H. Tomura et al., Loss-of-function and dominant-negative mechanisms associated with hepatocyte nuclear factor-1 beta mutations in familial type 2 diabetes mellitus, J BIOL CHEM, 274(19), 1999, pp. 12975-12978
Hepatocyte nuclear factor (HNF)-1 beta, a homeodomain-containing transcript
ion factor, regulates gene expression in a dimerized form in pancreas, live
r, and some other tissues. Recent genetic studies have identified two HNF-1
beta mutations, R177X and A263fsinsGG, in subjects with a monogenic form o
f type 2 diabetes. Despite the defects being in the same gene, diverse seve
rities of disease are observed in the affected subjects. To investigate the
molecular mechanism by which mutations might cause various phenotypic feat
ures, wild type and mutant proteins were transiently expressed in insulin-p
roducing (MIN6) and hepatic (HepG2) cells. Luciferase reporter assay showed
that both mutations resulted in a marked reduction of transactivation acti
vity. Because their dimerization activity was found to be intact by the yea
st two-hybrid system, it was possible that they were dominant-negative to w
ild type activity. When co-expressed with wild type, both of the mutants si
gnificantly decreased wild type activity in HepG2 cells. In contrast, altho
ugh A263fsinsGG functioned similarly in MIN6 cells, R177X failed to affect
wild type activity in this cell line. Immunohistochemical analysis of the m
utants suggests that this functional divergence might be generated by the m
odification of nuclear localization. These results suggest that HNF-1 beta
mutations may impair pancreatic beta-cell function by loss-of-function and
dominant-negative mechanisms.