Muriel Girard

miR-122, a paradigm for the role of microRNAs in the liver

Recent studies have uncovered profound and unexpected roles for a family of tiny regulatory RNAs, known as microRNAs (miRNAs), in the control of diverse aspects of hepatic function and dysfunction, including hepatocyte growth, stress response, metabolism, viral infection and proliferation, gene expression, and maintenance of hepatic phenotype. In liver cancer, misexpression of specific miRNAs suggests diagnostic and prognostic significance. Here, we review the biology of the most abundant miRNA in human liver, miR-122, and consider the diversity of its roles in the liver. We provide a compilation of all miRNAs expressed in the liver, and consider some possible therapeutic opportunities for exploiting miRNAs in the different settings of liver diseases.

Nuclear outsourcing of RNA interference components to human mitochondria.

MicroRNAs (miRNAs) are small non-coding RNAs that associate with Argonaute proteins to regulate gene expression at the post-transcriptional level in the cytoplasm. However, recent studies have reported that some miRNAs localize to and function in other cellular compartments. Mitochondria harbour their own genetic system that may be a potential site for miRNA mediated post-transcriptional regulation. We aimed at investigating whether nuclear-encoded miRNAs can localize to and function in human mitochondria. To enable identification of mitochondrial-enriched miRNAs, we profiled the mitochondrial and cytosolic RNA fractions from the same HeLa cells by miRNA microarray analysis. Mitochondria were purified using a combination of cell fractionation and immunoisolation, and assessed for the lack of protein and RNA contaminants. We found 57 miRNAs differentially expressed in HeLa mitochondria and cytosol. Of these 57, a signature of 13 nuclear-encoded miRNAs was reproducibly enriched in mitochondrial RNA and validated by RT-PCR for hsa-miR-494, hsa-miR-1275 and hsa-miR-1974. The significance of their mitochondrial localization was investigated by characterizing their genomic context, cross-species conservation and instrinsic features such as their size and thermodynamic parameters. Interestingly, the specificities of mitochondrial versus cytosolic miRNAs were underlined by significantly different structural and thermodynamic parameters. Computational targeting analysis of most mitochondrial miRNAs revealed not only nuclear but also mitochondrial-encoded targets. The functional relevance of miRNAs in mitochondria was supported by the finding of Argonaute 2 localization to mitochondria revealed by immunoblotting and confocal microscopy, and further validated by the co-immunoprecipitation of the mitochondrial transcript COX3. This study provides the first comprehensive view of the localization of RNA interference components to the mitochondria. Our data outline the molecular bases for a novel layer of crosstalk between nucleus and mitochondria through a specific subset of human miRNAs that we termed ‘mitomiRs’.

Adams–Oliver syndrome and hepatoportal sclerosis: Occasional association or common mechanism?

Adams–Oliver syndrome (AOS) is characterized by the association of scalp and skull defects and abnormalities of terminal limbs. Congenital heart malformations have also been reported. Hepatoportal sclerosis (HPS) is a rare cause of portal hypertension in children characterized by abnormalities of intra‐hepatic portal veins, portal fibrosis, and nodular regeneration. Etiopathogenesis of these rare disorders remains unclear, but the hypothesis of vascular thrombotic mechanism has been suggested. Association of both syndromes has been reported in only one child. We now report on two unrelated children with AOS and HPS, one child harboring a factor V Leiden mutation. We hypothesize that the association of both disorders may not be fortuitous and reinforces the idea that AOS and HPS may share a vascular thrombotic mechanism.

Mutation in a primate-conserved retrotransposon reveals a noncoding RNA as a mediator of infantile encephalopathy

The human genome is densely populated with transposons and transposon-like repetitive elements. Although the impact of these transposons and elements on human genome evolution is recognized, the significance of subtle variations in their sequence remains mostly unexplored. Here we report homozygosity mapping of an infantile neurodegenerative disease locus in a genetic isolate. Complete DNA sequencing of the 400-kb linkage locus revealed a point mutation in a primate-specific retrotransposon that was transcribed as part of a unique noncoding RNA, which was expressed in the brain. In vitro knockdown of this RNA increased neuronal apoptosis, consistent with the inappropriate dosage of this RNA in vivo and with the phenotype. Moreover, structural analysis of the sequence revealed a small RNA-like hairpin that was consistent with the putative gain of a functional site when mutated. We show here that a mutation in a unique transposable element-containing RNA is associated with lethal encephalopathy, and we suggest that RNAs that harbor evolutionarily recent repetitive elements may play important roles in human brain development.

