Transglutaminase 2 (TG2 or TGM2) is a multi-functional enzyme which catalyzes transamidation reactions or acts as a G-protein in intracellular signalling. Tgm2 -/-Mice lacking TG2 activity are glucose intolerant and show impairment of insulin secretion, suggesting an important physiological role for TG2 in the pancreatic β cell. We have previously described a TGM2 heterozygous missense mutation ((c.998A>G, p.N333S) in a 14 year-old patient with insulin-treated diabetes and in his diabetic father. The aim of this study was to further investigate the role of TG2 in early-onset type 2 diabetes. We analysed the TGM2 gene in 205 patients with clinically defined Maturity Onset Diabetes of the Young (MODY) or early-onset type 2 diabetes. We found two novel heterozygous mutations (c.989T>G, p.M330R; c.992T>A, p.I331N), which were not detected in 300 normoglycemic controls. All mutations were in residues which are located close to the catalytic site and impaired transamidating activity in vitro. Gene expression of TGM family genes and localization of TG2 in normal human pancreas indicated that TG2 is the only transglutaminase significantly expressed in human pancreatic islet cells. We conclude that reduced TG2 activity can contribute to disorders of glucose metabolism possibly via an impairment of insulin secretion. © 2007 Wiley-Liss, Inc.
INTRODUCTIONTransglutaminase 2 (TG2; EC 2.3.2.13) is an ubiquitous enzyme with two well established, reciprocally regulated, activities: it cross-links proteins by virtue of its transamidating activity and acts as a G-protein in intracellular signalling (Griffin et al., 2002;Lorand and Graham, 2003). TG2 has been involved in several cellular processes such as cell adhesion and cell death, but its physiological role is still a matter of debate (Griffin et al., 2002;Lorand and Graham, 2003). In order to shed light on the function of TG2 in mammals, TG2 knock-out mice (Tgm2 -/-) have been generated (Bernassola et al., 2002;Nanda et al., 2001). Surprisingly, mice lacking TG2 activity do not show any major defects in apoptosis, but rather are glucose intolerant and show impairment of insulin secretion, suggesting a physiological role for TG2 in the pancreatic β cell (Bernassola et al., 2002). This is in keeping with previous studies showing that agents known to inhibit transglutaminase activity (e.g. monodansylcadaverine) can decrease glucose-stimulated insulin secretion from rat pancreatic islets in a dose-related manner (Sener et al., 1985, Bungay et al., 1986. In addition, we have described a TG2 heterozygous missense mutation p.N333S (c.998A>G) located in the TG2 catalytic site in a family clinically classified as maturity onset diabetes of the young (MODY; MIM# 606391) (Fig. 1A) (Bernassola et al., 2002). Thus, the aim of this study was to further investigate whether additional mutations in TG2 were also associated with early-onset type 2 diabetes (MIM# 125853).
SUBJECTS AND METHODS
Families with maturity onset diabetes of the young (MODY) or early onset type 2 diabetesWe studied the ...