Mutations

Question 1

What is molecular pathology ?

Answer 1

Why a mutation result in a clinical phenotype

Question 2

What are the types of mutations ?

Answer 2

• deletions
• insertions
• substitutions

Question 3

What are the different types of substitutions ?

Answer 3

• missense = when an amino acid is replaced with a different amino acid
• nonsense = amino acid codon is replaced with a stop codon

Question 4

What do deletion and insertion mutations cause ?

Answer 4

They can cause splice shifts which is where intron or exon splice sites are lost or created resulting in a frame shift mutation

Question 5

What are the effects of mutations on humans ?

Answer 5

They can effect transcription or translation

Question 6

What are the effects of the different types of mutations ?

Answer 6

• loss of function mutation
• gain of function mutation
• dominant negative mutation
• mutations that effect gene dosage

Question 7

Describe loss of function mutation

Answer 7

• when a gene product has reduced or none of its normal function
• any mutation that inactivates gene product will result in clinical symptoms
• Duchenne muscular dystrophy is caused by loss of function mutation

Question 8

Describe gain of function mutation

Answer 8

• gene product acquires a new abnormal function
• only the specific mutation that gives the product it’s new function will result in the clinical phenotype
• Huntington disease is caused by a gain of function mutation

Question 9

Describe Huntingtons disease

Answer 9

• caused by a gain of function mutation
• the mutation occurs as the CAG triplet is expanded within the coding gene sequence for the protein huntingtin
• causes the death of medium spiny neurones in the striatum
• affected alleles have 36-100 CAG repeats
• strikes in midlife (30-50 years)
• motor, emotional and cognitive symptoms
• death within 15-20 years
• the greater the number of CAG repeats the earlier the age of onset

Question 10

Describe dominant negative mutations

Answer 10

• when the mutant gene product not only loses its own function but also prevents other gene products from functioning correctly
• occurs where the mutation affects a multimeric protein encoded by more than 1 gene
• osteogenesis imperfecta is an example of a disease caused by a dominant negative mutation

Question 11

Briefly describe osteogenesis imperfecta

Answer 11

• caused by a mutation in either COL1A1 or COL1A2 gene encoding type I collagen
• very mild OI occurs when the mutated gene product is excluded from type I collagen
• lethal OI occurs when the mutated gene product is incorporated into type I collagen

Question 12

Describe mutations that effect gene dosage

Answer 12

• the mutation varies the level of gene product
• it is dependent upon gene and cell type
• Down syndrome is an example of a disease caused by mutations that effect gene dosage

Question 13

Describe the characteristic features of Down syndrome

Answer 13

• occurs due to an extra but normal chromosome 21
• mental retardation
• characteristic faces

Question 14

What is a locus ?

Answer 14

When individual genes or DNA have a chromosomal location that defines its position

Question 15

What is an allele ?

Answer 15

An individual copy of a gene that is present at a locus on a single chromosome
- humans are diploid and therefore have two alleles for each chromosomal locus one inherited from the mother and the other from the father

Question 16

What is a genotype ?

Answer 16

• combination of alleles that a person possesses at a locus
• homozygous = both alleles are the same
• heterozygous = alleles are different

Question 17

What is meant by hemizygous ?

Answer 17

Males have only one allele for X and one allele for Y chromosomes and therefore are hemizygous

Question 18

What is pedigree analysis ?

Answer 18

• representation of a family tree that uses standard symbols to depict biological characteristics
• it is used to understand how a particular mutation is inherited

Question 19

What is a kindred ?

Answer 19

Extended families covering many generations are called kindreds

Question 20

What is proband ?

Answer 20

The family member through which the family is first brought to the attention of clinicians is called the proband and may be marked with an arrow

Question 21

What are the mandelian inheritance patterns ?

Answer 21

• autosomal dominant
• autosomal recessive
• X-linked dominant
• X-linked recessive
• Y-linked

Question 22

What type of mutation causes DMD ?

Answer 22

Loss of function mutation

Question 23

Briefly describe Duchenne muscular dystrophy

Answer 23

• progressive muscular weakness
• affects mostly males
• no treatment
• X-linked recessive disorder

Question 24

What is Beckers muscular dystrophy ?

Answer 24

• much milder compared to DMD
• patients still walk at 60 years old

Question 25

What is X-inactivation ?

Answer 25

It is a dosage compensation mechanism that ensures only one X chromosome is active in each cell
- occurs in all female mammals

Question 26

Why do females with X:autosome translocation have DMD ?

Answer 26

• it would be expected that females with X:21 translocation would be carriers as DMD is recessive
• but X inactivation favours DMD

Question 27

Describe the DMD gene

Answer 27

• it is the largest human gene at ~ 2.5 million base pairs
• locus is Xp21
• contains mostly introns and 86 exons
• the gene product is dystrophin
• mutations in dystrophin cause Duchenne and Beckers

Question 28

Describe the mutations that cause DMD

Answer 28

• 60-65% of patients have deletions
• 5-15% of patients have duplications
• 20-35% of patients have small mutations, intron deletions or exon insertions of repetitive sequences
• frameshift mutations in dystrophin cause DMD
• frame neutral mutations cause BMD

Question 29

What is the role of dystrophin ?

Answer 29

dystrophin maintains the strength, flexibility and stability of muscle fibres

Question 30

What are the problems and solutions associated with gene therapy to combat DMD ?

Answer 30

Problems -
- very large mRNA
- muscle cells are post mitotic
- immune response

Solutions -
- use mini genes with crucial domains
- use adenoviral vectors

Question 31

What is happloinsufficiency ?

Answer 31

A situation in diploid organisms when one copy of a gene is inactivated or deleted and the remaining functional copy of the gene is not sufficient to preserve normal function

Question 32

What is a polymorphism ?

Answer 32

A DNA sequence variation that is common in the population