An introduction to the Sonic Hedgehog Signalling Pathway

As an undergraduate, I did not study the topic "embryology"; how a single fertlised egg grows into an organised animal body, so I am just beginning to grasp the relevant science now, by reading online.  Biological science has progressed fast over recent decades.  Fortunately, the internet can bring resources from afar, for little effort.  The difficulty lies in interpreting what we read.

Do we GS people need to understand anything about the basic science related to this topic?  I think we need to try to grasp it to some extent, especially if we are thinking of taking part in trials of new drug therapies which manipulate the "Sonic Hedgehog" function in our cells, to attempt to reduce the symptoms of GS.  After all, to give "informed consent", a patient needs to know something about what is involved and what is being attempted.  If we are planning to use experimental drugs to alter the function of this pathway, I think it wise to understand something about what is already known about its function.

Some basics:

The Sonic Hedgehog Signalling Pathway is a set of protein molecules in the cell, all of which need to work well, in order to get normal operation of this pathway.  Sometimes, people talk as if the gene is the protein.  This is not quite correct, and may lead to confusion for those who are trying to understand what is involved.  A gene is made of a string of "bases" (these are often written as A, T, C and G) joined together to make DNA, which is found on a chromosome, and a gene can be inherited.  A protein is made of a string of amino acids joined together by the cell, using instructions encoded in the gene.  The protein does the "job" in the cell.  It may help if I use an analogy:

I have recently taken up knitting again.  The gene is quite like the knitting pattern; it is a series of instructions that tells the cell how to knit together amino acids (that come from our diet) to make a protein that can become part of the cell; there are instructions very like those in knitting, such as: when to change to a different colour, knit, purl, add stitches, cast off stitches, slip stitches, repeat a sequence etc.  The protein is the product of genetic instructions in the cell; in knitting, the product of all those crafty instructions is a scarf or a beanie etc.  The knitting pattern can be written on paper, but the knitted product is made of yarn, and can be worn.  I hope that helps readers to picture these minuscule complex things in our cells.

Gorlin Syndrome is caused by a mutation of genes involved the the Sonic Hedgehog Signalling Pathway.  It is a "dominant" characteristic, therefore you see effects of  such a mutation in single dose; where there is one chromosome carrying a relevant mutated gene, while the other chromosome of that pair carries a normal copy of the same gene.

In our BCCs, the mutated gene is not making its "gene product" (which is a protein, like many other gene products), so the rest of the Sonic Hedgehog Signalling Pathway cannot work properly, and so cells grow out of control; a tumour begins.  If it stops working in a few cells while our bodies are still small and just - forming before birth, we develop with a a malformation.

In our other cells, the Sonic Hedgehog Signalling Pathway (SHH) is operating slower than in normal cells, due to the cell having one copy of one of the genes instead of 2 copies; this causes the various malformations found in many people with GS, such as bifid ribs, anomalies of vertebrae, incomplete dentition, extra fingers, missing fingers or a large cranium.  However, most cells in our bodies behave almost normally, unless they lose the remaining copy of the gene; this transforms them into a malignant cell.  Will the body kill that cell, or will the cell multiply and form a significant tumour? What happens next depends on various factors, including the presence of other genes.

An article in the US group's "Quarterly Advocate", explained something about the genetics and mutations which lead directly to tumour formation:

http://www.bccns.org/news/2007Fall.pdf

"Basal cell nevus syndrome is caused by a tumor suppressor gene, called PTCH, located on chromosome 9. Mutations in this gene may increase the risk of ovarian cancer.

Tumor suppressor genes usually control cell growth and cell death. Both copies of a tumor suppressor gene must be altered, or mutated, before a person will develop cancer. With basal cell nevus syndrome, the first mutation is inherited from either the mother or the father in 60 percent to 80 percent of cases. In 20 percent to 40 percent of cases, the first mutation is not inherited and arises de novo (for the first time) in the fertilized egg from which the person with symptoms was conceived. Whether de novo or inherited, this first mutation is present in all of the cells of the body and, as such, is called a germline mutation.

Whether a person who has a germline mutation will develop cancer and where the cancer(s) will develop depends upon where (which cell type) the second mutation occurs. For example, if the second mutation is in the skin, then skin cancer may develop. If it is in the ovary, then ovarian cancer may develop. The process of tumor development actually requires mutations in multiple growth control genes. Loss of both copies of PTCH is just the first step in the process. What causes of these additional mutations to be acquired is unknown. Possible causes include chemical, physical, or biological environmental exposures (such as sunlight) or chance errors in cell replication.

