Class 11A: Single gene disorders: defects in structural proteins

Question 1

Define a mutation.

Answer 1

Mutation - a hereditary change

-process by which genes change from one allelic form to another

Question 2

What are the 2 different levels of mutations?

Answer 2

GENE MUTATIONS –> point mutations (ex) –> typically map to one chromosomal locus –> changes an allele

CHROMOSOME MUTATIONS –> segments, whole or entire sets of chromosomes change –> can involve gene mutations as a result

Question 3

What is the reference point for mutations?

Answer 3

the wildtype allele

Question 4

What are mutations away from the wildtype called?

Answer 4

forward mutations

Question 5

How can mutations be classified?

Answer 5

Mutations can be classified:

1. Somatic versus germinal mutations
2. Dominant versus recessive mutations
3. Loss of function versus gain of function mutations
   - -> dominant-negative mutations

Question 6

Compare a dominant versus recessive mutation.

Answer 6

dominant mutation - only one of the 2 alleles of the gene needs to be mutant in order for the mutant phenotype to manifest
= heterozygous for mutation = mutant phenotype

recessive mutation - both alleles need to be mutant alleles in order fro the phenotype to manifest
= homozygous for the mutation = mutant phenotype

Question 7

What is a loss of function mutation?

Answer 7

• the mutant allele leads to decreased or no function of the product generated –> ex: null allele
• most often are recessive mutations

-BUT if amount of normal protein synthesized from the normal
(unmutated) allele is not sufficient for normal phenotype, then NOT
recessive = dominant
= haploinsufficiency
= heterozygote shows phenotype

Question 8

What is a gain of function mutation?

Answer 8

• mutant allele changes the gene product such that it now has a new and abnormal function in the cell
• usually are dominant mutations

-a dominant negative mutation results in a gene product that
antagonizes the normal gene product

Question 9

What are single gene disorders? What are some examples?

Answer 9

Caused by mutation of a specific gene in the affected individuals

Examples:

• Cystic fibrosis
• Sickle cell anemia
• Duchenne muscular dystrophy

Question 10

Single gene disorders: ECM proteins

Defects in structural proteins = proteins of the _____

Answer 10

ECM

collagen, fibrillin, elastin

Question 11

Defects in structural proteins

________ make lots of extracellular matrix

Answer 11

Connective tissue cells make lots of extracellular matrix

• carry the mechanical load of these tissues
• composed of fibrous proteins
• collagen is the main member of this family

Question 12

Connective tissue cells go by various names.

_____ in skin

_____ in bone

Answer 12

fibroblasts in skin

osteoblasts in bone

Question 13

How are ECM proteins synthesized?

Answer 13

All ECM proteins are synthesized intracellularly (precursor form),

secreted by exocytosis and are then processed and assembled into large
aggregates (mature form) in the extracellular space

Question 14

Single gene disorders: Collagen

Defects in structural proteins: Collagen

Mammals have ___ diff collagen genes

Chief protein in?

These diseases affect what?

Answer 14

Mammals have 20 different collagen genes
-chief protein in bone, skin & tendons
-~ 25% of our total protein mass
-mutations in any of the many genes that encode the different types of
collagens lead to severe genetic diseases

These diseases affect connective tissues!

Question 15

Single gene disorders: Collagen

Describe the structure of collagen.

Answer 15

Three coiled subunits (two α1 (I) chains and one α2(I) chain= heterotrimer)

• form right-handed triple helix
• unusual abundance of amino acids glycine and proline in repeating units Gly-Pro-X
• Gly present at every 3rd position = crucial for helix formation
• collagen molecules formed can make side-by-side interactions to
  form fibrils

Question 16

The three coiled subunits in ______ form what is referred to as a _____________.

Answer 16

The three coiled subunits in collagen form what is referred to as a right-handed triple helix.

Question 17

Single gene disorders: Collagen

What is crucial to normal collagen fibril formation?

Answer 17

POST-TRANSLATIONAL MODIFICATIONS are crucial to normal collagen fibril
formation.

Ex: lack of proline hydroxylation results in unstable triple helices and
lack of collagen fibrils –> defect in prolyl hydroxylase

-Scurvy caused by lack of ascorbic acid (Vit C) –> necessary cofactor for prolyl hydroxylases

Question 18

Defects in structural proteins: Type I Collagen

Mutations in the α1 or α2 genes encoding Type I collagen leads to a
brittle-bone disease known as?

Answer 18

osteogenesis imperfecta (OI)

Question 19

What is osteogenesis imperfecta (OI)?

Answer 19

Mutations in the α1 or α2 genes encoding Type I collagen leads to a
brittle-bone disease

Question 20

What are two important genes when discussing OI?

Answer 20

COL1A1 or COL1A2

Question 21

What can cause the disease OI?

Answer 21

single amino acid change can cause disease

• recall that every 3rd amino acid must be glycine!

Question 22

How is OI mostly inherited?

