Fragile X protein reveals its RNA partners

The most common inherited form of mental retardation, fragile X syndrome, continues to baffle researchers even though the mutant gene responsible for the condition was identified a decade ago.

Now, several research groups are unraveling how this single flawed gene on the X chromosome alters a cell’s use of protein-building instructions from dozens, even hundreds, of other genes. The new findings could lead to ways of counteracting those changes and reducing some symptoms of fragile X syndrome.

The master gene behind the syndrome encodes a protein–called FMRP, for fragile X mental retardation protein–that binds to strands of messenger RNA (mRNA). These strands are intermediaries between genes and the proteins they encode: Cells read a gene’s DNA to make an mRNA and then use it to build the protein.

Neuroscientists have theorized that fragile X syndrome results when FMRP doesn’t properly ferry mRNAs within cells, particularly in brain cells, or fails to correctly regulate how mRNAs make proteins. However, investigators are still struggling to identify mRNAs affected in fragile X syndrome.

To address this, Jennifer C. Darnell of Rockefeller University in New York and her colleagues exposed FMRP to random strings of the subunits that join to form RNA. The experiments revealed that FMRP binds tightly to segments of RNA that form an odd structure dubbed a G quartet. “It looks like a cube made out of RNA,” says study coauthor Robert B. Darnell, also of Rockefeller University.

The investigators then searched a database of mRNAs for ones with the G-quartet structure. They identified more than a dozen, most of them with known roles in the development or workings of the brain.

Meanwhile, Stephen T. Warren of the Howard Hughes Medical Institute at Emory University School of Medicine in Atlanta and his colleagues used FMRP-binding antibodies to pull the protein and its attached mRNAs out of mouse-brain tissue.

They identified several hundred such mRNAs, about 70 percent of which showed the G-quartet feature.

In the Nov. 16 Cell, the Atlanta and New York groups jointly describe their results and attempt to confirm the relevance of some of the mRNAs in fragile X syndrome. For example, the scientists found that immune cells of people with this condition have an altered cellular distribution of more than 250 mRNAs when compared with normal immune cells.

Last week, at the Society of Neuroscience meeting in San Diego, James Eberwine of the University of Pennsylvania in Philadelphia and his colleagues described another antibody-based technique that identifies RNAs bound to FMRP. The researchers located such RNAs within single brain cells and even within single dendrites, the branchlike extensions of nerve cells that enable them to communicate with each other.

Eberwine’s group has identified about 100 mRNAs that bind to FMRP in nerve cells or their dendrites. Alterations in the function of dendrites may lie at the heart of the mental retardation in fragile X syndrome, says Eberwine.

The scientists have yet to compare the mRNAs they’ve identified with those isolated by the other groups. However, they’ve already found that some of the mRNAs are misregulated in mice with the equivalent of fragile X syndrome.

Lynne Regan of Yale University cautions that additional mRNAs affected by FMRP probably remain undiscovered. She’s also puzzled why many mRNAs with the quartet feature identified by the Darnell group don’t bind to FMRP.

Still, neuroscientists say the mRNAs already identified should help explain how FMRP dysfunction produces mental retardation. “The finding that a novel group of RNAs is bound by the fragile X protein is of major importance,” says R. Frank Kooy of the University of Antwerp in Belgium. “Now begins the formidable task of tracing one by one the fates of these bound RNAs.”