Weight loss allows the dissection of the interaction between abdominal fat and PNPLA3 (adiponutrin) in the liver damage of obese children
Authors: Pierluigi Marzuillo, Anna Grandone, Laura Perrone, Emanuele Miraglia del Giudice
Journal: Journal of Hepatology (2013)

In the patatin-like phospholipase domain-containing 3 gene (PNPLA3), there is a polymorphism named rs738409, which encodes for the adiponutrin I148M variant and is a genetic determinant of liver fat and non-alcoholic fatty liver disease (NAFLD). Many works support the correlation among the PNPLA3 rs738409 polymorphism, ALT serum levels and hepatic steatosis. A recent study has demonstrated that this variant is associated with increased levels of liver enzymes and with liver steatosis in obese children and adolescents, suggesting that it confers genetic susceptibility to liver damage from a young age [[1]Romeo S. Sentinelli F. Cambuli V.M. Incani M. Congiu T. Matta V. et al.The 148M allele of the PNPLA3 gene is associated with indices of liver damage early in life.J Hepatol. 2010; 53: 335-338Abstract Full Text Full Text PDF PubMed Scopus (129) Google Scholar]. We have shown that in obese children, the magnitude of the association of PNPLA3 with liver enzymes is driven by the size of abdominal fat expressed as waist to height ratio (W/Hr) and we stratified the individual risk of developing liver damage on the basis of the interaction between the PNPLA3 genotypes and waist circumference [[2]Miraglia del Giudice E. Grandone A. Cirillo G. Santoro N. Amato A. Brienza C. et al.The association of PNPLA3 variants with liver enzymes in childhood obesity is driven by the interaction with abdominal fat.PLoS One. 2011; 6: e27933Crossref PubMed Scopus (77) Google Scholar]. Furthermore, the interplay between visceral fat and PNPLA3 in producing liver damage has recently been confirmed in adults [[3]Graff M. North K.E. Franceschini N. Reiner A.P. Feitosa M. Carr J.J. et al.PNPLA3 gene-by-visceral adipose tissue volume interaction and the pathogenesis of fatty liver disease: the NHLBI family heart study.Int J Obes (Lond). 2013; 37: 432-438Crossref PubMed Scopus (42) Google Scholar] and patients with chronic hepatitis C [[4]Zampino R. Coppola N. Cirillo G. Boemio A. Pisaturo M. Marrone A. et al.Abdominal fat interacts with PNPLA3 I148M, but not with the APOC3 variant in the pathogenesis of liver steatosis in chronic hepatitis C.J Viral Hepat. 2013; 20: 517-523Crossref PubMed Scopus (51) Google Scholar]. In addition to PNPLA3, a number of predisposing genetic polymorphisms have been associated with NAFLD [[5]Adams L.A. White S.W. Marsh J.A. Lye S.J. Connor K.L. Maganga R. et al.Association between liver-specific gene polymorphisms and their expression levels with nonalcoholic fatty liver disease.Hepatology. 2013; 57: 590-600Crossref PubMed Scopus (66) Google Scholar]. In order to dissect the interaction between abdominal fat and PNPLA3, we studied 129 obese children before and after a weight loss program. The frequency of the distribution of the different PNPLA3 rs738409 genotypes was in Hardy-Weinberg equilibrium (p >0.05). Fifty-three patients were homozygous for the wild type allele (CC) while 51 were heterozygous (CG) and 25 were homozygous for the minor allele (GG). The frequency of the PNPLA3 minor allele (148M) was 0.30, which is in line with the frequencies reported in obese subjects belonging to the same geographic area [[1]Romeo S. Sentinelli F. Cambuli V.M. Incani M. Congiu T. Matta V. et al.The 148M allele of the PNPLA3 gene is associated with indices of liver damage early in life.J Hepatol. 