Mutational Mechanisms and Disease

Question 1

What are the four major mechanisms whereby genetic mutations lead to disease:

Answer 1

1) loss of function of the protein (most common)
2) gain of function of the protein
3) acquisition of a novel property by the mutant protein
4) perturbed expression of a gene at the wrong time (heterochronic expression) or in the wrong place (ectopic expression), or both.

Question 2

What are five examples of loss of protein function we have talked about this unit?

Answer 2

Turner syndrome, Duchenne Muscular Dystrophy, HNPP (Hereditary Neuropathy with Pressure Palsies), Osteogenesis Imperfecta Type 1, Alpha-thalassemia

Question 3

Duchenne Muscular Dystrophy genetic/biochemical basis. What is the inheritance?

Answer 3

DMD Xp21.2. Deletion, TOTAL loss of function.

[Large deletions of multiple exons, nonsens mutations, frameshift mutations] X-linked inheritance

Question 4

What is Becker Muscular Dystrophy? What distinguishes this from Duchenne’s?

Answer 4

Milder form of DMD, partial loss of function.

Question 5

Clinical manifestation of Duchenne’s?

Answer 5

• Boys with abnormal gait at 3-5 years
• Calf pseudohypertrophy
• Gower maneuver (YouTube)
• Progressive involvement of respiratory muscles
• Median age of death 18 years
• Women may ! cardiomyopathy

Question 6

What EXACTLY does HNPP stand for?

Answer 6

Hereditary Neuropathy with Liability to Pressure Palsies

Question 7

What is HNPP and what is its cause?

Answer 7

due to DELETION of PMP22 gene leading to a phenotype where patients have temporary (usually reversible) neuropathy when pressure is applied to various nerves. Just as your arm may go to sleep if left in a certain position, these patients are more sensitive to pressure on nerves and their limbs can ‘go to sleep’ for longer periods of time (hours, days, to months)

Question 8

Osteogenesis imperfecta type I:

Answer 8

Nonsense (stop) mutations / frameshift mutations in COL1A1 results in premature termination. Reduced amount of normal COL1A1 (collagen) protein causing a ‘milder’ form of osteogenesis imperfect. Clinically characterized by increased fractures, brittle bones, and blue sclera. Normal production of COL1A2 (ratio COL1A1/COL1A2 is 2:1, 2 copies of both genes normally present).

Question 9

4 Gain-of-Function Mutations we have talked about?

Answer 9

1) Hemoglobin Kempsey
2) Achondroplasia
3) Alzheimer disease in Trisomy 21
4) Charcot-Marie-Tooth

Question 10

Hemoglobin Kempsey

Answer 10

Beta hemoglobin gene, Asp99Asn missense mutation): leads to a hemoglobin molecule which has higher than normal oxygen affinity, and is less able to unload oxygen in the tissues. Polycythemia (overproduction of RBC to compensate).

Question 11

Achondroplasia

Answer 11

FGFR3 Gly380Arg mutation increases the normal signaling though intracellular tyrosine kinase domain (essentially having the receptor constitutively in the ‘turned-on’ state).

Question 12

Alzheimer disease in Trisomy 21

Answer 12

Patients with an extra copy of chromosome 21 have 3 total copies of the APP (21q21) leading to increased production of APP protein which contributes to early-onset Alzheimer disease in this patient population.

Question 13

Charcot-Marie-Tooth

Answer 13

due to DUPLICATION of PMP22 gene (HNPP is due to deletion of same)

Question 14

3 novel protein functions we have discussed?

