Gene Mutation and Molecular Medicine

1. Gene Mutation and Molecular Medicine

2. 15 Gene Mutation and Molecular Medicine 15.1 What Are Mutations? 15.2 What Kinds of Mutations Lead to Genetic Diseases? 15.3 How are Mutations Detected and Analyzed? 15.4 How Is Genetic Screening Used to Detect Diseases? 15.5 How Are Genetic Diseases Treated?

3. 15 Gene Mutation and Molecular Medicine A mutation in a bone marrow cell causes white blood cells to divide continuously, resulting in a type of leukemia. The protein encoded by the mutated gene stimulates cell division. A chemical has been found to bind and inactivate this protein. Opening Question: Are there other targeted therapies directed to specific types of cancer?

4. 15.1 What Are Mutations? A mutation is a change in the nucleotide sequence of DNA that can be passed on to the next generation. Some mutations arise when DNA polymerase makes errors that are not corrected.

5. 15.1 What Are Mutations? Two types of mutations: • Somatic mutations: occur in somatic (body) cells. Passed to daughter cells in mitosis but not to sexually produced offspring • Germ linemutations: occur in germ line cells that give rise to gametes. A gamete passes these mutations on at fertilization

6. 15.1 What Are Mutations? Mutations affect phenotypes: • Silent mutation: usually don’t affect protein function • May be in a non-coding region, or code for the same amino acid as the original • Common; a result in genetic diversity that isn’t expressed

7. 15.1 What Are Mutations? • Loss of function mutations: gene may not be expressed at all, or protein doesn’t function • It is nearly always recessive

8. 15.1 What Are Mutations? • Gain of function mutation: leads to a protein with altered function • Usually dominant • Common in cancer—the new protein may stimulate cell division

9. Figure 15.1 Mutation and Phenotype

10. 15.1 What Are Mutations? • Conditional mutation: phenotype is altered only under certain (restrictive) conditions, (e.g., a protein may be unstable at high temperatures) • Mutation is not detectable under permissive conditions • Example—point restriction phenotype in Siamese cats

11. 15.1 What Are Mutations? • Reversion mutation: most mutations can be reversed by mutating a second time • DNA reverts to the original sequence • The phenotype goes back to wild type

12. 15.1 What Are Mutations? • Point mutation: change in a single nucleotide • This results from the gain, loss, or substitution of a single nucleotide • There are two types of base substitutions—transition and transversion

13. 15.1 What Are Mutations? Transition: substitution of one purine for the other, or one pyrimidine for the other

14. 15.1 What Are Mutations? Transversion: substitution of a purine for a pyrimidine, or vice versa

15. 15.1 What Are Mutations? A point mutation in the coding region of a gene will alter the mRNA sequence, and may or may not result in a change in the protein. Silent mutations do not alter amino acid sequences.

16. Figure 15.2 Point Mutations (Part 1)

17. Figure 15.2 Point Mutations (Part 2)

18. 15.1 What Are Mutations? • Missense mutations: result in substitution of one amino acid for another in a protein • Example: sickle-cell disease. Sickle allele differs from normal allele by one base pair, which alters one subunit of hemoglobin • Homozygous recessives have defective, sickle-shaped red blood cells

19. Figure 15.2 Point Mutations (Part 1)

20. Figure 15.2 Point Mutations (Part 3)

21. Figure 15.3 Sickle and Normal Red Blood Cells

22. 15.1 What Are Mutations? Missense mutations may have no effect on protein function. Or, the protein functional efficiency may be reduced, but not completely inactivated.

23. 15.1 What Are Mutations? Gain of function missense mutations can also occur: TP53 codes for a tumor suppressor, but certain mutations cause the protein to promote cell division and prevent cell death. The TP53 protein gains an oncogenic (cancer-causing) function.

24. 15.1 What Are Mutations? • Nonsense mutations: a base substitution causes a stop codon to form somewhere in the mRNA • This results in a shortened protein, which is usually not functional • If it’s near the 3' end, it may have no effect

25. Figure 15.2 Point Mutations (Part 1)

26. Figure 15.2 Point Mutations (Part 4)

27. 15.1 What Are Mutations? • Frame-shift mutations: insertions or deletions of bases • These mutations alter the reading-frame for the 3-base codons during translation • Nonfunctional proteins are produced

28. Figure 15.2 Point Mutations (Part 1)

29. Figure 15.2 Point Mutations (Part 5)

30. 15.1 What Are Mutations? Chromosomal mutations result in extensive changes in DNA. DNA molecules can break and rejoin. This can be caused by damage to chromosomes by mutagens or by errors in chromosome replication.

