Abstract 845: Novel Extracellular Domain Mutations of TGFBRII Cause Aortic Aneurysms
TGF-B signaling via its type I and II receptors plays a critical role in diverse vascular cell processes. Activating mutations in the kinase domain of both TGFBRI and TGFBRII have been implicated in a complex connective tissue disorder that can include aortic aneurysms. A cohort of 100 probands with potentially genetically triggered thoracic aortic aneurysms/dissections but without known connective tissue disorders was screened for TGFBRI and TGFBRII mutations. No TGFBRI mutations were found. Only 1 proband was found to have a TGFBRII mutation, and this mutation localized in the receptor’s kinase domain, as do previously reported mutations. The cohort was then screened for mutations in a little understood alternatively spliced genomic segment of TGFBRII (segment 1a) which encodes a 25 amino acid region in the extracellular domain and which has been proposed to modulate receptor affinity for TGF-B2. Two probands were found to have 2 different 1a nonconservative variants (H56N, D40N); neither was found in ethnically matched control populations, and each co-segregated with aortic disease in families. Therefore, we concluded that H56N and D40N were disease-causing mutations. QRT-PCR revealed that the 1a isoform is expressed in the aortic wall and represents approximately 6% of the total TGFBRII mRNA. To determine activities of mutant compared with wildtype receptors, we expressed wildtype and mutant TGFBRII proteins in TGFBRII null DR26 cells and analyzed their activities using a p3TP-lux luciferase reporter construct. We confirmed previous findings that mutation of the TGFBRII kinase domain ablates receptor activity. Novel investigation, though, of the 1a domain demonstrates that 1a acts to amplify wildtype TGFBRII signaling. However, introduction of the H56N mutation into 1a ablates the amplification and reduces activity of the TGFBRII receptor to that of the wildtype isoform without 1a. We conclude that TGFB receptor mediated pathways to aneurysm pathogenesis can specifically modulate amplified TGFBRII signaling via TGF-B2. Pathogenetic models altering the TGFBRII extracellular domain ability to modify the receptor affinity for TGF-B2 merit further investigation to determine mechanisms of aortic aneurysm formation.