Claire Adams

Postreceptor insulin resistance contributes to human dyslipidemia and hepatic steatosis.

Metabolic dyslipidemia is characterized by high circulating triglyceride (TG) and low HDL cholesterol levels and is frequently accompanied by hepatic steatosis. Increased hepatic lipogenesis contributes to both of these problems. Because insulin fails to suppress gluconeogenesis but continues to stimulate lipogenesis in both obese and lipodystrophic insulin-resistant mice, it has been proposed that a selective postreceptor defect in hepatic insulin action is central to the pathogenesis of fatty liver and hypertriglyceridemia in these mice. Here we show that humans with generalized insulin resistance caused by either mutations in the insulin receptor gene or inhibitory antibodies specific for the insulin receptor uniformly exhibited low serum TG and normal HDL cholesterol levels. This was due at least in part to surprisingly low rates of de novo lipogenesis and was associated with low liver fat content and the production of TG-depleted VLDL cholesterol particles. In contrast, humans with a selective postreceptor defect in AKT2 manifest increased lipogenesis, elevated liver fat content, TG-enriched VLDL, hypertriglyceridemia, and low HDL cholesterol levels. People with lipodystrophy, a disorder characterized by particularly severe insulin resistance and dyslipidemia, demonstrated similar abnormalities. Collectively these data from humans with molecularly characterized forms of insulin resistance suggest that partial postreceptor hepatic insulin resistance is a key element in the development of metabolic dyslipidemia and hepatic steatosis.

Integrative genomic analysis implicates limited peripheral adipose storage capacity in the pathogenesis of human insulin resistance

Insulin resistance is a key mediator of obesity-related cardiometabolic disease, yet the mechanisms underlying this link remain obscure. Using an integrative genomic approach, we identify 53 genomic regions associated with insulin resistance phenotypes (higher fasting insulin levels adjusted for BMI, lower HDL cholesterol levels and higher triglyceride levels) and provide evidence that their link with higher cardiometabolic risk is underpinned by an association with lower adipose mass in peripheral compartments. Using these 53 loci, we show a polygenic contribution to familial partial lipodystrophy type 1, a severe form of insulin resistance, and highlight shared molecular mechanisms in common/mild and rare/severe insulin resistance. Population-level genetic analyses combined with experiments in cellular models implicate CCDC92, DNAH10 and L3MBTL3 as previously unrecognized molecules influencing adipocyte differentiation. Our findings support the notion that limited storage capacity of peripheral adipose tissue is an important etiological component in insulin-resistant cardiometabolic disease and highlight genes and mechanisms underpinning this link.

Complement Abnormalities in Acquired Lipodystrophy Revisited

CONTEXT Lipodystrophy is a heterogeneous condition characterized by an inherited or acquired deficiency in the number of adipocytes required for the storage of energy as triglycerides. Acquired lipodystrophy is frequently associated with other autoimmune disorders. One well-studied form is characterized by the selective loss of upper body fat in association with activation of the alternative complement pathway by C3 nephritic factor, low complement factor C3, and mesangiocapillary glomerulonephritis. OBJECTIVE We now describe an immunologically distinct form of acquired generalized lipodystrophy, with evidence of activation of the classical complement pathway (low C4) and autoimmune hepatitis. Patients and Research Design: Three unrelated patients with acquired lipodystrophy and low complement C4 levels are described. In vitro analysis of the complement pathway was undertaken to determine the reason for the low C4 complement levels. Biopsies were obtained from liver, bone marrow, and adipose tissue for histological analysis. RESULTS All three patients manifested near-total lipodystrophy, chronic hepatitis with autoimmune features, and low C4 complement levels. Additional autoimmune diseases, including severe hemolytic anemia, autoimmune thyroid disease, and polyneuropathy, were variably present. Detailed studies of complement pathways suggested constitutive classical pathway activation. CONCLUSIONS Although the previously described syndrome, which typically results in a cephalad pattern of partial lipodystrophy, results from activation of the alternative complement pathway, this form, in which lipodystrophy is generalized, is associated with activation of the classical pathway. Future therapeutic approaches to these disorders may benefit from being tailored to their distinct immunopathogenesis.