MYO5B and bile salt export pump contribute to cholestatic liver disorder in microvillous inclusion disease

Microvillous inclusion disease (MVID) is a congenital disorder of the enterocyte related to mutations in the MYO5B gene, leading to intractable diarrhea often necessitating intestinal transplantation (ITx). Among our cohort of 28 MVID patients, 8 developed a cholestatic liver disease akin to progressive familial intrahepatic cholestasis (PFIC). Our aim was to investigate the mechanisms by which MYO5B mutations affect hepatic biliary function and lead to cholestasis in MVID patients. Clinical and biological features and outcome were reviewed. Pretransplant liver biopsies were analyzed by immunostaining and electron microscopy. Cholestasis occurred before (n = 5) or after (n = 3) ITx and was characterized by intermittent jaundice, intractable pruritus, increased serum bile acid (BA) levels, and normal gamma‐glutamyl transpeptidase activity. Liver histology showed canalicular cholestasis, mild‐to‐moderate fibrosis, and ultrastructural abnormalities of bile canaliculi. Portal fibrosis progressed in 5 patients. No mutation in ABCB11/BSEP or ATP8B1/FIC1 genes were identified. Immunohistochemical studies demonstrated abnormal cytoplasmic distribution of MYO5B, RAB11A, and BSEP in hepatocytes. Interruption of enterohepatic BA cycling after partial external biliary diversion or graft removal proved the most effective to ensure long‐term remission. Conclusion: MVID patients are at risk of developing a PFIC‐like liver disease that may hamper outcome after ITx. Our results suggest that cholestasis in MVID patients results from (1) impairment of the MYO5B/RAB11A apical recycling endosome pathway in hepatocytes, (2) altered targeting of BSEP to the canalicular membrane, and (3) increased ileal BA absorption. Because cholestasis worsens after ITx, indication of a combined liver ITx should be discussed in MVID patients with severe cholestasis. Future studies will need to address more specifically the effect of MYO5B dysfunction in BA homeostasis. (Hepatology 2014;60:301–310)

Neonatal ichthyosis and sclerosing cholangitis syndrome: extremely variable liver disease severity from claudin-1 deficiency.

350 T ight junctions between hepatocytes and cholangiocytes play a fundamental role in separating bile flow from plasma. Secondary alterations of tight junctions are well described in many cholestatic disorders inducing an increase in paracellular permeability and subsequent liver damage (1). The core proteins of tight junctions are composed of strands of claudins and occludin. Claudin-1 is a member of the claudin family expressed in liver and skin and mapping on chromosome 3q27-q28. The CLDN1 gene homozygous mutation was shown to cause a rare autosomal recessive syndrome associating neonatal ichthyosis to sclerosing cholangitis (NISCH syndrome) and first described in 2 unrelated Moroccan families (2,3). Only 8 patients, from 4 different families, have been described as affected by this syndrome so far (2–5). They presented with ichthyosis and neonatal cholestatic jaundice. Cholestasis varied from a transient remitting form to more severe fibrogenic conditions (2–5). Two patients required liver transplantation (3,5). One of them showed regression of skin lesions and alopecia after the transplant (3). We describe here 4 new patients from an inbred family of Moroccan origins, all presenting with the clinical features of NISCH syndrome and a mutation of the CLDN1 gene. In all patients, other causes of neonatal cholestasis were excluded (6,7). Table 1 summarizes characteristics of patients described here and of previously reported patients (2–5).

Specificities of sclerosing cholangitis in childhood.

Sclerosing cholangitis (SC) is a chronic cholestatic disease characterized by inflammation and obliterative fibrosis of the bile ducts, leading to biliary cirrhosis and ultimately to liver failure. Four main clinical forms can be distinguished in children: i) neonatal SC, most probably a genetic disease transmitted by autosomal recessive inheritance; ii) SC associated with strong features of autoimmunity (referred as autoimmune sclerosing cholangitis) with quite good response to immuno-suppression iii) primary SC of unknown etiology (i.e. without features of autoimmunity) and iv) SC secondary to various diseases, including Langerhans cell histiocytosis and immunodeficiencies. Ursodesoxycholic acid is considered the treatment of choice for all forms of SC but without proof of its effectiveness in preventing progression to secondary biliary cirrhosis. In patients with immunodeficiencies, early bone marrow transplantation is the only way to prevent secondary SC. Liver transplantation remains the only validated treatment in children with biliary cirrhosis. Recurrence of SC after liver transplantation has not been clearly demonstrated in children; however, recurrence of Langerhans cell histiocytosis with bile duct injury has been reported. For patients with severe immunodeficiency, a two-step liver then bone marrow transplantation protocol may be proposed.

