Molecular Basis of Disease Flashcards
(36 cards)
What is a pathogenic mutation?
-Cause of a disorder
Describe the anatomy of a gene
- Exons and Introns
- Promoter, initiator and stop codons
- Polyadenylation signal
What are the post transcriptional modifications to translation?
- Produces mature RNA
- Splicing (introns out of mRNA), maturation and polyadenylation
- Exported to cytoplasm to Golgi apparatus, translation from ATG initiator to TGA stop
What is the Pathogenic Mutation Criteria?
-Does it affect the function of the protein?
-Is it in a conserved region of the protein?
=more likely to affect function if changes in a region conserved across species (orthologs) or between members of a gene family (paralogs)
=Indicative of critical function
-Does it co-segregate with the disorder in the family?
=Is the gene change only found in affected members
-Is the change seen in the normal population?
=Has a sample of the normal population been screened
What are the types of mutation in DNA sequences?
-Deletions
=Ranges from 1bp to megabases
-Insertions
=Ranges vary can be as small as 1bp up to megabases
=Duplication and inversions
-Single base pair substitutions (point mutations)
-Frameshifts
=Caused by deletions, insertions or splice site errors
-Dynamic mutations
=Tandem repeats
=Triplet expansion
How can the different types of mutations in genomic DNA cause disruption in messenger RNA?
-Insertion and deletion disrupts codon sequence
=frameshifts
-Nonsense= abrupt cessation of protein translation
-Missense= may still make sense, sometimes pathogenic, 1 bp changed so may change amino acid
What are point mutations?
Can be classified according to their effect on the product of translation
- Synonymous
- Nonsynonymous
What is a synonymous mutation?
- Changes a codon into another that specifies the same amino acid as the original codon
- Due to redundancies within genetic code
What is a nonsynonymous mutation?
-Changes a codon into another that specifies a different amino acid to that of the original codon
What are the types of nonsynonymous mutations?
-Missense mutations =Replace one amino acid with another -Nonsense mutations =Replace an amino acid codon with a stop codon -Splice site mutations =Create or destroy splicing signals
Describe how missense mutations within the exon can be pathogenic
-Has it caused a change in amino acid? =Some redundancy in the genetic code =20 amino acids and 64 possible codons -If there is a change in amino acid, has it caused a conserved or non-conservative change in amino acid? =Change in polarity =Change in hydrophobicity
What is the Grantham Matrix?
- Method in calculating the significance of the amino acid substitution
- The bigger the score the more likely that the missense mutation has caused a change in the resultant protein structure
What is segregation analysis?
-Determine whether there are 5 or more affected members within that family that carry the gene variant under investigation
=98% certainty of the event being pathogenic
=Highly unusual in modern day living
=Same degree of certainty if two families where two affected first degree relatives involved
Describe splice site mutations
-4 nucleotides highly conserved across species (GUAG)
=Mutation within 4 nucleotides= abnormal splicing
-Nucleotides on either side of the group also conserved to a variant manner depending on gene and intron involved
=Disruptions of these cause alteration in splicing
What happens if there is a mutation in the splice donor site?
- Inclusion of intron in mRNA
- Fails to recognise start of intron so failure to splice out
- Can be quite large so protein translation affected
What happens if there is a mutation in the splice acceptor site?
- Exon skipping
- Fails to recognise end of intron so will splice all material
- Large exons= lots of sequencing codons= severely affected protein= pathogenic effect
- Small exons= small amount of sequencing codons= less affected
How do mutations cause disease?
- Loss of function (abolition) of gene product
2. Modification of gene product
Describe how mutations lead to loss of function
-Due to non-functioning or truncated protein
=Usually due to intragenic mutations due to disruption of messenger RNA (Marfan syndrome, Duchennes muscular dystrophy)
-Haploinsufficiency
=Usually refer to submicroscopic chromosomal deletions, whole genes deleted (William syndrome)
-Dominant negative
=Deafness syndromes, Collagen disorders
How do dominant and recessive disease manifest?
Threshold for dominant conditions higher than threshold for recessive conditions (above 50% vs less than 50%)
- Both genes contribute to gene product
- Dominant= one gene disrupted= only 50% of gene product so manifest when only one gene is disrupted
- Recessive= only when both copies lost there is no gene product= manifestation
Describe Dominant negative mutations
-Mutation produces a non-functioning protein
-Nom-functioning protein interferes with the protein of the normal functioning homologous gene
=Resulting in no effective gene product
=Pathogenic effect
Describe how mutations lead to modification of the gene product
-Creating a poorly functioning protein
=Becker’s muscular dystrophy (milder phenotype)
-Abnormal activation of protein (overexpression)
=Cancer genes
-Gain of function of protein (novel function)
=Huntington disease, cancer genes (philadelphilia chromosome-fusion protein)
Describe the structure of DNA
- On average, a sequence of 16 bases is unique in the human genome.
- DNA methylation occurs most commonly at CpG dinucleotides.
- DNA is arranged in the chromosome in association with histones.
What features does a gene have?
- A promotor region, this may contain a TATA box or CpG island.
- Exons, which contain the sequence transcribed to mRNA.
- Introns, which contain sequence that is removed by splicing after transcription.
- 5’ and 3’ untranslated regions.
- Start and stop codons.
What factors control the amount of protein product from a gene?
- Rate of Transcription. Amount of mRNA produced.
- Splicing – controlled by splice consensus sequences at intron/exon boundaries.
- Stability of mRNA.
- Stability of protein product made.
- Correct localisation of protein product.
- Correct post-translational modification of protein product.
