Concept 18.5: Cancer results from genetic changes that affect cell cycle control The gene regulation systems that go wrong during cancer are the very same systems involved in embryonic development

Types of Genes Associated with Cancer Cancer can be caused by mutations to genes that regulate cell growth and division Tumor viruses can cause cancer in animals including humans

Oncogenes and Proto-Oncogenes Oncogenes are cancer-causing genes Proto-oncogenes are the corresponding normal cellular genes that are responsible for normal cell growth and division Conversion of a proto-oncogene to an oncogene can lead to abnormal stimulation of the cell cycle

within a control element within the gene Fig. 18-20 Proto-oncogene DNA Translocation or transposition: Gene amplification: Point mutation: within a control element within the gene New promoter Oncogene Oncogene Figure 18.20 Genetic changes that can turn proto-oncogenes into oncogenes Normal growth- stimulating protein in excess Normal growth-stimulating protein in excess Normal growth- stimulating protein in excess Hyperactive or degradation- resistant protein

Proto-oncogenes can be converted to oncogenes by Movement of DNA within the genome: if it ends up near an active promoter, transcription may increase Amplification of a proto-oncogene: increases the number of copies of the gene Point mutations in the proto-oncogene or its control elements: causes an increase in gene expression

Tumor-Suppressor Genes Tumor-suppressor genes help prevent uncontrolled cell growth Mutations that decrease protein products of tumor-suppressor genes may contribute to cancer onset Tumor-suppressor proteins Repair damaged DNA Control cell adhesion Inhibit the cell cycle in the cell-signaling pathway

Interference with Normal Cell-Signaling Pathways Mutations in the ras proto-oncogene and p53 tumor-suppressor gene are common in human cancers Mutations in the ras gene can lead to production of a hyperactive Ras protein and increased cell division

Figure 18.21 Signaling pathways that regulate cell division Growth factor MUTATION Hyperactive Ras protein (product of oncogene) issues signals on its own Ras 3 G protein GTP Ras GTP 2 Receptor 4 Protein kinases (phosphorylation cascade) NUCLEUS 5 Transcription factor (activator) DNA Gene expression Protein that stimulates the cell cycle (a) Cell cycle–stimulating pathway 2 Protein kinases MUTATION Defective or missing transcription factor, such as p53, cannot activate UV light 3 Active form of p53 1 DNA damage in genome Figure 18.21 Signaling pathways that regulate cell division DNA Protein that inhibits the cell cycle (b) Cell cycle–inhibiting pathway EFFECTS OF MUTATIONS Protein overexpressed Protein absent Cell cycle overstimulated Increased cell division Cell cycle not inhibited (c) Effects of mutations

(a) Cell cycle–stimulating pathway Fig. 18-21a 1 Growth factor 1 MUTATION Hyperactive Ras protein (product of oncogene) issues signals on its own Ras 3 G protein GTP Ras GTP 2 Receptor 4 Protein kinases (phosphorylation cascade) NUCLEUS 5 Transcription factor (activator) DNA Figure 18.21 Signaling pathways that regulate cell division Gene expression Protein that stimulates the cell cycle (a) Cell cycle–stimulating pathway

(b) Cell cycle–inhibiting pathway Fig. 18-21b 2 Protein kinases MUTATION Defective or missing transcription factor, such as p53, cannot activate 3 Active form of p53 UV light 1 DNA damage in genome DNA Figure 18.21 Signaling pathways that regulate cell division Protein that inhibits the cell cycle (b) Cell cycle–inhibiting pathway

(c) Effects of mutations Fig. 18-21c EFFECTS OF MUTATIONS Protein overexpressed Protein absent Cell cycle overstimulated Increased cell division Cell cycle not inhibited Figure 18.21 Signaling pathways that regulate cell division (c) Effects of mutations

Suppression of the cell cycle can be important in the case of damage to a cell’s DNA; p53 prevents a cell from passing on mutations due to DNA damage Mutations in the p53 gene prevent suppression of the cell cycle

The Multistep Model of Cancer Development Multiple mutations are generally needed for full-fledged cancer; thus the incidence increases with age At the DNA level, a cancerous cell is usually characterized by at least one active oncogene and the mutation of several tumor-suppressor genes

EFFECTS OF MUTATIONS Fig. 18-22 Colon EFFECTS OF MUTATIONS Loss of tumor- suppressor gene APC (or other) 1 Activation of ras oncogene 2 Loss of tumor-suppressor gene p53 4 Colon wall Loss of tumor-suppressor gene DCC 3 Additional mutations 5 Figure 18.22 A multistep model for the development of colorectal cancer Normal colon epithelial cells Small benign growth (polyp) Larger benign growth (adenoma) Malignant tumor (carcinoma)

