A Hypomorphic Mutation in Lpin1 Induces Progressively Improving Neuropathy and Lipodystrophy in the Rat

Joram D Mul,Karim Nadra,Noorjahan B Jagalur,Isaac J Nijman,Pim W Toonen,Jean-Jacques Médard,Sandra Grès,Alain De Bruin,Gil-Soo Han,Jos F Brouwers,George M Carman,Jean-Sébastien Saulnier-Blache,Dies Meijer,Roman Chrast,Edwin Cuppen

doi:10.1074/jbc.m110.197947

Abstract

The Lpin1 gene encodes the phosphatidate phosphatase (PAP1) enzyme Lipin 1, which plays a critical role in lipid metabolism. In this study we describe the identification and characterization of a rat model with a mutated Lpin1 gene (Lpin1(1Hubr)), generated by N-ethyl-N-nitrosourea mutagenesis. Lpin1(1Hubr) rats are characterized by hindlimb paralysis and mild lipodystrophy that are detectable from the second postnatal week. Sequencing of Lpin1 identified a point mutation in the 5'-end splice site of intron 18 resulting in mis-splicing, a reading frameshift, and a premature stop codon. As this mutation does not induce nonsense-mediated decay, it allows the production of a truncated Lipin 1 protein lacking PAP1 activity. Lpin1(1Hubr) rats developed hypomyelination and mild lipodystrophy rather than the pronounced demyelination and adipocyte defects characteristic of Lpin1(fld/fld) mice, which carry a null allele for Lpin1. Furthermore, biochemical, histological, and molecular analyses revealed that these lesions improve in older Lpin1(1Hubr) rats as compared with young Lpin1(1Hubr) rats and Lpin1(fld/fld) mice. We observed activation of compensatory biochemical pathways substituting for missing PAP1 activity that, in combination with a possible non-enzymatic Lipin 1 function residing outside of its PAP1 domain, may contribute to the less severe phenotypes observed in Lpin1(1Hubr) rats as compared with Lpin1(fld/fld) mice. Although we are cautious in making a direct parallel between the presented rodent model and human disease, our data may provide new insight into the pathogenicity of recently identified human LPIN1 mutations.

Highlights

Molecular Pathology Center, Faculty of Veterinary Medicine, Utrecht University, 3584 CL Utrecht, The Netherlands, the ‡‡Department of Food Science and Rutgers Center for Lipid Research, Rutgers University, New Brunswick, New Jersey 08901, the §§Department of Biochemistry and Cell Biology, Faculty of Veterinary Medicine, Utrecht University, 3508 TC Utrecht, The Netherlands, and the ¶¶Department of Medical Genetics, University Medical Center Utrecht, 3584 CX Utrecht, The Netherlands
Previous studies have shown that Lipin 1 is a Mg2ϩ-dependent phosphatidate phosphatase (PAP1)5 enzyme catalyzing the dephosphorylation of phosphatidic acid (PA), yielding inorganic phosphate and diacylglycerol needed for the synthesis of triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine (5– 8)
On PND 21, DNA was collected from wild-type/heterozygous and mutant Wistar/BN F5 rats, and a forward screen was performed using a Wistar/BN-specific SNP marker panel (17)

Summary

Introduction

Molecular Pathology Center, Faculty of Veterinary Medicine, Utrecht University, 3584 CL Utrecht, The Netherlands, the ‡‡Department of Food Science and Rutgers Center for Lipid Research, Rutgers University, New Brunswick, New Jersey 08901, the §§Department of Biochemistry and Cell Biology, Faculty of Veterinary Medicine, Utrecht University, 3508 TC Utrecht, The Netherlands, and the ¶¶Department of Medical Genetics, University Medical Center Utrecht, 3584 CX Utrecht, The Netherlands. D, relative expression of Lpin1␣ and Lpin1␤ is increased in sciatic nerve tissue of Lpin11Hubr rats as compared with wild-type rats at PND 21.

Results

Conclusion