The Sonic Hedgehog Pathway — An Unrealized Dream
This post was originally written on July 5, 2017. Now updated with the latest developments summarized at the bottom.
Perhaps the most important area of hair loss research that I have not yet covered on this blog relates to the Sonic Hedgehog (SHH) protein and signaling pathway. This Shh pathway is used by cells to communicate with each other. While the sonic hedgehog protein has numerous critical effects on a developing human embryo (brain, craniofacial, lungs, teeth and more), it also continues to be important in adulthood when it controls certain stem cell division activities.
Getting into too much more detail about this would go beyond the scope of this blog. Shh was named after SEGA’s famous video game character, Sonic the Hedgehog.
Sato, Leopold and Crystal (1999)
In the initial decade after the first identification of the hedgehog gene around 1980, there was almost no research devoted to the impact of the SHH pathway upon human hair. However, this started to change in the mid-1990s (e.g., this from 1998) and culminated in the seminal work on this subject that was published in the US in 1999: “Induction of the hair growth phase in postnatal mice by localized transient expression of Sonic hedgehog“.
Of the three authors of the above study, the most cited was Cornell based Dr. Ronald Crystal who remains in practice even today.
What these authors discovered was that after injecting balding mice hair cells with the sonic hedgehog gene using an adenovirus, resting hair follicles in the mice started growing robust hair of the mouse’s native hair color (they dyed all the fur blonde so as to differentiate the newly growing hair).
Moreover, upon final analysis, the team found that the SHH gene was active in the injected areas of the skin, but not elsewhere. A very humorous and at the same time highly informative article on this subject from 1999 can be read here.
Curis-Procter & Gamble Partnership RIP
After the groundbreaking 1999 study on mice, some researchers were mildly optimistic that SHH activation could also have positive implications on human hair growth in balding men and women. A new company that was formed in 2000 called Curis partnered with Procter & Gamble in 2005 to try and develop a topical Hedgehog agonist product for scalp hair growth.
However, this partnership ended in 2007 due to potential safety issues since SHH can potentially also cause basal cell carcinoma cancer. P&G was not willing to continue with the drug development work, since even a very minimal risk of developing cancer is not worth it for treating a cosmetic problem such as hair loss (at least in the eyes of government). Interesting comment from the at-the-time CEO of Curis:
“We are obviously disappointed that the collaboration with Procter & Gamble will come to an end. We believe that our topically administered Hedgehog agonists have demonstrated encouraging efficacy in preclinical hair growth models and we were hopeful that one of our Hedgehog agonist drug candidates under the program would have progressed.”
Current Status
While the initial excitement of a SHH based cure for hair loss has long ended, sporadic research activity in this area continues (e.g, this in 2016). Moreover, in 2013, scientists even found that SHH signaling regenerates ear hair cells.
On a related note, also make sure to read my post from earlier this year regarding ear hair regeneration and possible links with scalp hair regeneration (some controversy in that analogy if you read the comments to that post).
November 2018 Update
On November 21st 2018, an important new study related to the Sonic Hedgehog (Shh) pathway and hair growth in mice was published in Nature Communications:
The lead author of this study is the famous Dr. Mayumi Ito, who I have covered numerous times on this blog.
For some reason, a number of newspapers around the world only covered these results a week after publication. Even Dr. Ito’s own university only discussed the findings on November 28th. More here.
The study authors claim to have shown that activation of the Sonic hedgehog (Shh) pathway regenerates dermal papilla cells, which in turn ensures hair follicle neogenesis. This was done on wounded and damaged skin that was previously unable to grow hair.
Moreover, the authors have found a way to prevent cancerous tumor growth upon Shh pathway activation:
“To bypass the risk of tumors reported in other experiments that turned on the sonic hedgehog pathway, the NYU Langone team turned on only fibroblasts located just beneath the skin’s surface where hair follicle roots (dermal papillae) first appear.”
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