Rhabdomyolysis: pathogenic mechanisms of lipin-1-dependent disease in human and animal for therapeutic perspectives – RHAB

Lipin-1 defect is responsible for massive rhabdomyolysis episodes in children. The physiopathology of this disease is not elucidated yet. Lipin-1 is most abundantly expressed in adipocytes and skeletal muscle and plays a dual role, i) a phosphatidate phosphatase 1 for triacylglycerol and phospholipid biosynthesis and ii) a transcriptional co-activator associating with PPARa, SREBP1 and PGC-1a, to regulate the expression of genes encoding fatty acid synthesis and oxidation (FAO) in response to nutritional cues and the activation of the mTOR pathway. The lipin-1 mutant animals (mice and rats) are characterized by lipodystrophy, hypertriglyceridemia and peripheral neuropathy while muscle symptoms are predominant in human. The myolysis episodes appear mostly precipitated by febrile illnesses, consistent with the hypothesis that environment, mainly inflammatory stress, plays an important role in this disease. Currently medical management and research on lipin-1 deficiency faces severe drawbacks. First, the function of lipin-1 in skeletal muscle growth and metabolism is largely unknown. Second, in vivo studies in animal models developing rhabdomyolysis episodes are missing. Third, emergency management of rhabdomyolysis is the sole therapy currently available.
We (partner #1, P de Lonlay) collected muscle biopsies and/or DNA from a cohort of ~200 patients presenting with rhabdomyolysis of unknown origin. We identified LPIN1 mutations in 40 patients and showed that lipin-1 content and activity in primary myoblast cultures derived from several patients were dramatically reduced. These myoblasts display increased lipid droplets (LD) content further enhanced by proinflammatory treatments suggesting that lipin-1 contributes to the inflammatory response.
We (partner #2: K Nadra, M Pende) and others have generated animal models of lipin-1 deficiency and described the phenotype in Schwann cells and adipocytes. Interestingly lipodystrophy, multilocular lipid droplets and the absence of differentiation of preadipocytes are associated with decreased protein levels of PPAR-regulated genes and FAO activity. Human adipose tissue has not been explored in detail and little is known on lipin functions in human adipocytes so far, as lipodystrophy is negligible in children patients. In addition, skeletal muscle pathophysiology in animal models remains to be investigated.
The complex phenotypes presented by LPIN1 deficient animals and humans and the multiple roles of lipin-1 are likely to reflect an intricated interaction between metabolism, growth, inflammation and multiple tissues. Several mechanisms leading to rhabdomyolysis, alone or combined, may be considered. Our research network combines medical research and fundamental approaches on these different aspects, using powerful tools of human and mouse/rat genetics, with the final goal to understand and cure a severe genetic disease.
The tasks are: 1) To analyse the functional defects of muscle and adipose tissues from lipin-1 deficient patients and animals by compared histology, differentiation in culture, functional assays and transcriptomic studies; 2) To induce rhabdomyolysis signs in animal models and human cells, namely by using inflammatory responses, drugs or diets potentially triggering rhabdomyolysis, followed by histological analysis and proteins and transcriptional studies to determine the inflammatory signature and test potential therapies; 3) To determine the role of the mTOR pathway in the onset of the disease by rescuing lipin-1 expression with mutants that cannot be phosphorylated by mTOR; and 4) Finally to identify new genetic origins of rhabdomyolysis through exome sequencing of patient DNA and compare the pathogenic mechanisms with those of lipin-1-dependent rhabdomyolysis, paying special attention to the putative relationship with inflammatory responses as rhabdomyolysis triggers of these patients are similar.