Genetic basis of diseases (!)

Genetic Basis of Diseases

Atif H Khi l i d Ph DAtif Hassan Khirelsied Ph.D.

D f Bi h iDepartment of Biochemistry

Faculty of Medicine

l f f h dInternational University of Africa, Khartoum, Sudan

Learning objectivesLearning objectives

• Understand the common processes that lead to

t imutagenesis

• Appreciate how different classes of mutations yield

different effects on protein structure and function.


• Draw out example pedigrees representing

1 autosomal dominant1. autosomal dominant,

2. autosomal recessive,

3. X‐linked dominant,

4 X‐linked recessive holandric4. X linked recessive, holandric

5. and mitochondrial inheritance.


• Appreciate the concept of heritability in the context of complex diseases.

• Identify the common chromosomal disorders and define aneuploidy, triploidy, trisomy and monosomy, with examples of diseases.

• Understand the molecular biology of cancer andUnderstand the molecular biology of cancer and describe features suggestive of inherited cancer susceptibility.suscept b ty

Genetic disordersGenetic disorders

• Genetic disorders are illnesses caused by

b liti i ti d thabnormalities in genetic sequences and the

chromosome structures.


Burden

• Although each genetic disorder may be rare,

combined together genetic diseases are commoncombined together genetic diseases are common.

• Can affect any body system and have a major impact

on both morbidity and mortality.


Importance of medical genetics

• An understanding of genetics is important, not only

for the diagnosis and management of such

disorders, but also for the identification of genetic

disease ‘carriers’ for genetic counselingdisease carriers for genetic counseling .

MUTATIONMUTATION

• Mutations are permanent inheritable changes in the

t t t f ti t i lamount or structure of genetic material.

• They can be inherited or occur spontaneously and

can be subdivided into germline (gametes) or

somaticmutations.

MUTATIONMUTATION

• Mutation plays a key role in the pathogenesis of

ti di d b lt i dgenetic disorder, by altering gene sequences and

their protein products.p p

• Defective gene→ defective protein → altered

function→ disease

Mechanisms of mutationMechanisms of mutation

• At the single‐gene level mutations may result from:

– substitution (point mutation)

deletion– deletion

– insertioninsertion

– inversion

– triplet repeat expansion.

Substitution mutationSubstitution mutation

• Point mutations may arise as a result of:

1. Errors in DNA replication.

2 D f ti i f d d DNA2. Defective repair of damaged DNA.

3 Spontaneous deamination of methylated3. Spontaneous deamination of methylated

cytosine to thymine (most common) .


• Substitutions are classified as:


• Point mutations may be silent or deleterious

d di th i t d itdepending upon their type and site.

• Rarely, a mutation may be advantageous and favored

by natural selection.

Mutations

Deletion and insertionDeletion and insertion

• Deletion is loss of DNA involving from one to many

th d f b ithousands of base pairs.

• Sequences at the ends of deletions are often similar,

predisposing to recombination errors.


• Insertion is a gain of DNA.

• Duplication a type of insertion occurs when runs of• Duplication, a type of insertion, occurs when runs of

bases and repeated motifs predispose to duplication

by replication slippage


The effects on the protein of deletion and insertion

depend on:

The amount of material lost– The amount of material lost

– Whether the reading frame is affected.Whether the reading frame is affected.

Deletion

Duplication


• Deletion, Alport’s syndrome, a hereditary disease of

b t b h t i d bbasement membranes, characterized by

sensorineural deafness and renal failure.

• Duplication, Duchenne muscular dystrophy (DMD).

InversionInversion

• Inversions may involve anything from two to many thousands of base pairs.

• They occur in areas of sequence homologyThey occur in areas of sequence homology(sequences at each end of the inverted segment often resemble each other).o e ese b e eac o e )

• In haemophilia A 40% of mutations result from an• In haemophilia A, 40% of mutations result from an inversion of several hundred thousand base pairs within the factor VIII genewithin the factor VIII gene

Triplet repeat expansionsTriplet repeat expansions

T i l t t i l tid t i• Triplet or trinucleotide repeat expansions are typically involving CG‐rich trinucleotides (CGG, CCG, CAG CTG)CAG,CTG).

• Triplet expansion results in a defective gene product, yielding disease.

• These expansions may be inherited in an• These expansions may be inherited in an autosomally dominant or recessivemanner, or be X‐linkedlinked.

