Germline mutations in SMAD4 disrupt bone morphogenetic protein signaling.

INTRODUCTION: Juvenile polyposis (JP) is an autosomal dominant disease that predisposes to GI malignancies. Germline mutations in the tumor suppressor gene SMAD4 account for approximately 20% of JP cases. SMAD4 is the common intracellular mediator of the TGF-β and bone morphogenetic protein (BMP) pathways. Since mutations in BMP receptor 1A also cause JP, we hypothesize that altered BMP signaling is the underlying defect in JP. We therefore set out to investigate the effect of SMAD4 mutations on BMP signaling.
METHODS: SMAD4 mutations identified in JP patients were selected for analysis. These were created in SMAD4 pCMV expression vectors (EV) using a PCR-based, site-directed mutagenesis (SDM) approach. SDM clones were confirmed by direct sequencing, then co-transfected with an IdI-BMP Luciferase Responsive Element (BRE-Luc) vector and Renilla control vector into HEK-293T cells. Lysates were then collected after 48 hours, and luciferase activity was quantified using a luminometer. A pCMV empty vector was used as a negative control, and its luciferase activity was considered the baseline for cellular BMP signaling. Results obtained for each SDM clone were compared to those with the wild type (WT) vector. Statistical analysis was performed with the Student's t-test.
RESULTS: Eleven distinct mutations from 16 JP patients were analyzed; seven mutations were nonsense, and four were missense. Both type of mutations resulted in reduction of BMP signaling; missense mutations produced an 8-30% reduction in luciferase activity, whereas nonsense mutations led to 30-60% reduction in luciferase activity when compared to the WT clone (Figure 1). All nonsense mutations led to significantly reduced activity relative to WT (P < 0.05), while the reduction in signaling seen in missense mutations was not statistically significant.
CONCLUSION: SMAD4 germline mutations as seen in the JP patients appear to negatively impact downstream BMP signaling. Nonsense mutations resulted in significantly reduced luciferase activity when compared to missense mutations. These results support the hypothesis that disruption of the BMP signaling pathway is the likely etiology of JP in patients with SMAD4 mutations.