Lecture 25

Question 1

What are four clinical features of Cystic Fibrosis (CF)?

Answer 1

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

Question 2

What mode of inheritance is Cystic Fibrosis (CF)?

Answer 2

Autosomal Recessive

Question 3

What is the gene that when function, prevents Cystic Fibrosis?

Answer 3

Cystic Fibrosis Transmembrane Regulatory (CFTR) gene

Question 4

How can one calculate number of introns knowing number of exons?

Answer 4

exons - 1 = # introns

Question 5

What is the most common Cystic Fibrosis (CF) allele in Caucasians?

Answer 5

deltaF508 in the NBD1 (Nuclear Binding Domain 1) gene portion

Question 6

What are characteristics of the Cystic Fibrosis Transmembrane Regulatory (CFTR) proteins?

Answer 6

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)

Question 7

What is an important deletion in the CFTR gene that is Class II and causes defective processing (causes CF to arise)?

Answer 7

Deletion of 3 Ts coding for Phe in the F508 allele. Does not shift the reading frame or induce nonsense mediated decay

Question 8

Who should genetic testing for Cystic Fibrosis (CF) be offered to?

Answer 8

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

Question 9

Who should genetic testing for Cystic Fibrosis (CF) not be offered to?

Answer 9

1) General population

2) Newborn infants

Question 10

What is a characteristic advantageous example of why there are different carrier frequencies of CF in different populations (ethnicities)?

Answer 10

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)

Question 11

What cells in the lung epithelia make mucus?

Answer 11

Goblet cells

Question 12

What is the only way to get rid of anything you inhale?

Answer 12

Produce mucus that is carried up and out of the pulmonary tract by cilia

Question 13

How does water cross membranes?

Answer 13

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

Question 14

What class of aquaporins is expressed in lung epithelial tissue?

Answer 14

Aquaporin 1

Question 15

What happens when the CFTR proteins are dysfunctional or missing?

Answer 15

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

Question 16

What are treatments for CF?

Answer 16

1) Antibiotics
2) Physical therapy
3) Supplementation of digestive enzymes
4) Hypertonic saline spray
5) Gene therapy

Question 17

How can the gene for CF be isolated?

Answer 17

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

Question 18

What is the difference between Positional cloning and gene cloning?

Answer 18

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

Question 19

Can CF occur from random, new mutations?

Answer 19

Rarely. It is an autosomal recessive disorder that will most likely only occur if both parents are at least obligate carriers

Question 20

What is unique about the types of mutations that can occur to induce CF?

Answer 20

CF demonstrates extreme allelic heterogeneity among the population (1,200 known mutations)

Question 21

How many classes of mutations can induce CF?

Answer 21

Four. We only have to know deltaF508

Question 22

What is a marker for muscle cell damage?

Answer 22

High levels of Creatine phosphokinase (CPK)

Question 23

What are clinical features of Duchenne Muscular Dystrophy (DMD)?

Answer 23

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

Question 24

What is the mode of inheritance for Duchenne Muscular Dystrophy (DMD)?

Answer 24

X-linked recessive

Question 25

How can women obtain Duchenne Muscular Dystrophy (DMD) if they have one normal X chromosome and one affected X chromosome?

Answer 25

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)

Question 26

What is the largest known human gene?

Answer 26

The dystrophin gene (DMD)

Question 27

What is the function of the dystrophin protein?

Answer 27

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)

Question 28

Do exons have to be multiples of 3 base pairs to encode proper codon sequence?

Answer 28

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)

Question 29

What causes Duchenne Muscular Dystrophy?

Answer 29

1) Nonsense mutations (1/3)

2) Out of frame deletions (2/3)

Question 30

What causes Becker Muscular Dystrophy?

Answer 30

In-frame deletions

Question 31

What is aminoglycoside therapy?

Answer 31

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

Question 32

How can you tell if a mother is an obligate carrier for DMD?

Answer 32

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)

Question 33

What are there no missense mutations that cause DMD?

Answer 33

A change in an amino acid will not change the structure of dystrophin much