Lecture 25 Flashcards
What are four clinical features of Cystic Fibrosis (CF)?
1) Clogged and infected airways - conducive to chronic inflammation due to bacterial colonization in viscous mucus
2) Digestive problems - pancreatic duct dysfunction, plugged bile ducts, meconium ileus
3) Bilateral absence of the vas deferens - male infertility
4) Salty sweat - diagnostically useful, salt transport problem
What mode of inheritance is Cystic Fibrosis (CF)?
Autosomal Recessive
What is the gene that when function, prevents Cystic Fibrosis?
Cystic Fibrosis Transmembrane Regulatory (CFTR) gene
How can one calculate number of introns knowing number of exons?
exons - 1 = # introns
What is the most common Cystic Fibrosis (CF) allele in Caucasians?
deltaF508 in the NBD1 (Nuclear Binding Domain 1) gene portion
What are characteristics of the Cystic Fibrosis Transmembrane Regulatory (CFTR) proteins?
1) Membrane Spanning Domains (MSDs) along gene code for transmembrane proteins that transport chloride ions (Hydrophobic proteins in membrane)
2) Nuclear Binding Domains (NBDs) along gene code for ion channel membrane proteins that assist in opening or closing the ion channel (close when ATP binds)
3) R-domains along gene code for Regulatory domain proteins that also assist in opening or closing the ion channel (closed when R-domain is phosphorylated)
What is an important deletion in the CFTR gene that is Class II and causes defective processing (causes CF to arise)?
Deletion of 3 Ts coding for Phe in the F508 allele. Does not shift the reading frame or induce nonsense mediated decay
Who should genetic testing for Cystic Fibrosis (CF) be offered to?
1) Adults with a positive family history of cystic fibrosis
2) Partners of individuals with cystic fibrosis
3) Couples currently planning a pregnancy
4) Couples seeking prenatal care
Who should genetic testing for Cystic Fibrosis (CF) not be offered to?
1) General population
2) Newborn infants
What is a characteristic advantageous example of why there are different carrier frequencies of CF in different populations (ethnicities)?
1) Salmonella typhi (causes of typhoid fever) uses CFTR protein for entry into intestinal epithelium, while S. typhimurium (nonpathogenic relative) does not
2) CF is therefore an advantageous natural selection process that occurs more frequently in areas with greater exposure to Salmonella typhi
(deltaF508 homozygotes translocate no S. typhi and delta F508 heterozygotes translocate 86% fewer S. typhi than normals)
What cells in the lung epithelia make mucus?
Goblet cells
What is the only way to get rid of anything you inhale?
Produce mucus that is carried up and out of the pulmonary tract by cilia
How does water cross membranes?
1) Aquaporin channels assist the movement of water across membranes
2) Water will follow the transport of ions such as Na+/Cl- by the process of osmosis
What class of aquaporins is expressed in lung epithelial tissue?
Aquaporin 1
What happens when the CFTR proteins are dysfunctional or missing?
1) Cl- ion cannot exit lung epithelial cells
2) Water therefore does not have much of a gradient to diffuse out of epithelial cells
3) Mucus continues to be produced and becomes thick and dehydrated due to the lack of water outside of the epithelial cells
4) Bacteria colonize in the thick mucus
What are treatments for CF?
1) Antibiotics
2) Physical therapy
3) Supplementation of digestive enzymes
4) Hypertonic saline spray
5) Gene therapy
How can the gene for CF be isolated?
Positional cloning - The cloning or identification of a gene for a particular disease based on its location in the genome, determined by a collection of methods including linkage analysis, genomic (physical) mapping, and bioinformatics, when no information about the biochemical basis of the disease is known
What is the difference between Positional cloning and gene cloning?
Gene cloning begins with a known protein product, determining its amino acid sequence, and using that information to isolate the gene. One does not know the protein product in positional cloning
Can CF occur from random, new mutations?
Rarely. It is an autosomal recessive disorder that will most likely only occur if both parents are at least obligate carriers
What is unique about the types of mutations that can occur to induce CF?
CF demonstrates extreme allelic heterogeneity among the population (1,200 known mutations)
How many classes of mutations can induce CF?
Four. We only have to know deltaF508
What is a marker for muscle cell damage?
High levels of Creatine phosphokinase (CPK)
What are clinical features of Duchenne Muscular Dystrophy (DMD)?
1) 1/3000 incidence, nearly all boys
2) High CPK levels in blood - evidence of muscle cell damage
3) Proximal muscle hypertrophy and degeneration, respiratory failure
4) Frequent cardiomyopathy and sudden death
What is the mode of inheritance for Duchenne Muscular Dystrophy (DMD)?
X-linked recessive
How can women obtain Duchenne Muscular Dystrophy (DMD) if they have one normal X chromosome and one affected X chromosome?
1) Translocation occurs at Xp21 (gene that prevents DMD) with chromosome 3
2) If this affected X-chromosome with part of chromosome 3 is inactivated, the cell does not contain all the genes on chromosome 3 and will die
3) If the normal X-chromosome is inactivated, the cell lives with the disrupted DMD gene (causing DMD)
What is the largest known human gene?
The dystrophin gene (DMD)
What is the function of the dystrophin protein?
In skeletal and cardiac muscles, dystrophin is part of a group of proteins (a protein complex) that work together to strengthen muscle fibers and protect them from injury as muscles contract and relax. The dystrophin complex acts as an anchor, connecting each muscle cell’s structural framework (cytoskeleton) with the lattice of proteins and other molecules outside the cell (extracellular matrix)
Do exons have to be multiples of 3 base pairs to encode proper codon sequence?
No. The only requirement is that when splicing occurs, the paired exons together must read an inframe codon sequence (one codon can begin on one exon and continue on another)
What causes Duchenne Muscular Dystrophy?
1) Nonsense mutations (1/3)
2) Out of frame deletions (2/3)
What causes Becker Muscular Dystrophy?
In-frame deletions
What is aminoglycoside therapy?
1) Aminoglycoside antibiotics disrupt translation in bacteria by binding to 16S rRNA but at high does, they induce misreading of mRNA in eukaryotes
2) Misreading of a nonsense codon as a sense codon would prevent nonsense mediated mRNA degradation and allow translation of full length dystrophin
How can you tell if a mother is an obligate carrier for DMD?
If there are two or more sons that are affected. If only one son is affected, then 2/3 of the time she is a carrier by the remaining 1/3 of the time she is not (new mutation must occur instead)
What are there no missense mutations that cause DMD?
A change in an amino acid will not change the structure of dystrophin much
