The G380R mutation in the transmembrane domain of fibroblast growth factor receptor 3 (FGFR3) causes achondroplasia, the most common form of human dwarfism. Achondroplasia is a heterozygous disorder, and thus the affected individuals express both wild-type and mutant FGFR3. Yet heterodimerization in achondroplasia has not been characterized thus far. To investigate the formation of FGFR3 heterodimers in cellular membranes, we designed an FGFR3 construct that lacks the kinase domain, and we monitored the formation of inactive heterodimers between this construct and wild-type and mutant FGFR3. The formation of the inactive heterodimers depleted the pool of full-length receptors capable of forming active homodimers and ultimately reduced their phosphorylation. By analyzing the effect of the truncated FGFR3 on full-length receptor phosphorylation, we demonstrated that FGFR3 WT/G380R heterodimers form with lower probability than wild-type FGFR3 homodimers at low ligand concentration. These results further our knowledge of FGFR3-associated bone disorders.The G380R mutation in fibroblast growth factor receptor 3 (FGFR3) transmembrane (TM) 2 domain has been linked to achondroplasia, the most common form of human dwarfism (1). Achondroplasia, characterized by short stature, is a common (1 in 15,000 live births) autosomal dominant disorder that interferes with the maturation of the cartilage growth plate of long bones (2-4). The affected protein, FGFR3, is a receptor tyrosine kinase. Its activation, manifested in the autophosphorylation of its kinase domain, involves lateral dimerization and binding of FGF ligands and heparan sulfate (5, 6). Ligand binding increases receptor activation by stabilizing the dimers and perhaps altering the dimer structure (7-11).In cellular studies, the G380R mutation increases FGFR3 phosphorylation in the absence of ligand and at low ligand concentration (12-15). The increase, which is the likely cause for pathogenesis, has been linked to an increase in the phosphorylation of the unliganded dimer (13). However, FGFR3 dimerization does not appear affected by the G380R mutation, as both the wild-type FGFR3 and the G380R mutant show very similar cross-linking and dimerization propensities (13).Achondroplasia is a heterozygous disorder, and thus the affected individuals express both wild-type and mutant FGFR3. In this paper, we address the question whether mutant FGFR3 receptors heterodimerize with the wild type. Currently, the answer to this question is unknown. Most studies of FGFR3 activation thus far have focused on FGFR3 homodimers, because the detection of heterodimers is challenging. When both wild-type and mutant FGFR3 molecules are present in cells, three different dimeric species can co-exist as follows: 1) wild-type homodimers, 2) mutant homodimers, and 3) wildtype/mutant heterodimers (16). These dimers will be indistinguishable in many experiments, and their abundance will depend on the respective expression levels and on the dimerization propensities, which are unknown. Furthermore,...

Background. During the conduct of a cohort study intended to study the associations between mixed metal exposures and child health outcomes, we found that 78% of 309 children aged 20–40 months evaluated in the Munshiganj District of Bangladesh had blood lead concentrations ≥5 µg/dL and 27% had concentrations ≥10 µg/dL. Hypothesis. Environmental sources such as spices (e.g., turmeric, which has already faced recalls in Bangladesh due to high lead levels) may be a potential route of lead exposure. Methods. We conducted visits to the homes of 28 children randomly selected from among high and low blood lead concentration groups. During the visits, we administered a structured questionnaire and obtained soil, dust, rice, and spice samples. We obtained water samples from community water sources, as well as environmental samples from neighborhood businesses. Results. Lead concentrations in many turmeric samples were elevated, with lead concentrations as high as 483 ppm. Analyses showed high bioaccessibility of lead. Conclusions. Contamination of turmeric powder is a potentially important source of lead exposure in this population.

Background: CRL4Cdt2 requires that a substrate bind to proliferating cell nuclear antigen (PCNA) on DNA prior to ligase recruitment, but the precise role of PCNA is unclear. Results: A specific PCNA residue is required for destruction of CRL4 Cdt2 substrates. Conclusion: CRL4Cdt2 recognizes a composite surface composed of PCNA and substrate residues. Significance: This is the first ubiquitin ligase whose substrate recognition requires creation of a bipartite substrate surface.

The transmembrane (TM) domains of receptor tyrosine kinases (RTKs) are believed to be important players in RTK signal transduction. However, the degree of specificity and promiscuity of RTK TM domain lateral interactions in mammalian membranes has not been assessed in detail in the literature. A technique to probe the occurrence of interactions between TM domains and their biological significance is to evaluate the propensity for formation of heterodimers of a full-length RTK and its TM domain. Here we examine if specific inhibition of two RTK pathogenic mutants, Neu/V664E and FGFR3/A391E, can be achieved by the TM domains of Neu, Neu/V664E, FGFR3 and FGFR3/A391E. We show that the TM domain of Neu/V664E specifically inhibits the phosphorylation of full-length Neu/V664E, while the wild-type Neu TM domain does not. In addition, Neu/V664E TM domain does not affect the phosphorylation levels of full-length FGFR3/A391E. The results suggest that TM domain peptides could be exploited in the future for the development of specific inhibitors of mutant RTKs.

Two mutations in FGFR3, G380R and G375C are known to cause achondroplasia, the most common form of human dwarfism. The G380R mutation accounts for 98% of the achondroplasia cases, and thus has been studied extensively. Here we study the effect of the G375C mutation on the phosphorylation and the cross-linking propensity of full-length FGFR3 in HEK 293 cells, and we compare the results to previously published results for the G380R mutant. We observe identical behavior of the two achondroplasia mutants in these experiments, a finding which supports a direct link between the severity of dwarfism phenotypes and the level and mechanism of FGFR3 over-activation. The mutations do not increase the cross-linking propensity of FGFR3, contrary to previous expectations that the achondroplasia mutations stabilize the FGFR3 dimers. Instead, the phosphorylation efficiency within un-liganded FGFR3 dimers is increased, and this increase is likely the underlying cause for pathogenesis in achondroplasia. We further investigate the G346E mutation, which has been reported to cause achondroplasia in one case. We find that this mutation does not increase FGFR3 phosphorylation and decreases FGFR3 cross-linking propensity, a finding which raises questions whether this mutation is indeed a genetic cause for human dwarfism.