Some individuals who have inherited a germline tumor suppressor gene mutation may never develop cancer because they never get the second mutation necessary to knock out the function of the gene and start the process of tumor formation. This can make the cancer appear to skip generations in a family, when, in reality the mutation is present. Persons with a mutation, regardless of whether they develop cancer, however, have a 50/50 chance to pass the mutation on to the next generation.

It is also important to remember that the gene responsible for basal cell nevus syndrome is not located on the sex chromosomes. Therefore, mutations can be inherited from the mother or the father's side of the family."

Why does the PTCH1 gene sometimes completely stop working in a cell?  See my blog on “Genetics and Lifestyle”.


This recent article in "DermNet NZ", published by the New Zealand Dermatological Society Incorporated, is a good brief summary of the genetics involved, including Gorlin Syndrome:

http://dermnetnz.org/pathology/bcc-genetics.html

It was published 13 July 2011, and mentions several several genes:

"There is evidence that mutations in the PTCH1, PTCH2, SMO and SUFU genes predispose patients to BCC."

although later, it still attributed (all) GS to heritable mutations in PTCH1.  In the Orphanet paper, in the "Etiology" section:

http://www.ojrd.com/content/3/1/32

"The PTCH1 gene has recently been mapped to the long arm of chromosome 9 (q22.3-q31) with no apparent heterogeneity [8,96]. Approximately 50% of NBCCS patients have allelic losses including this site..."

Some other scientific resources say that around 80% of us have a mutated PTCH1 gene.  Anyhow,  the remainder have mutations in other genes that are involved in the Sonic Hedgehog Signalling Pathway; genes other than PTCH1, which is short for "patched homolog 1".  So far, there is a generally - available DNA test only for PTCH1, so genetic testing is of limited benefit in families affected by GS.  It may be worth testing the children, if testing of the adults in the family has already shown that family's GS problem is due to PTCH1 mutation.  Otherwise, the diagnosis still depends on detection of symptoms and signs.  After all, this is a "syndrome"; it is not defined by the presence or absence of a certain gene.

That paper says a few things about another "Patched" gene in humans; PTCH2.  It seems that losing one of your copies of PTCH2 does not cause GS on its own, but having a mutation in PTCH1 as well as a mutation in PTCH2 leads to the symptoms of GS being worse than when it was only PTCH1 that had mutated.  This is how I understand the paragraph where they say:

"In striking contrast to PTCH1-/- mice, PTCH2-/- animals were born alive and showed no obvious defects and were not cancer prone [135]. However, loss of PTCH2 markedly affected tumor formation in combination with PTCH1 haploinsufficiency. PTCH1+/-PTCH2-/- and PTCH1+/-PTCH2+/- animals showed a higher incidence of tumors and a broader spectrum of tumor types compared with PTCH1+/- animals [135]. Therefore, PTCH2 modulates tumorigenesis associated with PTCH1 haploinsufficiency [135]."

I think it can be "translated" like this:

- you can build a normal body without any functioning PTCH2 genes, but not without any functioning PTCH1 genes

- having only 1 normal copy of PTCH1 gave symptoms of GS, when there were 2 copies of the normal PTCH1 gene present

- in animals with only 1 normal copy of PTCH1, there were more symptoms of GS, when there was only one copy of the normal PTCH2 gene present; when there was a mutation in both PTCH1 and PTCH2

See also the section "Other genes of importance in BCC risk", as having fair skin is known to make BCCs a major feature of GS, while GS people with very dark skin have few BCCs.  Only sophisticated genetic tests (not yet widely available) can determine whether we have any mutations in the other relevant genes which will increase the risk of us developing tumours.

The drugs mentioned in the section "Experimental therapies based on BCC genetics" are giving hope to families affected by GS that the children might access effective therapies that will effectively neutralise the symptoms of GS, but it is important to be aware that these drugs are still experimental, and that altering the function of SHH can have very unpleasant, even catastrophic, side effects.
The Wikipedia article explains it in some detail, but you need a science education to follow it all.

http://en.wikipedia.org/wiki/Hedgehog_signaling_pathway

I will "translate" a few bits:

It says: "Mammals have three Hedgehog homologues, of which Sonic hedgehog is the best studied".

We have 3 versions of these sets of genes, in contrast to a single set of these genes in fruitflies, and each version might have its special role, but researchers so far have focussed on the set called "Sonic Hedgehog".   (The other sets in humans are called "Desert Hedgehog" and "Indian Hedgehog".)

"The pathway is equally important during vertebrate embryonic development."

These SHH pathways need to operate normally during the formation of our organs, before we are born, in order that the body will have the correct components in the correct places.

"In knockout mice lacking components of the pathway, the brain, skeleton, musculature, gastrointestinal tract and lungs fail to develop correctly."

