For this project, St. Jude sequenced just the portion of the genome called the exome, which carries instructions for making proteins. Researchers sequenced the exomes of two families affected by rare inherited degenerative disorders that target cells in the muscle, bone and brain. Neither family carried mutations previously tied to ALS or related diseases. The project built on the infrastructure developed by the St. Jude Children's Research Hospital -- Washington University Pediatric Cancer Genome Project, which played an important role in finding the mutations.
Researchers found the families carried a single, previously unknown mutation in a pair of RNA-binding proteins named hnRNPA2B1 and hnRNPA1. The proteins both bind RNA and help regulate its function. When researchers checked for the same mutations in 517 ALS patients they found hnRNPA1 protein mutated in two patients. One patient had the inherited form of ALS. The other ALS patient had no family history of the disease.
The new mutations occurred in a region of the proteins Taylor refers to as a prion-like domain because it has similarities with yeast proteins called prions. Prions are proteins that can alternate between shapes as needed for different functions. "Until recently we did not know these domains existed in humans and now we realize that hundreds of human proteins have them," Taylor said. "We're only beginning to understand their function in human cells."
Researchers showed the prion-like domains are responsible for the shape change that occurs when these proteins convert into slender threads called fibrils. The mutations accelerate fibril formation and recruit normal proteins to form fibrils. This phenomenon called propagation may explain how ALS and related diseases spread throughout the nervous system.Taylor speculated that the normal function of prion-like domains is to assemble RNAs into temporary structures called granules, which are part of the cell's normal protein production machinery. Granules are normally short lived, and the RNA-binding proteins involved in their formation are recycled. But in cells with hnRNPA2B1 or hnRNPA1 mutation, RNA granules accumulated in the cytoplasm instead of being disassembled. "That's bad news for RNA regulation, which is bad news for those cells," Taylor said.