Triplet repeat expansionsTriplet repeat expansions• EXAMPLEEXAMPLE

• Friedreich’s ataxia results from an expansion of the (GAA) within the first intron of the FXN gene(GAA) within the first intron of the FXN gene.

N ll h 8 30 i f hi i l id• Normally there are 8 to 30 copies of this trinucleotide, patients may have as many as 1000.

• This expansion is intronic and is thought to make the DNA ‘sticky’, interfering with the process of transcription.

Structural effects of mutationon protein

• Silent mutations

• Silent mutations, point mutations, have no effect on the aminoacid sequence of a protein.

• Considered to be ‘evolutionary neutral’, but recently demonstrated to exert an effect on the control ofdemonstrated to exert an effect on the control of differential splicing.


• Missense mutationsMissense mutations• A base change alters a codon, incorporation of a different

amino acid into the proteinamino acid into the protein.

• The effect of the mutation on protein function depends• The effect of the mutation on protein function depends upon its location relevant to the tertiary or quaternary structure of the proteinstructure of the protein

It l d d h th th t i id i l d• It also depend on whether the two amino acids involved are from the same or different groups (i.e. hydrophobic or hydrophilic)hydrophilic).


E lExammple

i kl ll di h b f b h• In sickle‐cell disease, the substitution of A by T at the 17th nucleotide of the β‐globin gene changes the codonfrom GAG to GTG (Glu to val)from GAG to GTG (Glu to val).

• This mutation changes the solubility and molecular• This mutation changes the solubility and molecular stability of the protein.

• Haemoglobin forms polymers under conditions of low oxygen tension, leading to sickling of red blood cells.yg , g g


Nonsense mutations

• Nonsense mutations are point mutations that lead to the conversion of a codon to a stop codon (UAG,the conversion of a codon to a stop codon (UAG,

• UAA, UGA).

• They lead to a truncated protein, with those that occur early in a gene sequence having a higher probability of completely inactivating a gene.


• Frameshift mutations• Frameshift mutations

i d d l i f l id if• Insertions and deletions of nucleotides, if not a multiple of three, lead to ‘frameshift’ mutations,

• The open reading frame of the gene and the di i id i l dcorresponding amino‐acid sequence is altered,

• Leading to complete inactivation of the gene.

Functional effects of mutationon protein

• With the exception of imprinted genes, genes on both the maternal and paternal chromosomes are

dexpressed.

• If either the maternal or the paternal gene contains a mutation, the cell will express two different protein products.


• Mutations exert their phenotypic effects by one of two mechanisms:

1. loss of function

2. or gain of function.


• Loss of function mutations

– Amorphic mutation also known as ‘null mutations’, are associated with a completemutations , are associated with a complete absence of gene product function.


L f f i i• Loss of function mutations

– Hypomorphic mutation, also known as ‘leaky mutations’, lead to a partial loss of function.

– They usually result from: 1. an altered amino acid that makes the polypeptide

less active. 2. a reduction in transcription that results in less

normal transcript.


HaploinsufficiencyHaploinsufficiency

h j i f h h l ffi i• The majority of heterozygous states are haplosufficient; that is one functional copy of a gene is adequate for the manifestation of a wild type phenotypemanifestation of a wild type phenotype.

• The term haploinsufficiency is a situation whereby a reduction of 50% of gene function results in an abnormal h tphenotype.


• Gain of function mutations• Gain of function mutations

h l h• These mutations result in either:

– increased activity of the gene product (hypermorphic)

– Or the gain of a novel function or a novel pattern of O e ga o a o e u c o o a o e pa e ogene expression of the gene product (neomorphic).


• Gain of function mutations• Gain of function mutations

l d f• Trinucleotide repeat expansions represent gain of function mutations.

• Usually a toxic gain of protein function, which predisposes to protein misfolding and protein aggregation and leads to neurodegeneration.


• Dominant negative mutations

• Dominant negative mutations are also known as antimorphicmutations.

• They arise when the null allele product of aThey arise when the null allele product of a heterozygote adversely affects the normal gene product, for example by dimerizing with andproduct, for example by dimerizing with and inactivating it.


• Dominant negative mutations

• The classical example is that of an amino‐acid change

that prevents a polypeptide from functioning in a

multimeric protein complex as seen with fibrillin inmultimeric protein complex, as seen with fibrillin in

Marfan syndrome.

Education

Genetic basis of diseases (!)