Twists and turns of the genetic story of mevalonate kinase-associated diseases: A review

Genetically transitional disease: conceptual understanding and applicability to rheumatic disease

In genomic medicine, the concept of genetically transitional disease (GTD) refers to cases in which gene mutation is necessary but not sufficient to cause disease. In this Perspective, we apply this novel concept to rheumatic diseases, which have been linked to hundreds of genetic variants via association studies. These variants are in the 'grey zone' between monogenic variants with large effect sizes and common susceptibility alleles with small effect sizes. Among genes associated with rare autoinflammatory diseases, many low-frequency and/or low-penetrance variants are known to increase susceptibility to systemic inflammation. In autoimmune diseases, hundreds of HLA and non-HLA genetic variants have been revealed to be modest- to moderate-risk alleles. These diseases can be reclassified as GTDs. The same concept could apply to many other human diseases. GTD could improve the reporting of genetic testing results, diagnostic yields, genetic counselling and selection of therapy, as well as facilitating research using a novel approach to human genetic diseases.

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.

The pyrin inflammasome, a leading actor in pediatric autoinflammatory diseases

The activation of the pyrin inflammasome represents a highly intriguing mechanism employed by the innate immune system to effectively counteract pathogenic agents. Despite its key role in innate immunity, pyrin has also garnered significant attention due to its association with a range of autoinflammatory diseases (AIDs) including familial Mediterranean fever caused by disruption of the MEFV gene, or in other genes involved in its complex regulation mechanisms. Pyrin activation is strictly dependent on homeostasis-altering molecular processes, mostly consisting of the disruption of the small Ras Homolog Family Member A (RhoA) GTPases by pathogen toxins. The downstream pathways are regulated by the phosphorylation of specific pyrin residues by the kinases PKN1/2 and the binding of the chaperone 14-3-3. Furthermore, a key role in pyrin activation is played by the cytoskeleton and gasdermin D, which is responsible for membrane pores in the context of pyroptosis. In addition, recent evidence has highlighted the role of steroid hormone catabolites and alarmins S100A8/A9 and S100A12 in pyrin-dependent inflammation. The aim of this article is to offer a comprehensive overview of the most recent evidence on the pyrin inflammasome and its molecular pathways to better understand the pathogenesis behind the significant group of pyrin-related AIDs.

Autoinflammatory Keratinization Diseases-The Concept, Pathophysiology, and Clinical Implications

Recent advances in medical genetics elucidated the background of diseases characterized by superficial dermal and epidermal inflammation with resultant aberrant keratosis. This led to introducing the term autoinflammatory keratinization diseases encompassing entities in which monogenic mutations cause spontaneous activation of the innate immunity and subsequent disruption of the keratinization process. Originally, autoinflammatory keratinization diseases were attributed to pathogenic variants of CARD14 (generalized pustular psoriasis with concomitant psoriasis vulgaris, palmoplantar pustulosis, type V pityriasis rubra pilaris), IL36RN (generalized pustular psoriasis without concomitant psoriasis vulgaris, impetigo herpetiformis, acrodermatitis continua of Hallopeau), NLRP1 (familial forms of keratosis lichenoides chronica), and genes of the mevalonate pathway, i.e., MVK, PMVK, MVD, and FDPS (porokeratosis). Since then, endotypes underlying novel entities matching the concept of autoinflammatory keratinization diseases have been discovered (mutations of JAK1, POMP, and EGFR). This review describes the concept and pathophysiology of autoinflammatory keratinization diseases and outlines the characteristic clinical features of the associated entities. Furthermore, a novel term for NLRP1-associated autoinflammatory disease with epithelial dyskeratosis (NADED) describing the spectrum of autoinflammatory keratinization diseases secondary to NLRP1 mutations is proposed.

Phosphomevalonate kinase deficiency expands the genetic spectrum of systemic autoinflammatory diseases

BACKGROUND

In the isoprenoid biosynthesis pathway, mevalonate is phosphorylated in 2 subsequent enzyme steps by MVK and PMVK to generate mevalonate pyrophosphate that is further metabolized to produce sterol and nonsterol isoprenoids. Biallelic pathogenic variants in MVK result in the autoinflammatory metabolic disorder MVK deficiency. So far, however, no patients with proven PMVK deficiency due to biallelic pathogenic variants in PMVK have been reported.

OBJECTIVES

This study reports the first patient with functionally confirmed PMVK deficiency, including the clinical, biochemical, and immunological consequences of a homozygous missense variant in PMVK.

METHODS

The investigators performed whole-exome sequencing and functional studies in cells from a patient who, on clinical and immunological evaluation, was suspected of an autoinflammatory disease.

