Mevalonate kinase-deficient THP-1 cells show a disease-characteristic pro-inflammatory phenotype

Objective

Bi-allelic pathogenic variants in the MVK gene, which encodes mevalonate kinase (MK), an essential enzyme in isoprenoid biosynthesis, cause the autoinflammatory metabolic disorder mevalonate kinase deficiency (MKD). We generated and characterized MK-deficient monocytic THP-1 cells to identify molecular and cellular mechanisms that contribute to the pro-inflammatory phenotype of MKD.

Methods

Using CRISPR/Cas9 genome editing, we generated THP-1 cells with different MK deficiencies mimicking the severe (MKD-MA) and mild end (MKD-HIDS) of the MKD disease spectrum. Following confirmation of previously established disease-specific biochemical hallmarks, we studied the consequences of the different MK deficiencies on LPS-stimulated cytokine release, glycolysis versus oxidative phosphorylation rates, cellular chemotaxis and protein kinase activity.

Results

Similar to MKD patients' cells, MK deficiency in the THP-1 cells caused a pro-inflammatory phenotype with a severity correlating with the residual MK protein levels. In the MKD-MA THP-1 cells, MK protein levels were barely detectable, which affected protein prenylation and was accompanied by a profound pro-inflammatory phenotype. This included a markedly increased LPS-stimulated release of pro-inflammatory cytokines and a metabolic switch from oxidative phosphorylation towards glycolysis. We also observed increased activity of protein kinases that are involved in cell migration and proliferation, and in innate and adaptive immune responses. The MKD-HIDS THP-1 cells had approximately 20% residual MK activity and showed a milder phenotype, which manifested mainly upon LPS stimulation or exposure to elevated temperatures.

Conclusion

MK-deficient THP-1 cells show the biochemical and pro-inflammatory phenotype of MKD and are a good model to study underlying disease mechanisms and therapeutic options of this autoinflammatory disorder.

A novel homozygous variant in PMVK is associated with enhanced IL1β secretion and a hyper-IgD syndrome-like phenotype

The evolutionarily conserved mevalonate pathway plays an important role in the synthesis of cholesterol and isoprenoid compounds. Mevalonate kinase (MVK) and phosphomevalonate kinase (PMVK) enzymes regulate key rate-limiting steps in this pathway by sequentially phosphorylating mevalonic acid to yield downstream metabolites that regulate protein prenylation and cell signaling. Biallelic pathogenic variants in MVK cause a spectrum of rare autoinflammatory disorders that encompass milder forms of hyper-IgD syndrome (HIDS) at one end and the more severe mevalonic aciduria on the other. In contrast, pathogenic variants reported in PMVK are heterozygous and associated with porokeratosis, a skin disorder with no systemic manifestations. Recently, biallelic variants in PMVK were reported as a cause for an autoinflammatory disorder for the first time in two unrelated patients. In this study, we describe a child with recurrent arthritis and a HIDS-like phenotype harboring a novel homozygous variant c.398 C>T (p.Ala133Val) in PMVK. Mononuclear cells isolated from the patient showed significantly elevated production of interleukin 1β, a key cytokine that shapes the inflammatory response in HIDS. Protein modeling studies suggested potential defects in PMVK enzyme activity. These results posit a further expanding of the genotypic spectrum of autoinflammatory disease to include biallelic PMVK variants.

Phenotypic diversity, disease progression, and pathogenicity of MVK missense variants in mevalonic aciduria

Mevalonic aciduria (MVA) and hyperimmunoglobulinemia D syndrome (MKD/HIDS) are disorders of cholesterol biosynthesis caused by variants in the MVK gene and characterized by increased urinary excretion of mevalonic acid. So far, 30 MVA patients have been reported, suffering from recurrent febrile crises and neurologic impairment. Here, we present an in-depth analysis of the phenotypic spectrum of MVA and provide an in-silico pathogenicity model analysis of MVK missense variants. The phenotypic spectrum of 11 MVA patients (age range 0-51 years) registered in the Unified European Registry for Inherited Metabolic Disorders database was systematically analyzed using terms of the Human Phenotype Ontology. Biochemical, radiological as well as genetic characteristics were investigated. Six of eleven patients have reached adulthood and four have reached adolescence. One of the adolescent patients died at the age of 16 years and one patient died shortly after birth. Symptoms started within the first year of life, including episodic fever, developmental delay, ataxia, and ocular involvement. We also describe a case with absence of symptoms despite massive excretion of mevalonic acid. Pathogenic variants causing MVA cluster within highly conserved regions, which are involved in mevalonate and ATP binding. The phenotype of adult and adolescent MVA patients is more heterogeneous than previously assumed. Outcome varies from an asymptomatic course to early death. MVK variants cluster in functionally important and highly conserved protein domains and show high concordance regarding their expected pathogenicity.