Colon Colon wall Normal colon epithelial cells Fig. 18-22a Figure 18.22 A multistep model for the development of colorectal cancer Normal colon epithelial cells

1 Small benign growth (polyp) Loss of tumor- suppressor gene Fig. 18-22b Loss of tumor- suppressor gene APC (or other) 1 Small benign growth (polyp) Figure 18.22 A multistep model for the development of colorectal cancer

Activation of ras oncogene 2 3 Larger benign growth (adenoma) Loss of Fig. 18-22c Activation of ras oncogene 2 Loss of tumor-suppressor gene DCC 3 Larger benign growth (adenoma) Figure 18.22 A multistep model for the development of colorectal cancer

Additional mutations Malignant tumor (carcinoma) Loss of 4 Fig. 18-22d Loss of tumor-suppressor gene p53 4 Additional mutations 5 Malignant tumor (carcinoma) Figure 18.22 A multistep model for the development of colorectal cancer

Inherited Predisposition and Other Factors Contributing to Cancer Individuals can inherit oncogenes or mutant alleles of tumor-suppressor genes Inherited mutations in the tumor-suppressor gene adenomatous polyposis coli are common in individuals with colorectal cancer Mutations in the BRCA1 or BRCA2 gene are found in at least half of inherited breast cancers

Fig. 18-23 Figure 18.23 Tracking the molecular basis of breast cancer

Operon Promoter Genes A B C Operator RNA polymerase A B C Polypeptides Fig. 18-UN1 Operon Promoter Genes A B C Operator RNA polymerase Fig. 18-UN1 A B C Polypeptides

no corepressor present Corepressor Fig. 18-UN2 Genes expressed Genes not expressed Promoter Genes Operator Active repressor: corepressor bound Inactive repressor: no corepressor present Corepressor Fig. 18-UN2

Genes not expressed Genes expressed Promoter Operator Genes Fig. 18-UN3 Genes not expressed Genes expressed Promoter Operator Genes Fig. 18-UN2 Active repressor: no inducer present Inactive repressor: inducer bound Fig. 18-UN3

Protein processing and degradation Fig. 18-UN4 Chromatin modification Transcription • Genes in highly compacted chromatin are generally not transcribed. • Regulation of transcription initiation: DNA control elements bind specific transcription factors. • Histone acetylation seems to loosen chromatin structure, enhancing transcription. Bending of the DNA enables activators to contact proteins at the promoter, initiating transcription. • DNA methylation generally reduces transcription. • Coordinate regulation: Enhancer for liver-specific genes Enhancer for lens-specific genes Chromatin modification Transcription RNA processing • Alternative RNA splicing: RNA processing Primary RNA transcript mRNA degradation Translation mRNA or Fig. 18-UN4 Protein processing and degradation Translation • Initiation of translation can be controlled via regulation of initiation factors. mRNA degradation • Each mRNA has a characteristic life span, determined in part by sequences in the 5 and 3 UTRs. Protein processing and degradation • Protein processing and degradation by proteasomes are subject to regulation.

Chromatin modification Fig. 18-UN5 Chromatin modification • Small RNAs can promote the formation of heterochromatin in certain regions, blocking transcription. Chromatin modification Transcription Translation RNA processing • miRNA or siRNA can block the translation of specific mRNAs. mRNA degradation Translation Protein processing and degradation Fig. 18-UN5 mRNA degradation • miRNA or siRNA can target specific mRNAs for destruction.

Enhancer Promoter Gene 1 Gene 2 Gene 3 Gene 4 Gene 5 Fig. 18-UN6 Enhancer Promoter Gene 1 Gene 2 Gene 3 Fig. 18-UN6 Gene 4 Gene 5

Fig. 18-UN7 Fig. 18-UN7

Fig. 18-UN8 Fig. 18-UN8

You should now be able to: Explain the concept of an operon and the function of the operator, repressor, and corepressor Explain the adaptive advantage of grouping bacterial genes into an operon Explain how repressible and inducible operons differ and how those differences reflect differences in the pathways they control

Explain how DNA methylation and histone acetylation affect chromatin structure and the regulation of transcription Define control elements and explain how they influence transcription Explain the role of promoters, enhancers, activators, and repressors in transcription control

Explain how eukaryotic genes can be coordinately expressed Describe the roles played by small RNAs on gene expression Explain why determination precedes differentiation Describe two sources of information that instruct a cell to express genes at the appropriate time

Explain how maternal effect genes affect polarity and development in Drosophila embryos Explain how mutations in tumor-suppressor genes can contribute to cancer Describe the effects of mutations to the p53 and ras genes