1. Mutation
Mutation is defined as any permanent, heritable change in sequence or arrangement of genomic DNA.
Somatic mutations occur in body cells, and cannot be passed on to offspring
A germline mutation occurs in the sex cells, and can be passed to offspring

2. Sources of DNA mutation
‘Endogenous’ versus ‘exogenous’
Endogenous sources include inherent instability of DNA molecule, errors of replication, errors in homologous pairing and recombination, and gene conversion.
Exogenous sources include ionizing radiation, UV radiation, mutagens/intercolators.
Exposures and damage followed by faulty or incomplete repair mechanisms.

3. Types of point mutations (single base changes)
Missense = point mutation that results in a codon that codes for a different amino acid
Nonsense = point mutation that results in a premature stop codon, which can truncate a protein
Silent mutations = point mutation that results in the same or different amino acid, but with no functional change in the protein

4. Other types of mutations
Frameshift = insertion or deletion of one or more nucleotides, such that the translational reading frame is shifted
Splice-site mutation = a mutation that inhibits the normal function of a splice donor or acceptor site
Regulatory mutations = affect where or how often a gene is expressed
Expansion or contraction of codon repeats by strand slippage

5. Other types of mutation
Gene amplification = duplication of a chromosomal region, increasing the dosage of the genes located within that region
Chromosomal translocations = interchange of genetic parts from nonhomologous chromosomes.
Interstitial deletions = an intra-chromosomal deletion that removes a segment of DNA from a single chromosome, thereby juxtaposing previously distant genes.
Chromosomal inversions = reversing the orientation of a chromosomal segment.
Loss of heterozygosity = loss of one allele in an organism that previously had two different alleles.

6. Strand slippage during replication can lead to net insertion or deletion

7. Strand slippage, continued

8. Examples of actual mutations in disease related genes

9. Mutations affecting splicing

10. More splice mutations

11. Cancer, basic concepts Cancer is not one disease, but many
Related by abnormal cell growth and proliferation with the potential for metastasis
1/3 to 1/2 of Americans will develop cancer
As burden of infectious and other disease decreases, relative importance of cancer greater
Cancer is clonal in origin
‘Natural’ end state of multi-cellular organism?

12. Cancer, basic concepts
Cancer is ALWAYS a genetic disease, but is only occasionally a hereditary disease
Distinction between germline and somatic mutation
Hereditary and sporadic cancers involve mutations in same classes of genes
Depending upon tumor type, ~5 to 10% of cancer cases are due to a strong hereditary predisposition

13. Cancer, selected definitions
Neoplasm is a mass or tumor
Malignant means that a tumor may invade tissue locally and/or metastasize
Metastasis is the spreading of cancer cells to distant sites
Benign tumors do not metastasize, but may still be very dangerous

14. Cancer types, broad categories
Carcinoma is derived from epithelial tissue
Ductal carcinoma
Ovarian carcinoma
Colorectal carcinoma
Sarcoma, from mesenchymal tissue
Bone, muscle, connective tissue, soft tissue
Hematopoietic and lymphoid affect marrow, blood, and lymphatic system

15. Proto-oncogenes Normal proto-oncogenes promote cell division
When abnormally activated, become oncogenic.
An oncogene is a gene that, when mutated or expressed at high levels, helps turn a normal cell into a tumor cell
Dominant at cell level, single mutant allele enough to cause problems

16. Proto-oncogenes in sporadic and hereditary cancers
SOMATIC abnormal activation of proto-oncogenes is a common feature of many tumors, sporadic and inherited
GERMLINE mutation in proto-oncogene leading to hereditary cancer syndrome is apparently rare
Often inviable?
Best known example, MEN2, AD cancer syndrome caused by mutations in ret proto-oncogene
Parathyroid, thyroid, adrenal tumors
Very high penetrance, prevention by prophylactic thyroidectomy in childhood

17. Tumor suppressor genes
Normal functions broadly divided
‘Gatekeepers’ directly down-regulate cell growth (cycle checkpoints, contact inhibition)
‘Caretakers’ maintain genomic integrity and have indirect effect (repair of DNA damage, replication errors, chromosome breaks)
Loss of function of both alleles
Recessive at cellular level

