genetics

GENETICS

Lec. 2

Mutations

defined as a permanent change in the DNA
Origin
germ cells – transmitted to progeny
somatic cells – cancer and some congenital malformations
Types of mutation
Chromosome mutation – structural changes within the chromosome – translocations, deletions, etc
Genome mutation – loss or gain of whole chromosomes: monosomy and trisomy
gene mutation – alterations at the level of the gene

Gene mutation could be1.Point mutation

result from substitution of a single base in the DNA
Coding portion of gene
Missense– result in substitution of one amino acid for another in the coded protein
conservative – function of protein is not affected
nonconservative – function of protein altered
Nonsense
stop codon
Noncoding portion of gene
promoter and enhancer regions
posttranslational processing


• 2.Frame-shift mutation
• Which results from insertion or deletion of one or 2 base pairs that alters the reading frame of the DNA strand.
• A change in 3 or more base pairs leads to missing of one or more amino acids.
• 3.Trinucleotide repeat mutation.
• Such type of mutation leads to amplification of a sequence of 3 nucleotides as in fragile X chromosome syndrome in which there is a 250 or more repeat of CGG. (normally up to 29).
• 4. Mutations within noncoding sequences
• Deleterious effects may also result from mutations that do not involve the exons. Point mutations or deletions involving these regulatory sequences may lead to total lack of transcription.

Genetic disorders are of 4 main types

• 1. Single-gene mutation with large effects. Most of these disorders follow the mendelian role, so called mendelian disorders.
• 2. Chromosomal disorders.
• Multifactorial also called polygenic. This group is applied to most human diseases like DM & HT.
• Miscellaneous disorders (disorders with non-classical single gene mutation)

Phenotype means the clinical expression.Could be physiological (height, color) or pathological (DM, HT).

Pleiotropism is a single defect that may lead to many phenotypic effects as in sickle cell anemia.

Genetic heterogeneity means mutations at several gene-loci that lead to the same clinical disorder as in Phenylketonuria.

Polymorphism, when a DNA change occurs in at least 1% of the population.

Features of Autosomal Dominant Disorders:

male

female

50% probability

Affected Patient
(heterozygous)

Manifested in the heterozygous state.

At least one parent is usually affected.

Males & females are affected & both can transmit the disorder.

When one parent is affected, half of their children have the chance of getting the condition.

New mutation plays an important role.

In some cases the condition appears as a new mutation (i.e. both parents are not affected). Many such mutations occur in the germ cells of older fathers.


• The onset may be delayed in some cases like in Huntington disease.
• Some persons inherit the disorder but are phenotypically normal this phenomenon is called reduced penetrance.
• In others the mutant gene is expressed differently among individuals this is called variable expressivity.
• Non-enzyme proteins are affected usually:
• Proteins involved in regulation of complex metabolic pathways.
• Key structural proteins such as collagen & spectrin are commonly affected.

E.g.

Neurofibromatosis,
Huntington disease,
adult polycystic kidney disease,
familial polyposis coli,
hereditary spherocytosis,
von Willebrand disease,
Marfan disease,
osteogenesis imperfecta,
Achondroplasia,
familial hypercholesterolemia.. etc

Autosomal Recessive Disorders:

male


female

25% diseased, 50% carriers

Autosomal Recessive
Disease occurs only when both alleles at given gene locus are present
Parents are usually normal
If the mutant gene is rare the proband may be the product of a consanguineous marriage or an example of the founder effect
Nearly all inborn errors of metabolism are recessive

Features of Autosomal Recessive Disorders

Expression of the disorder more uniform than with dominant diseases
Complete penetrance is common
Onset is frequently early in life
New mutations may occur but are rarely detected

X-Linked Disorders

Absence of father-son transmission
All daughters of affected male are obligate carriers
Dominant and recessive apply only to the female – males are hemizygous
Nearly all X-linked disorders are recessive
The only X-linked dominant disorders are vitamin D-resistant rickets and Alport's syndrome (Hereditary glomerulonephritis with nerve deafness)


X-linked disorders
X

X

X

Y

X

X

X

Y
Y
X

X


X

X

= affected X

Features of X-linked disorders
• Females are carriers & males are affected.
• The carrier females transmit the disorder only to their sons which have the chance of 50% to be affected.
• The affected male cannot transmit the disease to sons, but all the daughters are carriers.

