The novel loss of function Ile354Val mutation in PPARG causes familial partial lipodystrophy.

AIMS: Familial partial lipodystrophy (FPLD) is a rare autosomal dominant disorder, mostly due to mutations in lamin A (LMNA) or in peroxisome proliferator-activated receptor gamma (PPARG) genes. In the present study, we aimed to identify and functionally characterize the genetic defect underlying FPLD in an Italian family presenting with several affected individuals in three consecutive generations. METHODS: Mutational screening by direct Sanger sequencing has been carried out on both LMNA and PPARG genes. In silico analyses and functional in vitro studies on transfected cell lines have been also performed to evaluate the biological impact of the identified mutation. RESULTS: We identified a novel PPARG missense mutation (i.e., PPARγ2 Ile354Val) segregating with FPLD in the study family. In silico analyses and in vitro experiments showed that probably altering the PPARγ2 ligand binding domain conformation, the Ile354Val aminoacid change leads to a significant reduction (i.e., ~ 30-35%) of transcriptional activity in the mutant receptor, with no evidences of a dominant negative effect on the wild-type receptor. CONCLUSIONS: Our present data extend the spectrum of PPARG mutations responsible for FPLD3 and reinforce the notion that even loss of function mutations affecting transcriptional activity to an extent lower than that observed in the case of haploinsufficiency are able to cause a severe FPLD3 phenotype.

ENPP1 affects insulin action and secretion: evidences from in vitro studies.

The aim of this study was to deeper investigate the mechanisms through which ENPP1, a negative modulator of insulin receptor (IR) activation, plays a role on insulin signaling, insulin secretion and eventually glucose metabolism. ENPP1 cDNA (carrying either K121 or Q121 variant) was transfected in HepG2 liver-, L6 skeletal muscle- and INS1E beta-cells. Insulin-induced IR-autophosphorylation (HepG2, L6, INS1E), Akt-Ser(473), ERK1/2-Thr(202)/Tyr(204) and GSK3-beta Ser(9) phosphorylation (HepG2, L6), PEPCK mRNA levels (HepG2) and 2-deoxy-D-glucose uptake (L6) was studied. GLUT 4 mRNA (L6), insulin secretion and caspase-3 activation (INS1E) were also investigated. Insulin-induced IR-autophosphorylation was decreased in HepG2-K, L6-K, INS1E-K (20%, 52% and 11% reduction vs. untransfected cells) and twice as much in HepG2-Q, L6-Q, INS1E-Q (44%, 92% and 30%). Similar data were obtained with Akt-Ser(473), ERK1/2-Thr(202)/Tyr(204) and GSK3-beta Ser(9) in HepG2 and L6. Insulin-induced reduction of PEPCK mRNA was progressively lower in untransfected, HepG2-K and HepG2-Q cells (65%, 54%, 23%). Insulin-induced glucose uptake in untransfected L6 (60% increase over basal), was totally abolished in L6-K and L6-Q cells. GLUT 4 mRNA was slightly reduced in L6-K and twice as much in L6-Q (13% and 25% reduction vs. untransfected cells). Glucose-induced insulin secretion was 60% reduced in INS1E-K and almost abolished in INS1E-Q. Serum deficiency activated caspase-3 by two, three and four folds in untransfected INS1E, INS1E-K and INS1E-Q. Glyburide-induced insulin secretion was reduced by 50% in isolated human islets from homozygous QQ donors as compared to those from KK and KQ individuals. Our data clearly indicate that ENPP1, especially when the Q121 variant is operating, affects insulin signaling and glucose metabolism in skeletal muscle- and liver-cells and both function and survival of insulin secreting beta-cells, thus representing a strong pathogenic factor predisposing to insulin resistance, defective insulin secretion and glucose metabolism abnormalities.

Role of the ENPP1 K121Q polymorphism in glucose homeostasis.

OBJECTIVE: To study the role of the ENPP1 Q121 variant on glucose homeostasis in whites from Italy. RESEARCH DESIGN AND METHODS: We conducted case-control studies in 764 adults (from two independent samples of 289 nonobese and 485 obese individuals) and 240 overweight/obese children undergoing oral glucose tolerance testing (OGTT). Early-phase insulin secretion and insulin sensitivity (the insulinogenic index and the insulin sensitivity index) and their interplay (the disposition index) were calculated. RESULTS: In adult subjects, glucose profiles during OGTT were significantly (P = 2 x 10(-2)) different across K121Q genotype groups and higher in QQ than KK individuals (P = 5 x 10(-2)). The insulinogenic index was significantly reduced in QQ (18.5 +/- 3.4) compared with both KK (31.6 +/- 1.0; P = 2.2 x 10(-7)) and KQ (30.5 +/- 1.5; P = 3.2 x 10(-6)) individuals. KQ individuals also showed a reduced insulin sensitivity index compared with KK subjects (P = 3.6 x 10(-2)). The disposition index was lower in QQ carriers than in KQ and KK individuals (P = 8 x 10(-3) and 4 x 10(-4), respectively) and lower in KQ than in KK individuals (P = 3 x 10(-2)). Data obtained in overweight/obese children were very similar to those observed in adults, with QQ individuals showing (compared with KQ and KK subjects) a reduced insulinogenic index (P = 7 x 10(-3) and 2 x 10(-2), respectively) and disposition index (P = 2 x 10(-2) and 7 x 10(-3), respectively). CONCLUSIONS: Homozygous carriers of the ENPP1 Q121 variant are characterized by an altered glucose homeostasis. Reduced early-phase insulin secretion and inefficient interplay between insulin secretion and sensitivity, which occur at early ages, are major determinants of this defect.