Evidence against a blood derived origin for transforming growth factor beta induced protein in corneal disorders caused by mutations in the TGFBI gene

Several inherited corneal disorders in humans result from mutations in the transforming growth factor beta induced gene (TGFBI), which encodes for the extracellular transforming growth factor beta induced protein (TGFBIp) that is one of the most abundant proteins in the cornea. We previously reported a significant amount of TGFBIp in plasma by immunoblotting using the only TGFBIp antiserum (anti-p68βig-h3) available at that time (anti-p68βig-h3 was generated against residues Val210-His683 of TGFBIp).

This observation raised the possibility that a fraction of corneal TGFBIp may originate from the plasma. However, recent experiments in our laboratory indicated that the anti-p68βig-h3 antiserum cross-reacts with an environmental protein contaminant. Therefore, we investigated the specificity of the originally utilized anti-p68βig-h3 antiserum and re-evaluated the amount of TGFBIp in human plasma by immunoblotting using a new specific antiserum.

The observed cross-reactivity of the previously utilized anti-p68βig-h3 antiserum was tested by immunoblotting and the antigen identity was determined by mass spectrometry. A part of human TGFBI encoding an NH2-terminal 11.4 kDa fragment of TGFBIp (residues Gly134-Ile236) was amplified by polymerase chain reaction (PCR) and cloned in E. coli.

The TGFBIp fragment was expressed in E. coli, purified by Ni2+-affinity chromatography, and used to immunize rabbits to produce a specific antiserum (anti-TGFBIp134-236). To enhance the detection of possible TGFBIp in plasma by allowing a higher sample load, albumin and immunoglobulin G (IgG) were specifically depleted from normal human plasma by affinity chromatography.

The presence of TGFBIp in plasma was investigated by immunoblotting using the anti-TGFBIp134-236 antiserum. Purified TGFBIp from porcine corneas was used for estimation of the TGFBIp detection limit. The previously utilized TGFBIp antiserum, anti-p68βig-h3, cross-reacted with human keratin-1, a common environmental protein contaminant.

Thus, the anti-p68βig-h3 antiserum recognizes both TGFBIp and keratin-1. In contrast, the anti-TGFBIp134-236 antiserum reacted with TGFBIp but showed no indication of reactivity with other proteins in plasma. Using this antiserum, TGFBIp was not detected in crude or albumin/IgG-depleted human plasma and the detection limit of TGFBIp using immunoblotting was estimated to be 10 ng.

Our failure to detect TGFBIp in human plasma using a highly specific antiserum suggests that TGFBIp is not present in a physiologically relevant concentration in human plasma. The previous impression that normal human plasma contains a significant amount of TGFBIp by immunoblotting was due to the utilization of a less specific antiserum that recognizes both TGFBIp and human keratin-1.

Together with other results, our observation makes it unlikely that TGFBIp is imported into the cornea from the circulation as reported for other abundant extracellular corneal proteins and suggests corneal origin of TGFBIp deposits in individuals with inherited corneal diseases caused by mutations in the TGFBI gene.