Heterozygous females rarely express the disease when there is

Inactivation of the other X-chromosome.



Father is affected & mother is a carrier.






Female has one affected X-chromosome as in Turner syndrome.





E.g. of X-linked disorders; Duchenne muscular dystrophy, hemophilias, G6PD, fragile X syndrome.

Mechanisms of Single Gene Disorders

• Enzyme defects and their consequences
• Defects in membrane receptors and transport systems
• Alterations in structure, function, or quantity of nonenenzyme proteins
• Mutations resulting in unusual reactions to drugs

Enzyme Defects and Their Consequences

Accumulation of the substrate
Metabolic block and decreased amount of the product (± lack of feedback inhibition)
Failure to inactivate a tissue damaging substance
α1- antitrypsin deficiency
neutrophil elastase inactivation is deficient
unchecked activity of this protease leads to lung damage and liver damage





Downloaded from: Robbins & Cotran Pathologic Basis of Disease (on 18 July 2005 09:03 PM)
© 2005 Elsevier
Lesch-Nyhan Syndrome
Galactosemia: galactose-1-phosphate uridyltransferase
Albinism: tyrosinase

increased concentration of intermediate 2 may stimulate minor pathway result in excess of M1 & M2
tissue injury may result if the precursor, the intermediates or the products of alternative minor pathway are toxic in high concentration
example; galactosemia

enzyme defect causes metabolic block and decrease amount of end product (albinism)

if (P) is feedback inhibitor of involved enzyme, deficiency of (P) causes over-production of intermediates and their catabolic products, which may be injurious in high concentration (Lesch- Nyhan syndrome- Hyperuricemia).
failure to inactivate tissue-damaging substrate, (alpha 1 Antitrypsin deficiency)

Diseases caused by mutations in non-enzymatic structural proteins

Marfan syndrome:

Autosomal dominant disorder of connective tissue that affects fibrillin 1 (a glycoprotein that is secreted by fibroblast) encoded by FBN1 gene (mapped on chr.15).


Prevalence, 3 per 10 000 & 75% are familial.

Fibrillin 1 is a scaffold for deposition of elastin. Its mutation leads to multiple systemic effects.

The main 3 systems affected are the skeleton, eye & cardiovascular system.

Morphology of Marfan Syndrome
Skeletal abnormalities include long legs, arms & fingers, hyperextensibility of joints, depressed sternum

Ocular changes are bilateral subluxation of the lens

CVS abnormalities are aortic aneurysm, aortic incompetence, floppy mitral valve & heart failure

Marfan Pheotype

Other examples

Sickle cell anemia,
Thalassemia and
Hereditary spherocytosis


Diseases caused by mutations in receptor protein
Familial Hypercholestoleremilia
Possibly the most frequent Mendelian disorder, with a gene frequency of 1:500
Results from a mutation of the gene encoding the low density lipoprotein (LDL) receptor
Heterozygotes
2-3x elevation of serum cholesterol
tendon xanthomas and premature atherosclerosis in early adulthood
Homozygotes
5-6x elevation of serum cholesterol
tendon xanthomas and premature atherosclerosis develop earlier
may have myocardial infarction by age 20 years

Defects in proteins that regulate cell growth

Mutations in proto-oncogenes and tumor-suppressor genes causing neoplasia

Cancer-causing mutations usually affect somatic cells, only 5% of cancers are familial, mostly autosomal dominant

Mutations resulting in unusual reactions to drugs

Glucose -6-phoshpate dehydrogenase (G6PD)
G6PD activity is necessary to protect the red blood cell from oxidative stress
drugs that block G6PD (e.g. primaquine) can cause severe hemolysis in patients who lack this enzyme
Cytochrome P450 enzymes
used by the liver to metabolize many drugs
changes in CYP enzyme levels affect drug metabolism