Cyclopamine is a naturally occurring chemical that belongs in the family of steroidal alkaloids. It is a teratogen isolated from the corn lily that causes fatal birth defects. It prevents the embryonic brain from separating into two lobes, which in turn causes the development of a single eye. The chemical was named after this effect, as it was originally noted by Idaho lamb farmers who contacted the US Department of Agriculture after their herds gave birth to cycloptic lambs in 1957. It then took more than a decade to identify the corn lily as the culprit. Later work suggested that different rain patterns caused the sheep to graze differently, impacting the amount of corn lily ingested by pregnant sheep. The poison interrupts the sonic hedgehog signaling pathway during development, thus causing birth defects.
Discovery and naming
In 1957 Idaho sheep ranchers contacted the US Department of Agriculture when their sheep gave birth to lambs with a fatal singular eye deformity. After collecting local flora and feeding it to mice, they struggled to recreate the cyclopia. After a decade of trial and error, they came across wild corn lilies and advised the ranchers to avoid the corn lilies. Cyclopamine was one of three steroidal alkaloids isolated from corn lily, but the only unknown at the time, and it was named after its effects on sheep embryos. Four decades later a team lead by Professor Phillip Beachy related the sonic hedgehog gene to cyclopamine. Upon experimentation, they recreated cyclopia by silencing the sonic hedgehog gene. Professor Beachy then connected their cycloptic results to the cycloptic sheep noted four decades earlier. is induced by the cyclopamine present in the plant.|none
Mechanism
Cyclopamine impacts embryonic development by hindering the sonic hedgehog pathway. In healthy development, the Shh gene codes for Shh proteins. These proteins have a high affinity for a surface membrane protein called “Patched”. Upon binding, Shh proteins inhibit Patched. With Patch inhibited, another surface membrane protein called “Smoothened” may signal further cascades which impact development. Cyclopamine has a high affinity for Smoothened – and upon binding, inhibits the signal. Even though Shh may inhibit Patched, Smoothened cannot signal in the presence of Cyclopamine and thus the pathway is interrupted.
Embryological
Cyclopamine causes the most advanced form of holoprosencephaly. Because it blocks Shh signaling, the embryonic brain no longer divides into lobes. Thus, only one optical track develops, hence the cycloptic eye. Furthermore, this disease is fatal and presently has no cure. One can imagine one half of the healthy brain not dividing, but instead growing out and resembling the alobar brain. This occurs in cases of cyclopamine poisoning. This malformation is always fatal, and it is worth noting that there are lesser cases of holoprosencephaly that aren't always fatal. However, embryonic cyclopamine poisoning causes the most extreme and therefore fatal cases.
Medical potential
Cyclopamine is currently being investigated as a treatment agent in basal cell carcinoma, medulloblastoma, and rhabdomyosarcoma, tumors that commonly result from excessive SHh activity, glioblastoma, and as a treatment agent for multiple myeloma. For example, studies of epithelial cancers have demonstrated that tumor cells secrete Shh ligand to signal adjacent growth factors production by stromal cells which leads to angiogenesis, tumor cell proliferation, and tumor cell survival. With this in mind, one can imagine cyclopamine as a way of attenuating cancer’s mechanism. However, while cyclopamine has been demonstrated to inhibit tumor growth in mouse xenograft models, it never reached therapeutic potential as it caused many side effects including weight loss, dehydration, and death in mouse models. Having said that, two functional analogs of cyclopamine have been approved by the FDA; vismodegib in 2012, and sonidegib in 2015. Furthermore, vismodegib was the first Shh pathway drug approved for treating cancer. While cyclopamine and vismodegib do not appear very similar, the development of vismodegib revealed which aspects of cyclopamine give it functionality and used those results to make vismodegib. For example, the added chlorine group in Vismodegib gives the drug a much higher solubility than cyclopamine – low solubility is a hindrance towards making cyclopamine a practical drug. The development of vismodegib revealed structure activity relationships, and determined that hydrogen bonding in two sites, as well as solubility, impact the effectiveness of the drug. Specifically speaking, the two hydrogen bonds work in opposite ways; at one site a tyrosine residue on the Smoothened receptor offers a proton to be accepted whereas a separate arginine residue works as a hydrogen bond acceptor. While the accepting group is more impactful, having both makes for stronger binding.