Answer 22

Mostly autosomal dominant inheritance

• more than 1500 mutations in COL1A1 & COL1A2 identified!
• affect approx. 1 in every 20,000 births

Question 23

Single gene disorders: OI

What are some features of OI?

Answer 23

Clinically heterogeneous connective tissue disorder
-can affect structure, quantity, modification or assembly into ECM

-low bone mass
-reduced bone material strength
= bone fragility
- fractures, growth deficiency, bone deformities

-range in severity from lethal to subclinical

Question 24

Single gene disorders: OI

What are the OI types? (classification)

Answer 24

Most often deal with the OI classifications I through IV.

OI type I is a haploinsufficiency situation due to a null COL1A1 allele

• premature termination codon –> nonsense mediated decay
• autosomal dominant

Classification evolving. Clinical phenotype and type of mutation taken
into consideration.

Question 25

What is OI type I?

Answer 25

OI type I is a haploinsufficiency situation due to a null COL1A1 allele

• premature termination codon –> nonsense mediated decay
• autosomal dominant

Question 26

What is the overall result of OI?

Answer 26

Overall result is bone fragility and deformity!

-changes in the bone matrix (ECM) results in changes in the
presence of growth factors and cytokines that influence bone cell
proliferation and mineralization of bone

Question 27

What is recessive OI caused by?

Answer 27

Caused by mutations in genes that encode proteins that interact with collagen Type I

-for example: prolyl hydroxylases; factors involved in helical folding
(chaperones)

2-5% of OI cases in N. America and Europe

Phenotypes are very varied and can be difficult to classify/counsel/treat

Question 28

What are some possible treatments for OI?

Answer 28

-Rehab and physical therapy

-Orthopedic surgery =
“rodding”

• Bisphosphonate drugs
• pamidronate is used commonly
• recent issues with dosage and long-term effects
• compound has very long half life in bone

Question 29

What is Marfan’s?

Defects in structural proteins: fibrillin

Answer 29

• Large 350KDa protein with repeating calcium-binding EGF motifs
• motifs known to bind to TGF-β

-Fibrillin monomers form polymers with other proteins like elastin –>
together strengthen tissues (ex: aortic wall)

-Mutations in FBN1 gene à structurally inferior connective tissue

Question 30

Defects in structural proteins: Marfan’s syndrome

Answer 30

• defects in the skeletal, cardiovascular and ocular systems
• incidence 2-3 in 10,000 births
• 25% of cases are caused by de novo mutations!
• mutations in FBN1 gene most common

high incidence of aortic aneurysms

• weakness of the aortic wall
• risk of rupture especially if pressure is high
• serious hemorrhage –> fatal

Question 31

Marfan’s syndrome: molecular defects

Answer 31

-Fibrillin binds to TGF-β and maintains it in an inactive state in ECM

-lack of or decrease in fibrillin in connective tissue = excess of active
TGF-β

-inappropriate cell signaling cascades are activated

Question 32

Single gene disorders: Marfan’s

What are some of the syndrome’s treatment?

Answer 32

-Drugs that bind and inactivate TGF-β seem to improve the outcome of
Marfan’s patients

-Drugs that block the beta-adrenergic receptors (β-blockers) to delay
or prevent aortic aneurysms à slow aortic growth

• E.g. Losartan –FDA approved in 1995 = angiotensin receptor blocker
• blocking angiotensin, dilates blood vessels and reduces blood pressure

Question 33

Defects in structural proteins: collagen type III

Answer 33

• Encoded by COL3A1 gene
• mutations result in either lack of/severe decrease in collagen or abnormal collagen fibrils

-Haploinsufficiency versus abnormal fibrils

-In patients heterozygous for a mutation in COL3A1, only 1/8th of the collagen fibrils produced will be normal!
=homotrimer

Question 34

Defects in structural proteins: collagen type V

Answer 34

• Encoded by COL5A1 and COL5A2 genes
• Type V often assembles with Type I
• thought to initiate fibril assembly
• important structural role

Question 35

Collagen type III & V defects: Ehlers-Danlos syndrome

Answer 35

-Mutations in the genes encoding type V collagens lead to Ehlers-Danlos
syndrome
= mutations in COL5A1 and COL5A2 genes –> cause “classical” forms
-laxity and fragility of skin; hypermobility of joints; easy bruising
-1 in 10,000 people affected

• Very severe Ehlers Danlos = mutations in type III collagen = COL3A1
• referred to as the “vascular” type –> vascular and bowel rupture –> fatal
• 1 in 100,000 people

Question 36

What is EDS?

Answer 36

Collagen type III & V defects: Ehlers-Danlos syndrome

Question 37

What are EDS features?

Answer 37

Ehlers-Danlos syndrome

• loose joints
• elastic skin
• fragile skin

Question 38

What are EDS treatment options?

Answer 38

• Treatment depends on the symptoms that manifest in each patient
• anti-inflammatory drugs are often used
• physical therapy
• Careful monitoring for aneurysms is critical