RESULTS

The investigators identified a homozygous PMVK p.Val131Ala (NM_006556.4: c.392T>C) missense variant in the index patient. Pathogenicity was supported by genetic algorithms and modeling analysis and confirmed in patient cells that revealed markedly reduced PMVK enzyme activity due to a virtually complete absence of PMVK protein. Clinically, the patient showed various similarities as well as distinct features compared to patients with MVK deficiency and responded well to therapeutic IL-1 inhibition.

CONCLUSIONS

This study reported the first patient with proven PMVK deficiency due to a homozygous missense variant in PMVK, leading to an autoinflammatory disease. PMVK deficiency expands the genetic spectrum of systemic autoinflammatory diseases, characterized by recurrent fevers, arthritis, and cytopenia and thus should be included in the differential diagnosis and genetic testing for systemic autoinflammatory diseases.

Decreased Imiquimod-Induced Psoriasis-Like Skin Inflammation in a Novel MvdF250S/+ Knock-In Mouse Model

The mevalonate-diphosphate decarboxylase (MVD) gene, a member of the mevalonate pathway, plays a critical role in regulating the biosynthesis of cholesterol, steroid hormones, and non-steroid isoprenoids. Previous studies have suggested that the MVD c.746 T > C mutation is a major pathogenic gene of porokeratosis (PK), an autoinflammatory keratinization disease (AIKD) with unclear pathogenesis, few effective treatments, and no suitable animal model. To investigate the function of Mvd^F250S/+ mutation, we developed a novel Mvd^F250S/+ mouse model carrying an equivalent point mutation to the most common genetic variation among Chinese PK patients (MVD^F249S/+) using CRISPR/Cas9 technology, which exhibited reduced cutaneous expression of Mvd protein. In the absence of external stimulation, Mvd^F250S/+ mice did not display specific phenotypes. However, upon induction with imiquimod (IMQ), Mvd^F250S/+ mice exhibited decreased susceptibility to skin acute inflammation compared to wild-type (WT) mice, as evidenced by reduced cutaneous proliferation and lower protein levels of IL-17a and IL-1β. Additionally, after IMQ induction, the Mvd^F250S/+mice exhibited downregulated collagen generation and upregulated expression of Fabp3 compared to WT mice, whereas no significant changes in the key genes related to cholesterol regulation were found. Furthermore, the Mvd^F250S/+ mutation activated autophagy. Our findings provided insights into the biological function of MVD in the skin.

Renal involvement in monogenic autoinflammatory diseases: A narrative review

Autoinflammatory diseases (AIDs) are mostly caused by dysfunctions in single genes encoding for proteins with a prominent role in the regulation of innate immunity, such as complement factors, inflammasome components, tumour necrosis factor (TNF)-α, and proteins belonging to type I-interferon (IFN) signalling pathways. Due to the deposition of amyloid A (AA) fibrils in the glomeruli, unprovoked inflammation in AIDs frequently affects renal health. In fact, secondary AA amyloidosis is the most common form of amyloidosis in children. It is caused by the extracellular deposition of fibrillar low-molecular weight protein subunits resulting from the degradation and accumulation of serum amyloid A (SAA) in numerous tissues and organs, primarily the kidneys. The molecular mechanisms underlying AA amyloidosis in AIDs are the elevated levels of SAA, produced by the liver in response to pro-inflammatory cytokines, and a genetic predisposition due to specific SAA isoforms. Despite the prevalence of amyloid kidney disease, non-amyloid kidney diseases may also be responsible for chronic renal damage in children with AIDs, albeit with distinct characteristics. Glomerular damage can result in various forms of glomerulonephritis with distinct histologic characteristics and a different underlying pathophysiology. This review aims to describe the potential renal implications in patients with inflammasomopathies, type-I interferonopathies, and other rare AIDs in an effort to improve the clinical course and quality of life in paediatric patients with renal involvement.

Isolated neurological presentations of mevalonate kinase deficiency

Mevalonate kinase (MK) deficiency is a rare autosomal recessive metabolic disorder caused by pathogenic variants in the MVK gene with a broad phenotypic spectrum including autoinflammation, developmental delay and ataxia. Typically, neurological symptoms are considered to be part of the severe end of the phenotypical spectrum and are reported to be in addition to the autoinflammatory symptoms. Here, we describe a patient with MK deficiency with severe neurological symptoms but without autoinflammation and we found several similar patients in the literature. Possibly, the non-inflammatory phenotype is related to a specific genotype: the MVK p.(His20Pro)/p.(Ala334Thr) variant. There is probably an underdetection of the neurological MK deficient phenotype without inflammatory symptoms as clinicians may not test for MK deficiency when patients present with only neurological symptoms. In conclusion, although rare, neurological symptoms without hyperinflammation might be more common than expected in MK deficiency. It seems relevant to consider MK deficiency in patients with psychomotor delay and ataxia, even if there are no inflammatory symptoms.