The humble skin mole has at least one oncogenic mutation. But it is not cancer- why not?
We know that mutations cause cancer. But we also know that it takes multiple mutations, not just one, in virtually all cases. This is one reason why age is such a strong risk factor, providing the time to accumulate multiple "hits". One place where this is particularly apparent is the skin. Most people have moles (nevi) and other imperfections, which are no cause for alarm. We are also on the lookout for the unusual signs and forms that indicate melanoma- which truly is a cause for alarm. Moles typically have one of the key oncogenic mutations for melanoma, however: BRAF V600E (which means the 600th amino acid in its protein chain has been changed from valine to glutamic acid). So what is behind the difference? What systems do cells and organs have to keep this train on the tracks, despite a wheel or two coming off?
A recent paper (review) explored this issue, and tells a complicated technical and scientific story. But the bottom line is that certain miRNAs- a novel form a gene regulator discovered just in the last couple of decades- form a firewall against further proliferation. The BRAF mutation is an activating change, which disrupts the normal "off" state of this protein kinase. BRAF is a protein kinase that attaches phosphate groups to serines and threonines on other proteins. And some those other proteins are specifically other (MAP) protein kinases that form cascades promoting cell proliferation and differentiation. In the case of melanocytes in the skin, the BRAF mutation promotes just that: proliferation, mole formation, and, in some cases, progression to full blown melanoma.
What is a skin mole? Well, it clearly is composed of lots of cells, so whatever is arresting the mutant BRAF-activated proliferation is taking its sweet time. Proliferation goes for a while, but then stops for an unknown reason. It had been thought in the field (and by these researchers as well) that mole cells had gone into senescence- an irreversible division arrest that is frequently activated in cancer cells and is similar to age-dependent cell cycle arrest. But they show now that senescence is not the explanation. If the BRAF mutation state is reversed, the cells resume dividing. And they also have other hallmarks of a different form of (G2/M) cell division arrest. So something more dynamic is going on.
They do a few technical tours de force of modern DNA sequencing and large-scale molecular biology to find what unusual genes are being expressed in these cells, and find two: MIR211-5p and MIR328-3p. These are miRNAs, which are short RNA pieces that repress the expression of other genes. We have thousands of them, and each can repress hundreds of other genes, forming a somewhat crazy interdigitated regulatory network. They evolved from an immune function of repressing the expression of viruses and other foreign DNA, but have been repurposed to have broad regulatory effects, often in development and disease.
In BRAF-activated skin mole cells, these miRNAs have one effective target, which is AURKB (Aurora B kinase), another protein kinase that is needed for cell division. No AURKB, no cell division. Indeed, skin mole cells have a high rate of cells stuck in the last phase of cell division, with 4 genome equivalents. They found that AURKB has low expression in skin mole cells, but high expression, as expected, in melanoma cells, while the miRNAs had the reverse pattern. And tellingly, artificial inhibition of these miRNAs released mole cells from their proliferation arrest and allowed the BRAF mutation to have its way with them.
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| Model of this paper's findings about melanocytes. Starting with stem-like melanocytes, mutated BRAF can cause oncogenenic or pre-oncogenic proliferation. Separately, TPA, or some local tissue factor like TPA, can encourage stem melanocytes to grow and differentiate properly into mature melanocytes. But those same activators (TPA and its natural analog) increase miRNA expression of particularly MIR211-5p, which (by inhibiting AURKB) arrests growth as part of the differentiation program, and also shuts down proliferation caused by mutated BRAF, (at late mitosis / G2 arrest), at least most of the time. |
But there was still a problem- what activates the miRNA gene expression in the natural setting? It isn't the mutated BRAF protein, since it routinely drives cells through several replication cycles to form moles, and didn't have any regulatory effect on the miRNAs. The researchers focused on the kinds of local secreted hormones, like endothelin, that might locally inhibit overgrowth of cells, and logically lead to a mole-like pattern. What they hit on was TPA, an artificial analog of diacylglycerol, which is an activator of yet another protein kinase, PKC. TPA is paradoxically a tumor promoter, and is routinely used in cell culture systems to goose the proliferation of melanocytes. But for the mutated BRAF- driven cells from moles, TPA arrests their growth, and it does so because PKC activates the expression of MIR211-5p. They showed that taking TPA out of their cell culture mixes dramatically restarted the growth of mole-derived and other BRAF mutation-driven cells. So this closes the circle in some degree, explaining how it is that skin moles form as sort of arrested mini-cancers.
Unfortunately, TPA is not a natural chemical, and diacylglycerol is not hormone, though many hormones, such as thyroid hormone and oxytocin, do affect PKC activity. So the natural PKC and miRNA activator, and inhibitor of excess proliferation in these BRAF mutation-driven melanocytes remains unknown. I am sure that this research group will be hunting diligently for it, since it is an extremely interesting issue not just in oncology, but in skin and tissue development generally.
