Mouse Models of Melanoma
Malignant melanoma is thought to evolve in a series of steps, each accompanied by multiple genetic mutations with oncogene activation being temporally distinct from tumor suppressor gene loss. The earliest and most common identified genetic lesion (BRAFT1799A) encodes BRAFV600E (BRAFVE), a constitutively active protein serine/threonine kinase that elicits sustained BRAF → MEK1/2 → ERK1/2 MAP kinase pathway activation.
This mutation is detected in ~65% of metastatic melanomas yet in ~80% of benign nevi, consistent with the notion that BRAFVE expression is in itself insufficient for full malignant conversion owing to activation of oncogene induced senescence. Significantly, progression to malignant melanoma is invariably accompanied by silencing of one or more TSGs, most often in humans PTEN or CDKN2α. This combination of mutated BRAF and silenced of PTEN expression is common in human melanoma (~20%). There are two fundamental challenges to determine how melanoma can be overcome. (1) We need to understand the mechanisms by which early benign, senescent lesions (nevi) progress to cancers capable of sustained proliferation (melanoma). This includes determining which genes are involved and when they play a role in tumor progression. (2) We must also determine which genes are responsible for the acquisition of a metastatic phenotype and whether they are obligate targets for successful therapy.
There are a number of observations that drive our current melanoma research focus: (i) Melanocytic BRafVE expression causes a proliferative burst followed by senescence induction in both humans and mice. (ii) PTEN function is lost during melanoma progression due to direct mutation, deletion, or extinguished expression and this correlates with PI3K pathway activation (iii) BRAFVE and PTEN mutations are prevalent in human melanoma. (iv) Simultaneous PTEN ablation and BRafVE expression in mice cooperates to bypass growth arrest leading to metastatic disease. (vi) In mice specific pharmacological inhibition of Mek1/2 and/or mTORC1 downstream of BRafVE and PTEN loss respectively prevent, yet fail to eradicate, disease. (v) Current BRAFVE specific inhibitors fail to provide sustained responses in the clinic.
Together these data suggest that PTEN may play a duel role in melanoma progression: one regulating early senescence induction and later one regulating migration. We hypothesize that PTEN negatively regulates proliferation (ie. senescence induction), survival and/or migration through downstream PIP3 activated genes. We aim to uncover which “PI3K/PTEN network” genes are important in these processes and furthermore when and how they function.
Figure 1. (a) Mice carrying various conditional alleles of BRaf (BRafCA) and/or Pten (Ptenlox4-5 or Ptenlox5) were crossed to Tyr::CreER mice with melanocyte specific expression of a hormone dependent form of Cre recombinase (CreERT2) 17,20. 4-HT dependent activation of CreER leads to melanocyte specific conversion of BRafCA → BRafV600E and the conversion of the Ptenlox alleles to null alleles. BRafCA/+ (b), Tyr::CreER; BRafCA/+ (c) and Tyr::CreER; BRafCA/CA (d) mice were treated topically with 4-HT and monitored for 75 weeks for signs of melanocytic proliferation (i). Mice were euthanized and skin from the ear (ii, iii) and flank (iv) was stained with hematoxylin and eosin and inspected for the presence of pigmented cells. (e) A Tyr::CreER; BRafCA/+ mouse developed a papular pigmented lesion ~60 weeks after topical administration of 4-HT (i). This mouse was euthanized and skin sections encompassing the lesion were stained with hematoxylin and eosin and inspected for the presence of pigmented cells (ii & iii).