Name: SHH – sonic hedgehog
Location: Chromosome 7
Length: 12,288 bases
Role: Codes for a signalling protein crucial for determining the placement of limbs and organs in the developing embryo
Site of action: The embryo
The seating arrangements for any school trip by coach are always the same. The kids who are going to cause trouble sit at the back. Right at the front near the teacher are the more nerdy pupils. And in between, along the length of the bus, a strange gradient of cool is established, depending on how far away from the teacher each child is sitting.
The layout of the human body is established in much the same way. When an embryo first forms, concentration gradients of various signalling proteins are established – high concentrations where the protein is being secreted getting progressively lower with increasing distance from the source – that tell this amorphous blob of cells which end is the front, the back, the top, the bottom.
The interaction of these proteins lays down a basic body pattern, and this is where sonic hedgehog (SHH) comes in.
Imagine that the kids are all the same when they climb into the school bus and seat themselves at random. The "uncool" flowing from the teacher at the front of the bus sets up a concentrations gradient – from high concentration nearby to low at the back – that determines what kind of kid each becomes. So the kids at the back sprout cigarettes and spiky hair, while the kids at the front become the type to ask for extra homework. (I was one of the latter, in case you hadn't guessed.)
A similar thing happens with body cells. A cell has particular genes switched on depending on where in the body it happens to be: it knows where it is because of the concentrations of those signalling molecules, such as SHH.
This creates a huge problem if SHH doesn't work for any reason. Fruit fly embryos that had been genetically modified to lack this and similar genes were covered in spine-like projections, so the scientists called the group of genes "hedgehog genes". One of them was dubbed sonic hedgehog, which I think passes as a joke in scientific circles.
The consequences of a mutation in the SHH gene in humans are very serious indeed. Most of the time, the embryo simply doesn't survive because its body layout is completely disordered. If the embryos do survive, they have major problems with brain and facial development.
It is hard to believe that the layout of something as complex as the human body is determined by the diffusion of signalling proteins. And even harder to believe that it was a group of scientists who named one of them sonic hedgehog.