2010; 53: 335-338Abstract Full Text Full Text PDF PubMed Scopus (129) Google Scholar]. By analysing the children on the basis of their PNPLA3 genotypes, we observed that before the weight loss program, ALT and AST levels in the serum increased significantly in the children carrying the 148M allele (p = 0.01 and p = 0.02, respectively), adjusting for age, gender, pubertal stage, and BMI-SDS. Of 148M homozygotes, 24% had ALT levels above 40 IU/L compared to 16% of heterozygous and 9% of homozygous for the 148I major allele (p = 0.02). The prevalence of steatosis was also different according to the PNPLA3 genotypes, with 17% of obese children 148I homozygotes showing steatosis, 33% of heterozygotes and 76% of 148M homozygotes (p = 0.01), adjusting for age, gender, pubertal stage, and BMI-SDS. After weight loss, all children reduced their BMI SDS by at least 0.5. Reduction in BMI SDS and W/Hr were not statistically different among PNPLA3 genotypes. The group of homozygous subjects for the minor allele (148M) presented a greater reduction in serum ALT concentrations (Δ ALT) and in the prevalence of steatosis compared with the group of heterozygous or homozygous subjects for the major allele (p = 0.02 for ALT and p = 0.01 for steatosis). Moreover, after weight loss, no differences among PNPLA3 genotypes were found in ALT levels or in the prevalence of steatosis (p = 0.1 and p = 0.2, respectively). Both the relationships between ΔBMI SDS and ΔALT and between ΔW/Hr and ΔALT were analysed, based on the different PNPLA3 genotypes, and the slope of the relative regression lines were compared (Fig. 1). Homozygous children for the PNPLA3 minor allele (148M) showed a stronger correlation between ΔALT and ΔW/Hr than those carrying the other genotypes, as we observed in the comparison of regression lines (p = 0.002) (Fig. 1A). The same comparison performed for ΔBMI SDS was not significant (p = 0.38) (Fig. 1B). These results support the idea that it is the abdominal fat (as expressed by W/Hr) more than the overall adiposity (expressed by BMI), the external (not genetic) factor which modulates the effect of PNPLA3 148M allele on liver damage. Exploring whether the PNPLA3 148M allele influences the ability of weight loss to decrease liver fat, Sevastianova et al. [[6]Sevastianova K. Kotronen A. Gastaldelli A. Perttilä J. Hakkarainen A. Lundbom J. et al.Genetic variation in PNPLA3 (adiponutrin) confers sensitivity to weight loss-induced decrease in liver fat in humans.Am J Clin Nutr. 2011; 94: 104-111Crossref PubMed Scopus (107) Google Scholar] investigated 18 subjects placed on a hypocaloric low-carbohydrate diet for 6 days and demonstrated that weight loss is an effective way to decrease liver fat, irrespective of the PNPLA3 genotype. Indeed, short term weight loss decreases liver fat content more in homozygous carriers of PNPLA3 148M allele than in those carrying the 148I allele. These results are in agreement with our data. The mutant PNPLA3 148M allele is partially unable to hydrolyse intra-hepatocytic triglycerides [7He S. McPhaul C. Li J.Z. Garuti R. Kinch L. Grishin N.V. et al.A sequence variation (I148M) in PNPLA3 associated with nonalcholic fatty liver disease disrupts triglyceride hydrolysis.J Biol Chem. 2012; 285: 6706-6715Crossref Scopus (464) Google Scholar, 8Huang Y. Cohen J.C. Hobbs H.H. Expression and characterization of a PNPLA3 protein isoform (I148M) associated with nonalcoholic fatty liver disease.J Biol Chem. 2011; 286: 37085-37093Crossref PubMed Scopus (203) Google Scholar], which increases the risk of liver steatosis. Studies have also shown that there is about a three-fold increase of PNPLA3 expression in obese human subjects after the completion of a weight loss program [[9]Johansson L.E. Danielsson A.P. Parikh H. Klintenberg M. Norström F. Groop L. et al.Differential gene expression in adipose tissue from obese human subjects during weight loss and weight maintenance.Am J Clin Nutr. 2012; 96: 196-207Crossref PubMed Scopus (79) Google Scholar]. It is possible, therefore, that the interaction between adiposity and PNPLA3 may well be modulated by the effect of adipose tissue changes on PNPLA3 expression. An important clinical implication of the present study is that obese children with fat liver carrying the PNPLA3 148M allele are the patients who would most benefit from weight loss. The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.