Lecture 25

Question 1

what are the features of cystic fibrosis?

Answer 1

• autosomal recessive
• clogged and infected airways (prone to bacterial infection)
• digestive problems: pancreatic duct dysfunction, plugged bile ducts, meconium ileus (fetal blockage of intestine)
• bilateral absence of the vas deferens (99% male infertility)
• salty sweat due to salt transport problem
• usually die in ages between 3-5 due to mucus in lungs.

Question 2

what causes cystic fibrosis?

Answer 2

mutation in CFTR gene resulting in the misfolding of the membrane protein. This protein is located on the lung/air interface and is responsible for allowing chloride ions into the lung lumen. It is also responsible for co-regulating salt transport to the lung lumen (both important for establishing a water gradient). So the misfolding of this protein prevents sodium and chloride from getting onto the lung epithelial cell surface and therefore indirectly prevents water from passively diffusing through aquaporins onto the lung/air interface resulting in dehydrated mucus.

Question 3

why is there a high carrier frequency associated with CF?

Answer 3

• it confers a heterozygous advantage against typhoid fever since salmonella typhi use the CFTR membrane protein as a receptor to enter intestinal epithelial cells.

Question 4

what is the most common mutation resulting in CF?

Answer 4

deltaF508 with 70%

Question 5

describe the CF protein.

Answer 5

• It has 2 transmembrane domains rich in isoleucines and valines.
• It has two nucleotide membrane binding domains (bind ATP) and a regulatory domain which must be phosphorylated for it to work.
• it is also an ion transport protein for chloride made up of six transmembrane helices forming a pore (facilitated diffusion)

Question 6

who should be tested for cystic fibrosis?

Answer 6

• adults with positive family history of CF
• partners of individuals with CF
• couples currently planning a pregnancy
• couples seeking prenatal care

Question 7

what is the carrier frequency for caucasians, ashenazi jews, hispanics, and african americans, respectively (high, low, medium)?

Answer 7

• high (1/46)
• high (1/46)
• medium (1/46)
• low (1/60)

Question 8

what is the major problem with CF in children?

Answer 8

pancreatic deficiency and malnutrition.

Question 9

how does water move freely through biological membranes despite the fact that it is polar?

Answer 9

aquaporins. however it is not active transport, it is fascilitated diffusion so you need to always create a favorable gradient using SALT on the side you want the water to go to.

Question 10

what are the treatments for cystic fibrosis?

Answer 10

• antibiotics until bacterial resistance
• physical therapy
• supplementation of digestive enzymes for malnutrition
• hypertonic saline spray directly coats epithelial cells with salty solution which helps draw water out of the cells.
• gene therapy by supplying the normal protein

Question 11

what are the features of Duchenne Muscular Dystrophy?

Answer 11

• X-linked recessive on OMIM 310200
• effects begin at age 3, wheelchair by 8, dead by late teens usually due to respiratory failure or sudden cardiac myopathy.
• no normal muscle fibers.
• no difference among ethnic groups, males almost never reproduce.
• high Creatine phosphokinase (CPK) levels (evidence of muscle damage since they are only present in muscle)
• high rate of new mutations (1/3rd are de novo cases instead of inheritance)

Question 12

what is Becker Muscular dystrophy?

Answer 12

• rare milder form of DMD
• mutation is on the same gene (OMIM 310200)
• males can reproduce and aren’t wheelchair bound until 20

Question 13

what is the cause of muscular dystrophy?

Answer 13

mutation at the Xp21 gene which is 2,500,000 bp long (10 times longer than the CF gene). 99% of this gene is made up of introns with only 79 exons. These introns are full of repetitive sequences (necessary for promoter regions that have tissue specific transactivating factors) that are prone to interchromosomal deletions during meiosis and intrachromsomal recombination, leading to sequence deletion.

Question 14

what is the protein dystrophin responsible for?

Answer 14

• its an intracellular structural submembrane protein that lives inside the cell but close to the cell membrane.
• it cushions the movement of muscles and helps prevent shearing of the fibers.
• also associates with cytoskeleton proteins inside the cell, proteins on sarcolemma, and laminin and collagens, hooking them all together and providing structure.

Question 15

what happens if you lack laminin?

Answer 15

you get congenital muscular dystrophy

Question 16

what happens if you lack dystrophin?

Answer 16

you get duchenne or beckers muscular dystrophy

Question 17

what happens if you lack collagen VI?

Answer 17

you get bethlem myopathy

Question 18

what happens if you lack alpha-dystroglycan?

Answer 18

you get limb-girdle muscular distrophy

Question 19

what are the usual mutations that result in duchenne muscular dystrophy?

Answer 19

• 2/3 are out of frame large deletions resulting in premature stop codons and nonsense mediated decay. So no dystrophin protein at all.
• 1/3 are nonsense mutations with no protein production.
• never missense. Missense results in production of altered or defected dystrophin so you get the milder Becker Muscular Dystrophy.

Question 20

in summary which mutations lead to BMD vs DMD?

Answer 20

• in frame mutations lead to BMD since you still have the protein being translated only with altered amino acid structure.
• out of frame mutations lead to DMD since it leads to premature stop codons and nonsense mediated decay resulting in no protein production at all.

Question 21

what is a potential therapy for muscular dystrophy and how does it work?

Answer 21

• suppression of premature termination codons by aminoglycosides which are antibiotics that disrupt proper reading of mRNA in eukaryotes at high doses. This can lead to the misreading of a premature stop codon resulting in a full length protein.
• the difficulty is getting it to only misread the mutated premature stop codon and not any of the normal codons lol but as long as more of the normal length gene is being read than would’ve been read without the treatment, then it can be considered as a treatment (as seen in MDX mice)