Genetic variations creating microRNA target sites in the FXN 3'-UTR affect frataxin expression in Friedreich ataxia.

Friedreich’s ataxia (FRDA) is a severe neurodegenerative disease caused by GAA repeat expansion within the first intron of the frataxin gene. It has been suggested that the repeat is responsible for the disease severity due to impaired transcription thereby reducing expression of the protein. However, genotype-phenotype correlation is imperfect, and the influence of other gene regions of the frataxin gene is unknown. We hypothesized that FRDA patients may harbor specific regulatory variants in the 3′-UTR. We sequenced the 3′-UTR region of the frataxin gene in a cohort of 57 FRDA individuals and 58 controls. Seven single nucleotide polymorphisms (SNPs) out of 19 were polymorphic in our case-control sample. These SNPs defined several haplotypes with one reaching 89% of homozygosity in patients versus 24% in controls. In another cohort of 47 FRDA Reunionese patients, 94% patients were found to be homozygous for this haplotype. We found that this FRDA 3′-UTR conferred a 1.2-fold decrease in the expression of a reporter gene versus the alternative haplotype configuration. We established that differential targeting by miRNA could account for this functional variability. We specifically demonstrated the involvement of miR-124 (i.e hsa-mir-124-3p) in the down-regulation of FRDA-3′-UTR. Our results suggest for the first time that post-transcriptional regulation of frataxin occurs through the 3′-UTR and involves miRNA targeting. We propose that the involvement of miRNAs in a FRDA-specific regulation of frataxin may provide a rationale to increase residual levels of frataxin through miRNA-inhibitory molecules.

Early and Late Complications After Liver Transplantation for Propionic Acidemia in Children: A Two Centers Study

Propionic acidemia (PA) is a severe metabolic disorder with cardiac and neurologic complications and a poor quality of life. Liver transplantation (LT) was thus proposed in PA to increase enzyme activity. We studied retrospectively LT in PA in two European centers. Twelve patients underwent 17 LTs between 1991 and 2013. They developed severe, unusual and unexpected complications, with high mortality (58%). When present, the cardiomyopathy resolved and no acute metabolic decompensation occurred allowing dietary relaxation. Renal failure was present in half of the patients before LT and worsened in all of them. We suggest that cardiac and renal functions should be assessed before LT and monitored closely afterward. A renal sparing immunosuppression should be used. We speculate that some complications may be related to accumulated toxicity of the disease and that earlier LT could prevent some of these consequences. As kidney transplantation has been performed successfully in methylmalonic acidemia, a metabolic disease in the same biochemical pathway, the choice of the organ to transplant could be further discussed.

Influence of donor–recipient CYP3A4/5 genotypes, age and fluconazole on tacrolimus pharmacokinetics in pediatric liver transplantation: a population approach

AIM To characterize the effect of donor and recipient CYP3A4, CYP3A5 and ABCB1 genotypes as well as relevant patient characteristics on tacrolimus pharmacokinetics in pediatric liver transplantation. PATIENTS & METHODS Data from 114 pediatric liver transplant recipients were retrospectively collected during the first 3 months following transplantation. Population pharmacokinetic analysis was performed using nonlinear mixed effects modeling, including characterization of influential covariates. RESULTS A two-compartment model with first order elimination best fitted the data. Estimates of apparent volume of the central compartment, intestinal clearance, hepatic clearance and intercompartmental clearance were 79 l, 0.01 l/h, 10.9 l/h and 105 l/h, respectively. Time post-transplantation, recipient age, donor CYP3A5 and CYP3A4 genotypes and fluconazole administration significantly influenced tacrolimus apparent clearance while bodyweight influenced volume of distribution. CONCLUSION The proposed model displayed acceptable fitting performances and enabled identification of statistically significant and clinically relevant covariates on tacrolimus pharmacokinetics in the early pediatric post liver transplantation period.