Mutations disrupting the Kennedy phosphatidylcholine pathway in humans with congenital lipodystrophy and fatty liver disease

Significance The characterization of rare monogenic human disorders can and has yielded unique biological insights. Our phenotypic description and functional characterization of human loss-of-function mutations in PCYT1A is both clinically important for patients and their families afflicted with this rare but serious metabolic disease and biologically helpful in advancing understanding of the physiological consequences of impaired PCYT1A activity in humans. Phosphatidylcholine (PC) is the major glycerophospholipid in eukaryotic cells and is an essential component in all cellular membranes. The biochemistry of de novo PC synthesis by the Kennedy pathway is well established, but less is known about the physiological functions of PC. We identified two unrelated patients with defects in the Kennedy pathway due to biallellic loss-of-function mutations in phosphate cytidylyltransferase 1 alpha (PCYT1A), the rate-limiting enzyme in this pathway. The mutations lead to a marked reduction in PCYT1A expression and PC synthesis. The phenotypic consequences include some features, such as severe fatty liver and low HDL cholesterol levels, that are predicted by the results of previously reported liver-specific deletion of murine Pcyt1a. Both patients also had lipodystrophy, severe insulin resistance, and diabetes, providing evidence for an additional and essential role for PCYT1A-generated PC in the normal function of white adipose tissue and insulin action.

Mechanistic insights revealed by lipid profiling in monogenic insulin resistance syndromes

BackgroundEvidence from several recent metabolomic studies suggests that increased concentrations of triacylglycerols with shorter (14–16 carbon atoms), saturated fatty acids are associated with insulin resistance and the risk of type 2 diabetes. Although causality cannot be inferred from association studies, patients in whom the primary cause of insulin resistance can be genetically defined offer unique opportunities to address this challenge.MethodsWe compared metabolite profiles in patients with congenital lipodystrophy or loss-of-function insulin resistance (INSR gene) mutations with healthy controls.ResultsThe absence of significant differences in triacylglycerol species in the INSR group suggest that changes previously observed in epidemiological studies are not purely a consequence of insulin resistance. The presence of triacylglycerols with lower carbon numbers and high saturation in patients with lipodystrophy suggests that these metabolite changes may be associated with primary adipose tissue dysfunction. The observed pattern of triacylglycerol species is indicative of increased de novo lipogenesis in the liver. To test this we investigated the distribution of these triacylglycerols in lipoprotein fractions using size exclusion chromatography prior to mass spectrometry. This associated these triacylglycerols with very low-density lipoprotein particles, and hence release of triacylglycerols into the blood from the liver. To test further the hepatic origin of these triacylglycerols we induced de novo lipogenesis in the mouse, comparing ob/ob and wild-type mice on a chow or high fat diet, confirming that de novo lipogenesis induced an increase in relatively shorter, more saturated fatty acids.ConclusionsOverall, these studies highlight hepatic de novo lipogenesis in the pathogenesis of metabolic dyslipidaemia in states where energy intake exceeds the capacity of adipose tissue.

Truncation of POC1A associated with short stature and extreme insulin resistance

We describe a female proband with primordial dwarfism, skeletal dysplasia, facial dysmorphism, extreme dyslipidaemic insulin resistance and fatty liver associated with a novel homozygous frameshift mutation in POC1A, predicted to affect two of the three protein products of the gene. POC1A encodes a protein associated with centrioles throughout the cell cycle and implicated in both mitotic spindle and primary ciliary function. Three homozygous mutations affecting all isoforms of POC1A have recently been implicated in a similar syndrome of primordial dwarfism, although no detailed metabolic phenotypes were described. Primary cells from the proband we describe exhibited increased centrosome amplification and multipolar spindle formation during mitosis, but showed normal DNA content, arguing against mitotic skipping, cleavage failure or cell fusion. Despite evidence of increased DNA damage in cells with supernumerary centrosomes, no aneuploidy was detected. Extensive centrosome clustering both at mitotic spindles and in primary cilia mitigated the consequences of centrosome amplification, and primary ciliary formation was normal. Although further metabolic studies of patients with POC1A mutations are warranted, we suggest that POC1A may be added to ALMS1 and PCNT as examples of centrosomal or pericentriolar proteins whose dysfunction leads to extreme dyslipidaemic insulin resistance. Further investigation of links between these molecular defects and adipose tissue dysfunction is likely to yield insights into mechanisms of adipose tissue maintenance and regeneration that are critical to metabolic health.

Human biallelic MFN2 mutations induce mitochondrial dysfunction, upper body adipose hyperplasia, and suppression of leptin expression

MFN2 encodes mitofusin 2, a membrane-bound mediator of mitochondrial membrane fusion and inter-organelle communication. MFN2 mutations cause axonal neuropathy, with associated lipodystrophy only occasionally noted, however homozygosity for the p.Arg707Trp mutation was recently associated with upper body adipose overgrowth. We describe similar massive adipose overgrowth with suppressed leptin expression in four further patients with biallelic MFN2 mutations and at least one p.Arg707Trp allele. Overgrown tissue was composed of normal-sized, UCP1-negative unilocular adipocytes, with mitochondrial network fragmentation, disorganised cristae, and increased autophagosomes. There was strong transcriptional evidence of mitochondrial stress signalling, increased protein synthesis, and suppression of signatures of cell death in affected tissue, whereas mitochondrial morphology and gene expression were normal in skin fibroblasts. These findings suggest that specific MFN2 mutations cause tissue-selective mitochondrial dysfunction with increased adipocyte proliferation and survival, confirm a novel form of excess adiposity with paradoxical suppression of leptin expression, and suggest potential targeted therapies. DOI: http://dx.doi.org/10.7554/eLife.23813.001

