Genetic Basis of Diseases Atif Hassan Khirelsied Ph.D. Atif H Khi l i d Ph D Department of Biochemistry D f Bi h i Faculty of MedicineInternational University of Africa, Khartoum, Sudan l f f h d
Learning objectives Learning objectives• Understand the common processes that lead to mutagenesis t i• Appreciate how different classes of mutations yield different effects on protein structure and function.
Learning objectives Learning objectives• Draw out example pedigrees representing 1. 1 autosomal dominant dominant, 2. autosomal recessive, 3. X‐linked dominant, 4. X linked recessive, holandric 4 X‐linked recessive holandric 5. and mitochondrial inheritance.
Learning objectives Learning objectives• 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 and Understand the molecular biology of cancer and describe features suggestive of inherited cancer suscept b ty susceptibility.
Genetic disorders Genetic disorders• Genetic disorders are illnesses caused by abnormalities in genetic sequences and the b liti i ti d th chromosome structures.
Genetic disorders Genetic disordersBurden• Although each genetic disorder may be rare, combined together genetic diseases are common. combined together genetic diseases are common• Can affect any body system and have a major impact on both morbidity and mortality.
Genetic disorders Genetic disordersImportance 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 counseling disease ‘carriers’ for genetic counseling .
MUTATION• Mutations are permanent inheritable changes in the amount or structure of genetic material. t t t f ti t i l• They can be inherited or occur spontaneously and can be subdivided into germline (gametes) or somatic mutations.
MUTATION• Mutation plays a key role in the pathogenesis of genetic disorder, by altering gene sequences and ti di d b lt i d their protein products. p p• Defective gene→ defective protein → altered function → disease
Mechanisms of mutation Mechanisms of mutation• At the single‐gene level mutations may result from: – substitution (point mutation) – deletion – insertion – inversion – triplet repeat expansion.
Substitution mutation Substitution mutation• Point mutations may arise as a result of: 1. Errors in DNA replication. 2. Defective repair of damaged DNA. 2 D f ti i fd d DNA 3. Spontaneous deamination of methylated 3 Spontaneous deamination of methylated cytosine to thymine (most common) .
Substitution mutation Substitution mutation• Substitutions are classified as:
Substitution mutation Substitution mutation• Point mutations may be silent or deleterious depending upon their type and site. d di th i t d it• Rarely, a mutation may be advantageous and favored by natural selection.
Deletion and insertion Deletion and insertion• Deletion is loss of DNA involving from one to many thousands of base pairs. th d fb i• Sequences at the ends of deletions are often similar, predisposing to recombination errors.
Deletion and insertion Deletion and insertion• 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
Deletion and insertion Deletion and insertionThe 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 and insertion Deletion and insertion• Deletion, Alport’s syndrome, a hereditary disease of basement membranes, characterized by b t b h t i db sensorineural deafness and renal failure. • Duplication, Duchenne muscular dystrophy (DMD).
Inversion• Inversions may involve anything from two to many thousands of base pairs. • They occur in areas of sequence homology They occur in areas of sequence homology (sequences at each end of the inverted segment o e ese b e eac o e ) often resemble each other).• 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 gene within the factor VIII gene
Triplet repeat expansions Triplet repeat expansions• T i l t t i l tid repeat expansions are Triplet or trinucleotide t i 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 recessive manner, or be X‐ linked. linked
Triplet repeat expansions Triplet repeat expansions• EXAMPLE• 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 Normally there are 8 to 30 copies of this trinucleotide, ll h 8 30 i f hi i l id 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 mutation on 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 of demonstrated to exert an effect on the control of differential splicing.
Structural effects of mutation on protein• Missense mutations• A base change alters a codon, incorporation of a different amino acid into the protein. amino 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 protein structure of the protein• It l d It also depend on whether the two amino acids involved d h th th t i id i l d are from the same or different groups (i.e. hydrophobic or hydrophilic). hydrophilic)
Structural effects of mutation on proteinExammpleE l• In sickle‐cell disease, the substitution of A by T at the i kl ll di h b f b h 17th nucleotide of the β‐globin gene changes the codon from 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
Structural effects of mutation on proteinNonsense mutations• Nonsense mutations are point mutations that lead to the conversion of a codon to a stop codon 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.
Structural effects of mutation on protein• Frameshift mutations• Insertions and deletions of nucleotides, if not a i dd l i f l id if multiple of three, lead to ‘frameshift’ mutations,• The open reading frame of the gene and the corresponding amino‐acid sequence is altered, di i id i l d• Leading to complete inactivation of the gene.
Functional effects of mutation on protein• With the exception of imprinted genes, genes on both the maternal and paternal chromosomes are expressed. d• If either the maternal or the paternal gene contains a mutation, the cell will express two different protein products.
Functional effects of mutation on protein• Mutations exert their phenotypic effects by one of two mechanisms: 1. loss of function 2. or gain of function.
Functional effects of mutation on protein• Loss of function mutations – Amorphic mutation also known as ‘null mutations , are associated with a complete mutations’, are associated with a complete absence of gene product function.
Functional effects of mutation on protein• L Loss of function mutations ff i i – 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.
Functional effects of mutation on proteinHaploinsufficiency• The majority of heterozygous states are haplosufficient; h j i fh h l ffi i that is one functional copy of a gene is adequate for the manifestation of a wild type phenotype. manifestation of a wild type phenotype• The term haploinsufficiency is a situation whereby a reduction of 50% of gene function results in an abnormal phenotype. h t
Functional effects of mutation on protein• Gain of function mutations Gain of function mutations• These mutations result in either: h l h – 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 o gene expression of the gene product (neomorphic).
Functional effects of mutation on protein• Gain of function mutations Gain of function mutations• Trinucleotide repeat expansions represent gain of l d f function mutations.• Usually a toxic gain of protein function, which predisposes to protein misfolding and protein aggregation and leads to neurodegeneration.
Functional effects of mutation on protein• Dominant negative mutations• Dominant negative mutations are also known as antimorphic mutations. • They arise when the null allele product of a They arise when the null allele product of a heterozygote adversely affects the normal gene product, for example by dimerizing with and product, for example by dimerizing with and inactivating it.
Functional effects of mutation on protein• 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 in protein complex, as seen with fibrillin in Marfan syndrome.