016) On the mechanism of gating defects caused by the R117H mutation in cystic fibrosis transmembraneconductance regulator. J Physiol. 594(12): 3227-3244. https://doi.org/10.1113/JP271723.

Genotype-phenotype discord in Cystic Fibrosis

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Genotype-phenotype discord in Cystic Fibrosis

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