Molecular Defects: Genetic Disorders Quiz Mastery

The CFTR gene encodes the cystic fibrosis transmembrane conductance regulator protein, which functions as a chloride ion channel in epithelial cell membranes. In normal cells, CFTR allows chloride ions to move out of cells, which draws water into the airway lining and keeps mucus thin and fluid. When CFTR is absent or nonfunctional due to mutation, chloride and water transport is impaired, producing the thick, sticky mucus characteristic of cystic fibrosis.

Cystic fibrosis is an autosomal recessive condition. An individual must inherit two defective copies of the CFTR gene, one from each parent, to develop the condition. Individuals who carry only one mutated copy are heterozygous carriers who produce enough functional CFTR protein from their normal allele to remain unaffected. Carriers do not show symptoms of cystic fibrosis but have a 50 percent chance of passing the mutated allele to each of their children.

The most common cystic fibrosis mutation is deltaF508, a deletion of three nucleotides that removes a single phenylalanine residue at position 508 of the CFTR protein. Because the deletion is exactly three nucleotides, it does not cause a frameshift. Instead, the protein is produced but misfolds due to the missing amino acid. The misfolded CFTR protein is recognized by the cell's quality control system and degraded before it can reach the plasma membrane, resulting in absent chloride channel function.

Sickle cell disease is caused by a single missense point mutation in the HBB gene encoding the beta-globin subunit of hemoglobin. The mutation changes a single nucleotide, converting the codon for glutamic acid to a codon for valine at position 6 of the protein. This single amino acid substitution changes the surface chemistry of hemoglobin, causing the molecules to polymerize under low oxygen conditions and distort red blood cells into the characteristic rigid sickle shape.

Polymerized sickle hemoglobin causes red blood cells to become rigid and sickle-shaped. These cells block small blood vessels, causing vaso-occlusive pain crises and ischemic organ damage. Sickled cells are also fragile and have a shortened lifespan of about 10 to 20 days compared to the normal 120 days, leading to hemolytic anemia. Sickled cells do not have increased flexibility; they are significantly more rigid than normal biconcave red blood cells.

Individuals who are heterozygous carriers of the sickle cell allele, a condition called sickle cell trait, show a significant reduction in the severity of malaria caused by Plasmodium falciparum. The parasite completes part of its life cycle inside red blood cells, and the altered hemoglobin environment in carrier cells impairs parasite survival and replication. This protective effect explains the high frequency of the sickle cell allele in populations from malaria-endemic regions of sub-Saharan Africa and the Mediterranean.

Even though the deltaF508 mutation removes only one amino acid, the resulting protein misfolds in the endoplasmic reticulum. The cell's quality control machinery, specifically the endoplasmic reticulum-associated degradation pathway, identifies the misfolded CFTR protein and targets it for destruction via the proteasome. Because the protein never reaches the plasma membrane, no functional chloride channel is present in the epithelial cells of the airways, pancreas, and other affected organs, causing the full cystic fibrosis phenotype.

CFTR is expressed in the epithelial cells of multiple organ systems. In the pancreas, blocked ducts prevent digestive enzyme secretion, causing exocrine pancreatic insufficiency and malnutrition. The liver can develop biliary cirrhosis due to blocked bile ducts. In males, the vas deferens is often absent or obstructed, causing infertility. In females, thickened cervical mucus can reduce fertility. These multi-organ effects reflect the widespread expression of the CFTR protein throughout the body.

Cystic fibrosis and sickle cell disease are both autosomal recessive conditions, requiring two mutated gene copies for disease expression. Cystic fibrosis results from mutations in the CFTR gene, which encodes a chloride channel essential for epithelial fluid regulation. Sickle cell disease results from a mutation in the HBB gene, encoding the beta-globin subunit of hemoglobin. Both conditions demonstrate how single gene mutations can produce widespread and serious effects on human physiology.

When sickle hemoglobin is deoxygenated, the valine substitution at position 6 exposes a hydrophobic patch on the protein surface. This patch binds to a complementary hydrophobic region on an adjacent deoxygenated sickle hemoglobin molecule, triggering the formation of long rigid polymeric fibers. These fibers push against the internal surface of the red blood cell membrane, deforming the normally flexible biconcave disc into a stiff, elongated sickle shape that obstructs blood flow.

CFTR encodes a chloride ion channel, and the most common mutation, deltaF508, deletes the phenylalanine at position 508, causing protein misfolding and degradation. The resulting absence of CFTR at the cell membrane impairs chloride and water transport, producing thick mucus in the airways and other organs. The CFTR gene is located on chromosome 7, an autosome, not the X chromosome, which is consistent with the autosomal recessive inheritance pattern of cystic fibrosis.

The high frequency of the HbS sickle cell allele in malaria-endemic regions is a classic example of balancing selection, specifically heterozygote advantage or overdominance. Heterozygous carriers of sickle cell trait are protected from the most severe effects of malaria, giving them a survival and reproductive advantage. This advantage offsets the fitness cost of homozygous sickle cell disease, maintaining the allele at a relatively high frequency in the population across many generations.

CFTR modulator drugs, such as ivacaftor, lumacaftor, and their combinations, represent a major advance in treating cystic fibrosis by directly targeting the underlying molecular defect. Potentiators such as ivacaftor improve the opening probability of CFTR channels, while correctors such as lumacaftor help the misfolded deltaF508 protein reach the cell membrane. These therapies address the root molecular cause of the disease rather than only treating symptoms such as lung infections and mucus buildup.

Hemoglobin electrophoresis separates different forms of hemoglobin based on their electrical charge and migration speed through a gel or other medium. Hemoglobin S migrates differently from normal hemoglobin A due to the amino acid substitution that changes the charge of the protein. This technique can distinguish between individuals who are unaffected, those with sickle cell trait who carry one HbS allele, and those with sickle cell disease who carry two HbS alleles, making it a standard diagnostic tool.

Both cystic fibrosis and sickle cell disease are caused by mutations in single genes, CFTR and HBB respectively. In cystic fibrosis the mutant protein misfolds and fails to reach the membrane, a trafficking defect, while in sickle cell disease the mutant hemoglobin polymerizes under low oxygen. Both conditions affect multiple organ systems, illustrating the broad physiological impact a single gene defect can have. The two conditions are caused by different mutation types, a deletion versus a point missense substitution.