31. 15.1 What Are Mutations? Chromosomal mutations: • Deletions—chromosome may break in two places and rejoin, leaving out part of the DNA • Duplications—can occur with deletions when homologous chromosomes break at different places

32. Figure 15.4 Chromosomal Mutations

33. 15.1 What Are Mutations? Chromosomal mutations: • Inversions—chromosome breaks and rejoins, with one segment flipped • Translocations—segment of DNA breaks off and attaches to another chromosome; can cause duplications and deletions Down syndrome is caused by translocation of chromosome 21.

34. 15.1 What Are Mutations? Translocations can involve reciprocal exchanges of chromosome segments, as in chronic myelogenous leukemia (CML).

35. 15.1What Are Mutations? Retroviruses insert their DNA into the host genome at random. If the insertion is within a gene, it can cause a loss of function mutation. The viral DNA can remain in the host genome and be passed from one generation to the next. It’s called an endogenous retrovirus.

36. 15.1What Are Mutations? Transposons (transposable elements) also insert themselves into genes and cause mutations. They can move from one position in a genome to another, and usually carry genes to encode enzymes for this movement. Short sequences can be left behind and become mutations.

37. 15.1What Are Mutations? Some transposons replicate and the copies are inserted into new sites in the genome. Some genomic DNA is sometimes carried along with the transposon when it moves, resulting in gene duplication. These gene duplication events play an important role in evolution.

38. 15.1What Are Mutations? Mutations are caused in two ways: • Spontaneous mutations occur with no outside influence, and are permanent (movement of transposons, imperfect cellular processes) • Induced mutations are due to outside agents, or mutagens such as chemicals or radiation, or retroviruses

39. Figure 15.5 Spontaneous and Induced Mutations (Part 1)

40. 15.1What Are Mutations? Spontaneous mutation mechanisms: • The four bases can exist in different forms (tautomers). One form is rare If a base forms its rare tautomer, it can pair with the wrong base, resulting in a point mutation.

41. 15.1What Are Mutations? • Chemical reactions may change bases Example: loss of an amino group (deamination) from cytosine. If not repaired, DNA polymerase will add an A instead of G.

42. 15.1What Are Mutations? • Errors in replication by DNA polymerase Most errors are repaired by the proofreading function, but some become permanent. • Imperfect meiosis: nondisjunction and random breaking and rejoining of chromosomes

43. 15.1What Are Mutations? Inducedmutation mechanisms: • Chemicals can alter bases Example: nitrous acid can deaminate cytosine and convert it to uracil, which has the same result as spontaneous deamination.

44. 15.1What Are Mutations? • Some chemicals add other groups to bases Example: benzopyrene in cigarette smoke adds a chemical group to guanine and prevents base pairing. DNA polymerase will add any base at that point.

45. 15.1What Are Mutations? • Radiation damages DNA Ionizing radiation (X-rays, gamma rays, radiation from unstable isotopes) creates highly reactive free radicals. Free radicals can change bases into forms not recognized by DNA polymerase.

46. 15.1What Are Mutations? Ionizing radiation can also break the sugar-phosphate bonds of DNA, causing chromosomal abnormalities. UV radiation (from sun or tanning beds) is absorbed by thymine, causing it to form covalent bonds with adjacent bases and disrupt DNA replication.

47. Figure 15.5 Spontaneous and Induced Mutations (Part 2)

48. 15.1What Are Mutations? Mutagens may be human-made or natural. Plants make many small molecules for various functions, including defense; some are mutagenic and carcinogenic. Nitrites are human-made mutagens (used to preserve meats). They are converted to nitrosamines in the smooth ER and can deaminate cytosine.

49. 15.1What Are Mutations? Aflatoxin is made by the mold Aspergillus. When mammals ingest the mold, it is converted by the ER into a product that, like benzopyrene, binds to guanine.

50. 15.1What Are Mutations? Radiation can be human-made or natural. Isotopes from nuclear reactors and bombs can increase mutation rates. Natural UV radiation in sunlight can cause mutations.