Gene mutation and molecular medicine Chapter 15

Transcription

1 Gene mutation and molecular medicine Chapter 15 Lecture Objectives What Are Mutations? How Are DNA Molecules and Mutations Analyzed? How Do Defective Proteins Lead to Diseases? What DNA Changes Lead to Genetic Diseases? How Is Genetic Screening Used to Detect Diseases? How Are Genetic Diseases Treated? What are Mutations? Changes in DNA nucleotide sequence My or may not have phenotypic consequences SOMATIC MUTATIONS GERM LINE MUTATIONS Phenotypic Effects SILENT MUTATIONS LOSS OF FUNCTION MUTATIONS GAIN OF FUNCTION MUTATIONS CONDITIONAL MUTATIONS MOLECULAR LEVEL MUTATIONS POINT MUTATIONS

2 CHROMOSOMAL MUTATIONS PHENOTYPIC EFFECTS POINT MUTATIONS Missense mutations Are substitutions by one amino acid for another in a protein. Example: Sickle-cell disease allele differs from normal by one base pair. Nonsense mutations Involve a base substitution that causes a stop codon to form somewhere in the mrna. This results in a shortened protein, which is usually not functional if near the 3' end may have no effect. Frame-shift mutations Are insertions or deletions of bases in DNA. These mutations interfere with translation and shift the reading-frame. Nonfunctional proteins are produced. Chromosomal mutations Deletions have severe consequences unless affect noncoding DNA, unnecessary genes, or are masked by normal alleles Duplications homologous chromosomes break in different places and recombine with wrong partners; one may have two copies of segment CHROMOSOMAL MUTATIONS CHROMOSOMAL MUTATIONS Inversions result from breaking and rejoining, but segment is flipped Translocations segment of DNA breaks off and is inserted into another chromosome, can cause duplications and deletions. May result in sterility if chromosome pairing is impossible and meiosis is interrupted

3 CHROMOSOMAL MUTATIONS How do Mutations Happen? Spontaneous mutations occur with no outside influence, and are permanent Induced mutations are due to an outside agent, a mutagen Spontaneous Mutation Induced Mutations Mutations Errors in DNA polymerase Non-disjunction during meiosis Chemicals (deamination or adding a group to guanine, like benzopyrene) Radiation X-rays create free radicals UV creates thymine dimers Artificial or Natural mutations HOT SPOTS FOR MUTATIONS BENEFITS AND COSTS OF MUTATIONS Genetic diversity Benefit for own or offspring Germ line can me lethal Public health (ozone layer being destroyed; second hand smoking, etc) MOLECULAR GENETICS Study of DNA changes and possible effects on proteins Use of restriction enzymes

4 DNA fingerprinting For identification uses restriction digestion and gel electrophoresis. It works best with highly polymorphic sequences having multiple alleles that are likely to differ between individuals. POLYMORPHISMS Single nucleotide polymorphisms (SNPs): Inherited variations involving a single base point mutations Short tandem repeats (STRs): Short repetitive sequences occurring side by side on chromosomes, usually in noncoding regions DNA FINGERPRINTING DNA FINGERPRINTING IN THE ROYAL RUSSIAN FAMILY THE BAR CODE PROJECT Evolution research Species diversity issues Identification of new species Identification of undesirable microbes or bioterrorism agents DNA BAR CODE Genetic mutations may make proteins dysfunctional Phenylketonuria (PKU): abnormal enzyme phenylalanine hydroxylase normally catalyzes conversion of dietary phenylalanine to tyrosine. Has tryptophan instead of arginine in position 408. It is inherited in a mosaic type

5 PHENYLKETONURIA GENETIC DISORDERS Sickle cell anemia: β-chain replacement of glutamic acid for valine Hemoglobin Polymorphisms PRIONS Prions, or proteinaceous infective particles (not DNA or RNA) Transmissible spongiform encephalopathies (TSEs) are degenerative brain diseases the brain develops holes TSEs are a rare case of a mutant phenotype without a mutant gene. Normal brain cell membranes contain the protein PrP c. TSE-affected brains have a protein with the same amino acid sequence, but a different 3-D shape called PrP sc. PrP sc is insoluble and accumulates as fibers, causing cell death PrP vs PrPsc REVERSE GENETICS Previously: As in sickle-cell anemia: Clinical phenotype protein phenotype gene Now: as in Cystic Fibrosis Clinical phenotype gene protein phenotype GENETIC MARKERS Reference points for gene isolation Knowledge of two mutations is needed: the disease phenotype and the marker mutation Genetic linkage Identifying DNA sequences of genetic markers RE to identify STRs Restriction Fragment Polymorphisms (RFLPs).

6 DNA GEL BLOTTING Single nucleotide polymorphisms (SNPs) are widespread one for every 1,330 base pairs in the human genome. SNPs can be detected by sequence comparisons, PCR amplification, or by mass spectrometry. SNPS STRs, and RFLPs are used as landmarks to find genes of interest genes must be polymorphic. To narrow the gene location, a marker and gene must always be inherited together. Human Genome Information about the linked markers near the mutation Same in all humans for different genetic disorders Genetic disease-causing mutations can be a single base or a whole chromosome Entire chromosome (Aneuploidy) or a piece (Fragile-X) FRAGILE-X (CGG repetition) Fragile-X gets amplify with each generation More than 200 repeats results in transcriptional inactivation of FMR1. Normal function of protein made by FMR1 is to bind to mrnas involved in neuron function and regulate translation. If the mrnas are not translated in sufficient amounts, the nerve cells die and the result is mental retardation. Expanding Triplets Repeat Myotonic dystrophy Huntington s disease.

7 How the repeats expand is unknown; possibly DNA polymerase slips after copying the triplet, and copies it again GENETIC SCREENING Prenatal screening Screening of newborns Screening asymptomatic people with relatives who have genetic diseases NEWBORNS ARE SCREENED FOR PKU SCREENING FOR SICKEL CELL ANEMIA OLIGONUCLEOTIDE HYBRIDIZATION Modifying the disease phenotype can be done in three ways: Restricting the substrate as in PKU, reducing phenylalanine in the diet Metabolic inhibitors, such as drugs that can target specific proteins Supplying the missing protein blood factor VIII in hemophilia TREATING GENETIC DISEASES GENE THERAPY (Ex-Vivo) GENE THERAPY (In-Vivo) Example: DNA can be introduced to lungs as an aerosol to treat lung cancer. Hopefully the DNA will be incorporated into the patient s genome, and the disease will improve or disappear.