Answer 14

Sickle cell anemia, Huntington disease, Osteogenesis imperfecta types (2, 3, 4–all more serious that type I)

Question 15

Sickle cell anemia

Answer 15

the Glu6Val mutation of the beta globin gene results in a hemoglobin molecule which transports oxygen essentially normally.
However under low oxygen states the Val residue leads to polymerization of hemoglobin into long protein-fibers which deform and restrict the normally flexible red blood cells

Question 16

Huntington disease:

Answer 16

A triplet repeat disorder where by expansion of CAG repeat ‘triplets’ in the gene increase the number of glutamine residues (the CAG codon codes for glutamine). Increased polyglutamine residues above a certain threshold leads to a novel toxic effect on the huntingtin protein

Question 17

2 examples of Ectopic or Heterochronic Expression Mutations (altered expression)

Answer 17

Cancer, hereditary persistence of fetal hemoglobin

Question 18

8 steps at which mutations can disrupt normal protein formation:

Answer 18

1. Transcription 2. Translation 3. Polypeptide folding 4. Post-transcriptional modification 5. Assembly of monomers into a holomeric protein 6. Subcellular localization of the polypeptide or the holomer 7. Cofactor or prosthetic group binding to the polypeptide 8. Function of a correctly folded, assembled, and localized protein produced in normal amounts

Question 19

Error in Step 1, transcription (2 examples)

Answer 19

• Thalassemias due to reduced or absent production of a globin mRNA because of deletions or mutations in regulatory or splice sites of a globin gene
• Hereditary persistence of fetal hemoglobin, which results from increased postnatal transcription of one or more γ-globin genes

Question 20

Error in Step 2 translation (1)

Answer 20

Thalassemias due to nonfunctional or rapidly degraded mRNAs with nonsense or frameshift mutations

Question 21

Error in Step 3 Polypeptide folding

Answer 21

More than 70 hemoglobinopathies are due to abnormal hemoglobins with amino acid substitutions or deletions that lead to unstable globins that are prematurely degraded, e.g., Hb Hammersmith

Question 22

Error in Step 4. Post-transcriptional modification

Answer 22

I-cell disease, a lysosomal storage disease that is due to a failure to add a phosphate group to mannose residues of lysosomal enzymes. The mannose 6- phosphate residues are required to target the enzymes to lysosomes.

Question 23

Error in step 5. Assembly of monomers into a holomeric protein

Answer 23

Types of osteogenesis imperfecta in which an amino acid substitution in a procollagen chain impairs the assembly of a normal collagen triple helix

Question 24

Error in Step 6 Subcellular localization of the polypeptide or the holomer

Answer 24

Familial hypercholesterolemia mutations (class 4), in the carboxyl terminus of the LDL receptor, that impair the localization of the receptor to clathrin- coated pits, preventing the internalization of the receptor and its subsequent recycling to the cell surface

Question 25

Error in Step 7 Cofactor or prosthetic group binding to the polypeptide

Answer 25

Types of homocystinuria due to poor or absent binding of the cofactor (pyridoxal phosphate) to the cystathionine synthase apoenzyme

Question 26

Error in Step 8 Function of a correctly folded, assembled, and localized protein produced in normal amounts

Answer 26

Diseases in which the mutant protein is normal in nearly every way, except that one of its critical biological activities is altered by an amino acid substitution; e.g., in Hb Kempsey, impaired subunit interaction locks hemoglobin into its high oxygen affinity state

Question 27

Fragile X Syndrome is caused by?

Answer 27

CGG repeats in 5’UTR which silence translation. Lack of functional protein. (>200 repeats)

Question 28

Fragile X Tremor/Ataxia Syndrome (FXTAS) caused by?

Answer 28

60-200 CGG repeats in 5’UTR. 2-5 fold increase in FMR1 mRNA lead to neuronal intranuclear inclusions

Question 29

Friedreich ataxia caused by?

Answer 29

GAA repeats in intron, impaired transcriptional elongation, loss of frataxin function. Increased Fe in mitochondria, reduced heme synthesis, reduced activity of Fe-S complex containing proteins.

Question 30

Huntington disease caused by?

Answer 30

CAG repeats in exon confer novel properties on protein, protein overinteracts with cofactors causing loss of fxn? Expansion is PATERNALLY INHERITED THANK YOU

Question 31

Myontonic dystrophy caused by?

Answer 31

Expanded CUG repeats bind increased amounts of mRNA binding proteins, cause impaired RNA splicing. Expansion is MATERNALLY INHERITED YAH WELCOME