18. The cancer-car analogy
Mutations in proto-oncogenes jam on the accelerator
Mutations in tumor suppressor genes
In gatekeepers, cause brakes to fail
In caretakers, sabotage
Loosen nuts and bolts

19. Retinoblastoma tumor

20. Retinoblastoma – familial vs. sporadic

21. Retinoblastoma as prototype for tumor-suppressor mutation
Caused by loss of both alleles of crucial gatekeeper, RB1.
40% of cases are familial, inherited as autosomal dominant w/ ~90% penetrance.
What is being inherited?
Not cancer per se, but strong single-gene predisposition
Every cell has one RB1 allele knocked out constitutionally in the germline
Familial cases have earlier onset and may be bilateral

22. Knudson’s “2 hit” hypothesis

23. Hereditary non-polyposis colorectal cancer, HNPCC

24. HNPCC, continued Accounts for ~2-5% of all colon cancer
AD disease w/ high penetrance
Lifetime risk of crc ~70-90% (general population ~
For women, also risk of endometrial and ovarian cancers
Caused by mutations in one of several mismatch repair genes: hMLH1, hMSH2 family
Defect in ‘caretaker’ function, LOH observed
Genetic testing is available

25. Multi-step model of sporadic colorectal tumorigenesis

26. Chronic Myeloid Leukemia

27. Chronic Myeloid Leukemia
Cancer of the white blood cells
Increased and unregulated growth of myeloid cells in the bone marrow and in the blood
Detected from an acquired chromosomal translocation Philadelphia chromosome
Symptoms include: anemia, increased susceptibility to infections, malaise, low-grade fever

28. Characteristics of Bcr-Abl
Continuously active
Fusion gene product is a tyrosine kinase
Speeds up cell division
Inhibits DNA repair, which causes genomic instability

29. Bcr-Abl: Philadelphia Chromosome
Bcr-Abl is the most common cause of CML
Chromosome translocation between chromosome 9 and 22
Part of the BCR (breakpoint cluster region) gene from chromosome 22 is fused with the ABL gene on chromosome 9
Abl carries a domain that can add phosphate groups to tyrosine residues

30. Burkitt’s Lymphoma

31. 8;14 translocation in Burkitt’s lymphoma
B-cell malignancy
endemic type in equatorial Africa, relationship to malaria and Epstein-Barr Virus
Also sporadic or HIV associated
Puts myc gene under control of highly active IGH promoter (fusion protein!)

32. Amplification of MycN, a proto-oncogene, in a neuroblastoma
Not transcriptional up-regulation, rather many excess genomic copies of region containing MycN gene
‘double minutes’ appearance
Homogeneously staining region (HSR)

33. Hereditary breast and ovarian cancer Syndrome (HBOC)
BRCA1 (chrom 17) and BRCA2 (chrom 13)
account for ~50% of hereditary br ca and ~90% of hereditary br/ov cancer families
AD transmission
mutations in ~1/500 – 1/800 in most populations, many different mutations

34. Functions of BRCA1 and BRCA2
repair of DNA damage
key trigger in BRCA1 activation
Interaction w/ RAD51 DNA repair protein
cell cycle checkpoint control
ubiquitylation (tagging of proteins for degradation by the proteosome)

35. BRCA1-Associated Cancers: Lifetime Risks
ASCO Curriculum: Cancer Genetics & Cancer Predisposition Testing
BRCA1-Associated Cancers: Lifetime Risks
breast cancer, ~50-85%, often early onset
compare to general population risk ~12%
second breast primary, ~40-60%
ovarian cancer, ~30-45%
compare to general population risk ~1-2%
possible modest increases in colon, pancreatic, and male breast cancers
Breast and Ovarian Cancer 6

36. BRCA2-Associated Cancers: Lifetime Risk
ASCO Curriculum: Cancer Genetics & Cancer Predisposition Testing
BRCA2-Associated Cancers: Lifetime Risk
breast cancer, ~50-85%
ovarian cancer, ~10-20%
male breast cancer, 5-10%
Also increased risks of pancreatic cancer and melanoma
BRCA2 accounts for ~6% of familial pancreatic ca
Breast and Ovarian Cancer 10

37. Cancer and the Environment
“Several obstacles, I believe, prevent us from addressing cancer’s environmental roots. An obsession with genes and heredity is one.”