A Pharmacogenetic Approach to the Treatment of Patients with PPARG Mutations

Loss-of-function mutations in PPARG cause familial partial lipodystrophy type 3 (FPLD3) and severe metabolic disease in many patients. Missense mutations in PPARG are present in ∼1 in 500 people. Although mutations are often binarily classified as benign or deleterious, prospective functional classification of all missense PPARG variants suggests that their impact is graded. Furthermore, in testing novel mutations with both prototypic endogenous (e.g., prostaglandin J2 [PGJ2]) and synthetic ligands (thiazolidinediones, tyrosine agonists), we observed that synthetic agonists selectively rescue function of some peroxisome proliferator–activated receptor-γ (PPARγ) mutants. We report on patients with FPLD3 who harbor two such PPARγ mutations (R308P and A261E). Both PPARγ mutants exhibit negligible constitutive or PGJ2-induced transcriptional activity but respond readily to synthetic agonists in vitro, with structural modeling providing a basis for such differential ligand-dependent responsiveness. Concordant with this finding, dramatic clinical improvement was seen after pioglitazone treatment of a patient with R308P mutant PPARγ. A patient with A261E mutant PPARγ also responded beneficially to rosiglitazone, although cardiomyopathy precluded prolonged thiazolidinedione use. These observations indicate that detailed structural and functional classification can be used to inform therapeutic decisions in patients with PPARG mutations.

The metabolic syndrome- associated small G protein ARL15 plays a role in adipocyte differentiation and adiponectin secretion

Common genetic variants at the ARL15 locus are associated with plasma adiponectin, insulin and HDL cholesterol concentrations, obesity, and coronary atherosclerosis. The ARL15 gene encodes a small GTP-binding protein whose function is currently unknown. In this study adipocyte-autonomous roles for ARL15 were investigated using conditional knockdown of Arl15 in murine 3T3-L1 (pre)adipocytes. Arl15 knockdown in differentiated adipocytes impaired adiponectin secretion but not adipsin secretion or insulin action, while in preadipocytes it impaired adipogenesis. In differentiated adipocytes GFP-tagged ARL15 localized predominantly to the Golgi with lower levels detected at the plasma membrane and intracellular vesicles, suggesting involvement in intracellular trafficking. Sequencing of ARL15 in 375 severely insulin resistant patients identified four rare heterozygous variants, including an early nonsense mutation in a proband with femorogluteal lipodystrophy and non classical congenital adrenal hyperplasia, and an essential splice site mutation in a proband with partial lipodystrophy and a history of childhood yolk sac tumour. No nonsense or essential splice site mutations were found in 2,479 controls, while five such variants were found in the ExAC database. These findings provide evidence that ARL15 plays a role in adipocyte differentiation and adiponectin secretion, and raise the possibility that human ARL15 haploinsufficiency predisposes to lipodystrophy.

Magnetic resonance spectroscopy analysis of intramyocellular lipid composition in lipodystrophic patients and athletes

Context Paradoxically, intramyocellular lipid (IMCL) accumulation has been linked to both insulin-resistant and to insulin-sensitive (athletes) states. The composition of this lipid store is unknown in these states. Design and Methods We used a recently validated and potentially widely applicable 1H magnetic resonance spectroscopy method to compare the compositional saturation index (CH2:CH3 ratio) and concentration independent of composition (CH3) of intramyocellular lipid in the soleus and tibialis anterior muscles of 16 female insulin-resistant lipodystrophic patients with that of age- and gender-matched athletes (n=14) and healthy controls (n = 41). Main Outcome IMCL compositional saturation index (CH2:CH3 ratio). Results The IMCL CH2:CH3 ratio was significantly higher in both muscles of the lipodystrophic patients compared with age- and gender-matched controls but not compared to athletes. IMCL CH2:CH3 was dependent on IMCL concentration in the controls and after adjusting the composition index for quantity (CH2:CH3adj) was able to distinguish patients from athletes. With groups pooled, this CH2:CH3adj marker had the strongest relation to insulin resistance (HOMA-IR) compared to other measures of lipid concentration and composition, especially in the soleus muscle. Contrary to the ‘athlete’s paradox’, IMCL in athletes was similar in tibialis anterior (p>0.05) and significantly lower in the soleus (p < 0.004) compared to both controls and patients. Conclusions The IMCL saturation index adjusted for quantity, which likely reflects accumulation of saturated IMCL, is more closely associated with insulin resistance than concentration alone.