Lecture #15 | Tumor Suppressors

Tumor suppressors overview

• Negative controls of cell growth and proliferation.

• Inactivating mutations cause a loss of function.

• Act in a recessive fashion (both copies need to be lost).

• Detected in vitro in somatic cell hybrids.

• Detected in vivo in familial cancers in humans.

Somatic Cell Hybrids

• used to identify tumor suppressor genes.

• Plasma membranes fuse, followed by nuclear fusion, resulting in two sets of chromosomes in one cell.

• Mouse NIH 3T3 and monkey kidney cells are used for fusion.

  • These cells can be treated under different conditions

  • One cell is a cancer cell

Henry Harris et al. (1969)

Cell fusions show suppression of transformed phenotype, suggesting cancer is recessive and that genes prevent uncontrolled cell proliferation.

Familial Cancer Syndromes

• Isolation of tumor suppressor genes aided by studying rare familial cancer syndromes.

• Inherited as autosomal dominant:

  • Retinoblastoma (Rb)

  • Li-Fraumeni (p53)

  • Familial Breast-Ovarian (BRCA1 and BRCA2).

• In vivo evidence of tumor suppressors from inherited cancer syndromes

Retinoblastoma (Rb)

• Most common cancer of infants and children (1 in 20,000).

• “Cat’s eye” reflection (leukocoria).

• Heritable and nonheritable forms.

• Early identification reduces morbidity and mortality.

• Survival rate > 90% with diagnosis and treatment.

• Retinoblastoma is the prototype for understanding tumor suppressor genes.

Alfred Knudson’s Studies

• Examined cases of retinoblastoma in Houston from 1944-69.

• Sporadic (Not Inherited):

  • Single, unilateral tumors.

  • No family history.

  • Late diagnosis.

  • No increased risk.

  • Somatic mutations in one cell.

  • Not susceptible.

• Familial (Heritable):

  • Multiple, bilateral tumors.

  • 20% of cases.

  • Early diagnosis (~2 years).

  • Increased risk of sarcomas, melanoma.

  • Germline mutations in all cells.

  • 50% of offspring susceptible.

Two-hit hypothesis

Familial (inherited) Rb results from germline inactivation of one copy, followed by random inactivation of the second functional allele

• two mutant leads to Rb

• Very rare to be hit by mutations in same spot

• → led to the idea of loss of heterozygosity

  • arose from the observation that there is a high penetrance of Rb indicating that there are robust mechanisms for losing 2nd Rb gene

Ways that a normal Rb gene is eliminated? Loss of heterozygosity

Evidence that the 2nd hot involves los of Rb genes

• Deletion of 13q14 is associated with inheritance of Rb.

• Deletion at 13q14 in the other chromosome 13 is often observed in tumor, but not normal cells.

Retinoblastoma and Other Tumors

• Patients with the familial form of Retinoblastoma (bilateral) have a much greater risk of developing other tumors, unlike those with the sporadic form (unilateral).

• However, RB is mutated in many human cancers, even without genetic predisposition.

• cBioPortal in The Cancer Genome Atlas (TCGA) shows very few amplifications.

RB1 Gene Mutations

• Normal cells: 4.7 kb transcript.

• Tumor cells: transcript is often absent or truncated.

• Large gene spanning 27 exons, ~1700 known mutations across all cancer types.

• Truncations are the most frequent mutation type, followed by missense mutations.

• Many mutations are seen in multiple patients (hot spots).

• RB1 has 2 homologs -- p130 and p107 – their role in cancer is less clear.

• Mutations in Rb have been mapped across various tumor types (cBioPortal, 2017).

Mismatch repair genes

• MSH2, MLH1, PMS1, PMS2 (colon cancer).

NER genes

XPB, XPD, XPA (skin cancer)

• Bloom’s Syndrome

• BLM helicase (solid tumors

• DS break repair:

• ATM (lymphoma).