Respiratory deficiency in yeast mevalonate kinase deficient may explain MKD-associate metabolic disorder in humans

Mevalonate kinase deficiency (MKD) an orphan drug rare disease affecting humans with different clinical presentations, is still lacking information about its pathogenesis; no animal or cell model mimicking the genetic defect, mutations at MVK gene, and its consequences on the mevalonate pathway is available. Trying to clarify the effects of MVK gene impairment on the mevalonate pathway we used a yeast model, the erg12-d mutant strain Saccharomyces cerevisiae (orthologous of MKV) retaining only 10% of mevalonate kinase (MK) activity, to describe the effects of reduced MK activity on the mevalonate pathway. Since shortage of isoprenoids has been described in MKD, we checked this observation using a physiologic approach: while normally growing on glucose, erg12-d showed growth deficiency in glycerol, a respirable carbon source, that was not rescued by supplementation with non-sterol isoprenoids, such as farnesol, geraniol nor geranylgeraniol, produced by the mevalonate pathway. Erg12-d whole genome expression analysis revealed specific downregulation of RSF2 gene encoding general transcription factor for respiratory genes, explaining the absence of growth on glycerol. Moreover, we observed the upregulation of genes involved in sulphur amino acids biosynthesis that coincided with the increasing in the amount of proteins containing sulfhydryl groups; upregulation of ubiquinone biosynthesis genes was also detected. Our findings demonstrated that the shortage of isoprenoids is not the main mechanism involved in the respiratory deficit and mitochondrial malfunctioning of MK-defective cells, while the scarcity of ubiquinone plays an important role, as already observed in MKD patients.

Pyrin inflammasome activation and RhoA signaling in the autoinflammatory diseases FMF and HIDS

Mutations in the genes encoding pyrin and mevalonate kinase (MVK) cause distinct interleukin-1β (IL-1β)-mediated autoinflammatory diseases: familial Mediterranean fever (FMF) and hyperimmunoglobulinemia D syndrome (HIDS). Pyrin forms an inflammasome when mutant or in response to bacterial modification of the GTPase RhoA. We found that RhoA activated the serine-threonine kinases PKN1 and PKN2 that bind and phosphorylate pyrin. Phosphorylated pyrin bound to 14-3-3 proteins, regulatory proteins that in turn blocked the pyrin inflammasome. The binding of 14-3-3 and PKN proteins to FMF-associated mutant pyrin was substantially decreased, and the constitutive IL-1β release from peripheral blood mononuclear cells of patients with FMF or HIDS was attenuated by activation of PKN1 and PKN2. Defects in prenylation, seen in HIDS, led to RhoA inactivation and consequent pyrin inflammasome activation. These data suggest a previously unsuspected fundamental molecular connection between two seemingly distinct autoinflammatory disorders.

To Extinguish the Fire from Outside the Cell or to Shutdown the Gas Valve Inside? Novel Trends in Anti-Inflammatory Therapies

Cytokines are the most important soluble mediators of inflammation. Rare pediatric diseases provided exemplar conditions to study the anti-inflammatory efficacy of new generation therapies (biologics/biopharmaceuticals) selectively targeting single cytokines. Monoclonal antibodies and recombinant proteins have revolutionized anti-inflammatory therapies in the last two decades, allowing the specific targeting of single cytokines. They are very effective in extinguishing inflammation from outside the cell, even with the risk of an excessive and prolonged immunosuppression. Small molecules can enter the cell and shutdown the valve of inflammation by directly targeting signal proteins involved in cytokine release or in response to cytokines. They are orally-administrable drugs whose dosage can be easily adjusted to obtain the desired anti-inflammatory effect. This could make these drugs more suitable for a wide range of diseases as stroke, gout, or neurological impairment, where inflammatory activation plays a pivotal role as trigger. Autoinflammatory diseases, which have previously put anti-cytokine proteins in the limelight, can again provide a valuable model to measure the real potential of small inhibitors as anti-inflammatory agents.

Cholesterol metabolism and immunity

A study in mice implicates a cholesterol derivative in damping down the inflammatory response mediated by interleukin-1β and explaining, at least in part, the immunosuppressive effect of type I interferon, which is used in the treatment of multiple sclerosis.

