Physical Basis behind Achondroplasia, the Most Common Form of Human Dwarfism

Lijuan He,William Horton,Kalina Hristova

doi:10.1074/jbc.m109.094086

Abstract

Fibroblast growth factor receptor 3 (FGFR3) is a receptor tyrosine kinase that plays an important role in long bone development. The G380R mutation in FGFR3 transmembrane domain is known as the genetic cause for achondroplasia, the most common form of human dwarfism. Despite many studies, there is no consensus about the exact mechanism underlying the pathology. To gain further understanding into the physical basis behind the disorder, here we measure the activation of wild-type and mutant FGFR3 in mammalian cells using Western blots, and we analyze the activation within the frame of a physical-chemical model describing dimerization, ligand binding, and phosphorylation probabilities within the dimers. The data analysis presented here suggests that the mutation does not increase FGFR3 dimerization, as proposed previously. Instead, FGFR3 activity in achondroplasia is increased due to increased probability for phosphorylation of the unliganded mutant dimers. This finding has implications for the design of targeted molecular treatments for achondroplasia.

Highlights

Mutations in Fibroblast growth factor receptor 3 (FGFR3) are known to affect long bone development, which proceeds via endochondral ossification along a pathway involving differentiation of mesenchymal stem cells into cartilage, followed by bone invasion
3) The achondroplasia mutation does not affect the phosphorylation of FGFR3 at high ligand concentration, as probed by the Tyr-653/Tyr-654 antibody, which recognizes Tyr-647 and Tyr-648 in FGFR3
4) The mutation does not affect the cross-linking of FGFR3 at any ligand concentration

Summary

To whom correspondence should be addressed

Of the cartilage die, the space is invaded by bone). FGFR3 works as a negative regulator of bone development by mediating prodifferentiation signals in chondrocytes (4 – 6). Li et al [14] have further shown that the mutant does not need a ligand to become activated in L6 cells and induces transformations in NIH3T3 cells These authors concluded that the mutation likely “produces a dominant oversignaling receptor that is no longer regulated by FGF binding.”. We seek to determine whether the increase in FGFR3 activation occurs due to enhanced dimerization or due to a different physical mechanism We do this using a new approach that bridges biophysics and cell biology and has the power to provide mechanistic understanding of the effect of pathogenic mutations on different steps in FGFR3 activation. We propose that the underlying reason for the increase in FGFR3 activation in achondroplasia is not increased dimerization but an elevated phosphorylation within the unliganded FGFR3 dimers, most probably due to a structural change

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION