FRAGILE WHAT?

We have all gotten that response from time to time when discussing fragile X with others who may not be familiar with what the disorder actually is. In fact, many of us who deal with fragile X every day still feel that much of what causes fragile X remains a mystery. To help us all learn more about the basic science behind the cause of fragile X, Dr. Karen Usdin, one of our Scientific Advisors, has been working on a series of educational articles published exclusively in our CFX newsletter. Dr. Usdin, a senior Scientist at the National Institutes of Health, and her colleague, Ali Entezam co-authored this, the second installment of this exclusive series. Please Click Here to read the first installment, published in the Spring 2004 newsletter.

A FRAGILE X PRIMER
by KAREN USDIN AND ALI ENTEZAM

In the first installment of this column we used an encyclopedia analogy to describe the human genome. We talked about how this encyclopedia contains the complete set of instructions for making a human being. We discussed how these instructions are encoded by various combinations of the bases or letters A, G, C and T that make up DNA or deoxyribonucleic acid, and how the resultant “text” is organized into chapters or genes each containing the instructions for making a specific protein molecule. We talked about how the different proteins encoded by these genes do much of the work needed for the cell to function normally and to generate new cells. We learned that gene chapters are organized into separate chromosomes, or volumes in our encyclopedia analogy. The complete set of encyclopedia, our genome, consists of 46 volumes, 23 of which are maternally inherited and 23 paternally inherited. We also discussed how the gene affected in Fragile X syndrome, FMR1, is located in one such volume, the “X” chromosome, and that girls have 2 such chromosomes, one inherited from each parent. Boys on the other hand have only one “X” chromosome that is inherited from their mothers and a Y chromosome that is paternally inherited. This is why boys are generally more severely affected by problems in genes on the “X” chromosome: unlike girls they don’t have a second “X” chromosome to “dilute out” the effect of problems with the first “X” chromosome.

In this installment we are going to discuss the underlying changes that occur in the genomes of people with Fragile X syndrome. First we need to emphasize that our genomic encyclopedia is too valuable to risk damage or loss by too frequent use or transport from place to place. So, it is stored in a special place in the cell called the nucleus, which we can think of the as the cell's equivalent of the Library of Congress. Like the Library of Congress, you cannot actually remove the original volumes from the cell library. You can however make a photocopy of individual genes for use outside the nucleus. This copy, which we call RNA, or ribonucleic acid, can then be taken from the library to other parts of the cell and used to make protein. A DNA copy of the entire genome is also made once every cell cycle so that when a cell divides, each new cell has a complete copy of the encyclopedia. If errors occur during copying so that parts of the text are altered, lost, or duplicated, these changes or mutations become part of the new cell’s instruction set.

An unusual copying error occurs in the families of children with Fragile X syndrome. The text of the FMR1 gene chapter normally contains a paragraph with the 3 letters C-G-G repeated about 23 times. Copying errors don’t occur very often when the repeat number is this small and the repeat is said to be “stable”. However, for reasons that we still don’t fully understand, when the repeat number is more than 58, the risk of errors goes up. Specifically this error involves a big increase in the repeat number. For reasons that we don’t yet understand this increase or expansion, occurs much more often in women than it does in men and the risk of expansion increases with increasing repeat number. Children that inherit an FMR1 gene that has more than 200 repeats are at very high risk of having Fragile X syndrome.

So more is not necessarily better at least when it comes to C-G-Gs. How do these extra C-G-Gs cause problems? To understand this you need to know that each gene in our genome is made up of 2 parts. The first part tells the cell when to make the RNA copy and how much of it to make. The second part is a list of ingredients for the protein itself. So, depending on its location in the gene, a change in the "text" of any one chapter can result in a change in the amount of RNA (and thus protein) that is made, a change in when it is made, or a change in the protein itself. In the case of fragile X syndrome, the C-G-G repeats are located outside of the region of the FMR1 gene that specifies the ingredients for protein production. So it doesn’t change the protein itself. How then does it cause the symptoms of Fragile X syndrome? We don’t yet know exactly why this happens, but the repeats somehow result in a chemical modification of the DNA that we call methylation. This in turn causes the gene to be switched off so that little or no RNA copies are made of the gene. This happens in most but not all people with more than 200 repeats. In those lucky enough not to have their FMR1 genes methylated, fragile X symptoms can be mild or even absent. We also know that RNA with lots of C-G-G repeats makes less protein than RNA with fewer repeats. This probably happens because the extra repeats cause the RNA to fold into a hairpin-like structure that blocks the protein synthesis machinery. The net result of the DNA methylation and the effect of the repeats on protein synthesis is a drop in the amount of protein that is made from the FMR1 gene.

In the next installment of this column we will discuss what is known about this protein and how a deficiency in this protein results in the symptoms of Fragile X syndrome.