Block of the mevalonate pathway triggers oxidative and inflammatory molecular mechanisms modulated by exogenous isoprenoid compounds

Deregulation of the mevalonate pathway is known to be involved in a number of diseases that exhibit a systemic inflammatory phenotype and often neurological involvements, as seen in patients suffering from a rare disease called mevalonate kinase deficiency (MKD). One of the molecular mechanisms underlying this pathology could depend on the shortage of isoprenoid compounds and the subsequent mitochondrial damage, leading to oxidative stress and pro-inflammatory cytokines’ release. Moreover, it has been demonstrated that cellular death results from the balance between apoptosis and pyroptosis, both driven by mitochondrial damage and the molecular platform inflammasome. In order to rescue the deregulated pathway and decrease inflammatory markers, exogenous isoprenoid compounds were administered to a biochemical model of MKD obtained treating a murine monocytic cell line with a compound able to block the mevalonate pathway, plus an inflammatory stimulus. Our results show that isoprenoids acted in different ways, mainly increasing the expression of the evaluated markers [apoptosis, mitochondrial dysfunction, nucleotide-binding oligomerization-domain protein-like receptors 3 (NALP3), cytokines and nitric oxide (NO)]. Our findings confirm the hypothesis that inflammation is triggered, at least partially, by the shortage of isoprenoids. Moreover, although further studies are necessary, the achieved results suggest a possible role for exogenous isoprenoids in the treatment of MKD.

Severe phenotypic spectrum of mevalonate kinase deficiency with minimal mevalonic aciduria

Mevalonate kinase deficiency is a rare autosomal recessively inherited organic aciduria with a complex multi-systemic phenotype. We describe two deceased patients with clinically severe mevalonate kinase (MK) deficiency confirmed by MK mutation analysis. The phenotype in our patients ranged from neonatal hydrops in the first patient to severe failure to thrive, hepatosplenomegaly, recurrent febrile episodes and lymphadenopathy in the second. Both infants excreted relatively low amounts of mevalonic acid intermittently.

Aberrant expression of costimulatory molecules in splenocytes of the mevalonate kinase-deficient mouse model of human hyper-IgD syndrome (HIDS)

OBJECTIVE

We sought to determine the activation status and proliferative capacities of splenic lymphocyte populations from a mevalonate kinase-deficient mouse model of hyper-IgD syndrome (HIDS). We previously reported that murine mevalonate kinase gene ablation was embryonic lethal for homozygous mutants while heterozygotes (Mvk (+/-)) demonstrated several phenotypic features of human HIDS including increased serum levels of IgD, IgA, and TNFα, temperature dysregulation, hematological abnormalities, and splenomegaly.

METHODS AND RESULTS

Flow cytometric analysis of cell surface activation markers on T and B lymphocytes, and macrophage populations, demonstrated aberrant expression of B7 glycoproteins in all splenic cell types studied. Differences in expression levels between Mvk (+/-) and Mvk (+/+) littermate controls were observed in both the basal state (unstimulated) and after Concanavalin A (Con-A) stimulation in vitro of whole splenocyte cultures. In Mvk (+/-) CD4 and CD8 T cells, alterations in expression of CD25, CD80, CD152, and CD28 were observed. Mvk (+/-) splenic macrophages expressed altered levels of CD80, CD86, CD40, and CD11c while Mvk (+/-) B lymphocytes had differential expression of CD40, CD80, and CD86. Mvk (+/-) splenocyte subpopulations also exhibited altered proliferative capacities in response to in vitro stimulation.

CONCLUSION

We postulate that imbalances in the expression of cell surface proteins necessary for activation, proliferation, and regulation of the intensity and duration of an immune response may result in defective T cell activation, proliferation, and effector functions in our model and potentially in human HIDS.

Coenzyme Q(10) is decreased in fibroblasts of patients with methylmalonic aciduria but not in mevalonic aciduria

The content of coenzyme Q(10) (CoQ(10)) was examined in skin fibroblasts of 10 patients with mevalonic aciduria (MVA) and of 22 patients with methylmalonic aciduria (MMA). Patients with these inborn errors of metabolism are thought to be at risk for CoQ(10) depletion either by direct inhibition of the proximal pathway of CoQ(10) synthesis (MVA) or indirectly by inhibition of mitochondrial energy metabolism (MMA). We demonstrated that CoQ(10) concentrations were not significantly different from controls in MVA patients, suggesting that there may be upregulatory effects. On the other hand the CoQ(10) content in fibroblasts of patients with MMA was significantly reduced.