BMPR1A mutations in juvenile polyposis affect cellular localization. (Howe et al., 2013)

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Background: Juvenile polyposis (JP) is characterized by the development of hamartomatous polyps of the gastrointestinal tract that collectively carry a significant risk of malignant transformation. Mutations in the bone morphogenetic protein receptor type 1A (BMPR1A) are known to predispose to JP. We set out to study the effect of such missense mutations on BMPR1A cellular localization.

Methods: We chose eight distinct mutations for analysis. We tagged a BMPR1A wild-type (WT) expression plasmid with green fluorescent protein on its C-terminus. Site-directed mutagenesis was used to recreate JP patient mutations from the WT-green fluorescent protein BMPR1A plasmid. We verified mutant expression vector sequences by direct sequencing. First, we transfected BMPR1A expression vectors into HEK-293T cells; then, we performed confocal microscopy to determine cellular localization. Four independent observers used a scoring system from 1 to 3 to categorize the degree of membrane versus cellular localization.

Results: Of the eight selected mutations, one was within the signaling peptide, four were within the extracellular domain, and three were within the intracellular domain. The WT BMPR1A vector had strong membrane staining, whereas all eight mutations had much less membrane and much more intracellular localization. Enzyme-linked immunosorbent assays for BMPR1A demonstrated no significant differences in protein quantities between constructs, except for one affecting the start codon.

Conclusions: Bone morphogenetic protein receptor type 1A missense mutations occurring in patients with JP affected cellular localization in an in vitro model. These findings suggest a mechanism by which such mutations can lead to disease by altering downstream signaling through the bone morphogenetic protein pathway.

In Plain English:

BMPR1A is a receptor on the surface of the cell that receives chemical signals from outside the cell and communicates them to various elements within the cell, resulting in a number of changes, most notably in which genes become what amount of each protein.   These changes control a wide variety of processes, but are especially important in preventing cells from becoming cancer. When the BMPR1A gene is mutated, people tend to develop juvenile polyposis, where they form small tumors within their stomach and/or intestines that are likely to become cancerous. In this study, we found eight different BMPR1A mutations in patients with cancerous juvenile polyposis, and we looked at where these BMPR1A variants could be found in the cell. Across these mutations, we saw that none actually made it to the surface of the cell, instead staying in the interior. These results show mutations in BMPR1A could prevent signaling by preventing it from traveling to the outside of the cell,   which would keep it from receiving signals from outside the cell in the first place. Cells with these mutations would then be unable to respond to these signals, which could be responsible for these cells becoming cancerous.