These are mice in which scientists have destroyed these genes, while the mouse babies are tiny embryos, to see what would happen to the mouse foetuses as they develop.  They do not form organs correctly.

"Recent studies point to the role of hedgehog signaling in regulating adult stem cells involved in maintenance and regeneration of adult tissues."

These genes also need to operate sometimes after we are born, but just when and where they are needed, eg to repair damage to body parts; to make the nearby cells multiply in order to rebuild damaged bits.

"The pathway has also been implicated in the development of some cancers."

If they operate at the wrong times, it results in cancer.

"Drugs that specifically target hedgehog signaling to fight this disease are being actively developed by a number of pharmaceutical companies."

These drugs turn off the "hedgehog" genes, which offers hope to those people with Gorlin Syndrome in whom the tumours are really badly out of control ..... but there are going to be side effects to turning off this important "repair" pathway.

Once we have grown, it is involved in repair of damaged body areas, according to Wikipedia: see the parts on "Role" and "Human disease" eg:

"Hedgehog signaling remains important in the adult. Sonic hedgehog has been shown to promote the proliferation of adult stem cells from various tissues, including primitive hematopoietic cells[25], mammary[26] and neural[27] stem cells. Activation of the hedgehog pathway is required for transition of the hair follicle from the resting to the growth phase.[28] Curis Inc. together with Procter & Gamble are developing a hedgehog agonist to be used as a drug for treatment of hair growth disorders.[29] This failed due to toxicities found in animal models.[30]"

NB: Oxford Dictionary:

http://oxforddictionaries.com/view/entry/m_en_gb0013990#m_en_gb0013990
"agonist

noun
1 Biochemistry; a substance which initiates a physiological response when combined with a receptor.  Compare with antagonist"
(I think that makes the "hedgehog agonist" something that turned the hedgehog gene ON.)
Scientists need to know more about these matters, if they are ever to manipulate stem cells to fulfil their theoretical potential that receives so much "hype" in the media.  Meanwhile, they aim at simpler goals eg:

"Biotech companies are also attempting to turn this pathway on after a patient has a stroke or heart attack."


Safety first:

There has been a "real life" example of the unfortunate effects, on unborn animals, of cyclopamine; a drug that affects the SHH pathway.   An article about this can be read online in the "Quarterly Advocate":

http://www.bccns.org/news/2006Spring.pdf

"The Curious Case of The One Eyed Sheep
Matthew Herper. Forbes. New York: Nov 28

How a freakish birth defect among Idaho lambs 50 years ago has led to a powerful new cancer treatment.

Idaho sheep ranchers couldn't figure out why, in the decade after World War II, a random batch of their lambs were being born with strange birth defects.  The creatures had underdeveloped brains and a single eye planted, cyclopslike, in the middle of their foreheads. In 1957 they called in scientists from the U.S. Department of Agriculture to investigate.

The scientists worked for 11 years to solve the mystery. One of them, Lynn James, lived with the sheep for three summers before discovering the culprit: corn lilies. When the animals moved to higher ground during droughts, they snacked on the flowers. The lilies, it turned out, contained a poison, later dubbed cyclopamine, that stunted developing lamb embryos. The mothers remained unharmed. The case of the cyclopamine and the one-eyed Idaho lambs remained a freakish chemistry footnote for the next 25 years; researchers never could uncover why cyclopamine caused birth defects...."


See the article for the rest of the story.  It will become clear why drugs affecting the SHH pathway may never be safe in pregnancy.  In the present experimental trials of other drugs affecting the SHH pathway, children are not allowed to participate.  I assume this is normal in trials where the effects might be seen in adults, but there would also be some concern that such drugs, known to be able to disrupt normal body construction, might permanently harm children.  Among adult volunteers, there are reports of severe side effects such as loss of hair and sense of taste.  On the other hand, some GS people taking the SHH inhibitors in trials are thrilled with the good effects; see:

http://www.skinandallergynews.com/resources/rss-feeds/single-article/aad-oral-vismodegib-promising-for-basal-cell-nevus-syndrome/545d88a3f3.html


In this article about Vismodegib (formerly known as GDC-0449), they mentioned something that has been said at a few of our GS group's lunches: "These patients have terrible phobias after undergoing tons and tons of biopsies and surgeries, so they're really incredibly grateful for this drug."  I can understand why!  I have come to dread having more surgery, but I had more excisions in recent months.  I will put up with a lot of trouble from Aldara side effects, if it keeps me away from the scalpel, and I hope to try PDT one day, but I am nervous of these new drugs; they are powerful and may upset various body systems.

Meanwhile, we can try to minimise our numbers of cancers, by other means such as sunscreen.  Diet is probably important, too.  I will write more about that later.