Fm. Sladek et al., MODY1 MUTATION Q268X IN HEPATOCYTE NUCLEAR FACTOR 4-ALPHA ALLOWS FOR DIMERIZATION IN SOLUTION BUT CAUSES ABNORMAL SUBCELLULAR-LOCALIZATION, Diabetes, 47(6), 1998, pp. 985-990
Recent studies have shown that mutations in the hepatocyte nuclear fac
tor (HNF)-4 alpha gene give rise to maturity-onset diabetes of the you
ng, type 1 (MODY1). HNF-4, an orphan member of the nuclear receptor su
perfamily, contains a DNA-binding domain (DBD) and a putative ligand-b
inding domain (LBD) that can act independently of each other. The firs
t MODY1 mutation identified creates a stop codon at amino acid 268 in
the LED of HNF-4 (Q268X) that leaves the DBD intact, suggesting that t
he mutant protein may retain some of the properties of the wild-type p
rotein. To determine the functional properties of this mutant, we cons
tructed HNF4.Q268X and tested it in vitro and in vivo for DNA binding,
protein dimerization, and transactivation activity. Results of an ele
ctrophoretic mobility shift assay showed that HNF4.Q268X neither binds
DNA alone nor binds it as a dimer with wild-type HNF-4 (HNF4.wt). In
contrast, a co-immunoprecipitation assay showed that HNF4.Q268X is cap
able of dimerizing in solution with HNF4.wt. Transient transfection as
says, however, indicated that HNF4.Q268X does not affect transactivati
on by HNF4.wt in vivo, supporting the argument against a dominant nega
tive effect. Additional results suggest that the lack of a dominant ne
gative effect could be due to a striking differential subcellular loca
lization of the HNF4.Q268X protein: HNF4.Q268X could be extracted from
transfected cells only when treated with SDS. Taken together, our res
ults suggest that the MODY1 phenotype is due to a loss of functional H
NF-4 protein that is aggravated in tissues that express relatively low
amounts of HNF-4, such as pancreas.