Mutations in mevalonate pathway genes in patients with familial or sporadic porokeratosis

Porokeratosis comprises heterogeneous keratinization disorders that are characterized by one or more atrophic patches surrounded by a ridge-like cornoid lamella. In this study, we evaluated seven families affected by porokeratosis and five sporadic patients of the disease in a Chinese population. We performed Sanger sequencing of exons and flanking intron-exon boundaries of mevalonate pathway genes (MVD, MVK, PMVK and FDPS) and of SLC17A9. In five familial and three sporadic patients, we detected six variations, including four novel mutations (MVD c.1A>G; p.Met1?, c.916G>A; p.Ala306Thr, c.1013+1G>A, and PMVK c.65A>G; p.Lys22Arg) and two recurrent mutations (MVD c.746T>C; p.Phe249Ser, and MVK c.1028T>C; p.Leu343Pro). We then applied I-TASSER and iGEMDOCK to assess these variants for probable functional impacts. The findings of this study extend the mutation spectrum of porokeratosis and provide further evidence for the genetic basis of this disease.

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.

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.

Increases in 4-Acetaminobutyric Acid Generated by Phosphomevalonate Kinase Suppress CD8+ T Cell Activation and Allow Tumor Immune Escape

Certain metabolites in the tumor microenvironment (TME) can alter innate immunity. Here, it is shown how phosphomevalonate kinase (PMVK) allows hepatocellular carcinoma (HCC) cells to overcome the anti-tumor immunity mediated by CD8+ T cells. In HCCs, depletion of PMVK is required to facilitate CD8+ T cell activation and their subsequent suppression of tumor growth. Mechanistically, PMVK phosphorylates and stabilizes glutamate decarboxylase 1 (GAD1), thus increasing the synthesis of γ-aminobutyric acid (GABA), which normally functions as a neurotransmitter. However, PMVK also recruits acetyl-CoA acetyltransferase 1 (ACAT1) and allows it to convert GABA, to 4-acetaminobutyric acid (4-Ac-GABA), which is released into the TME. There, 4-Ac-GABA activates the GABAA receptor (GABAAR) on CD8+ T cells, which inhibits AKT1 signaling. This in turn suppresses CD8+ T cell activation, intratumoral infiltration, and the anti-tumor response. Inhibiting PMVK or GABAAR in HCC mouse models overcomes resistance to anti-PD-1 immune checkpoint therapy. These findings reveal non-canonical and cooperative functions among the key metabolic enzymes PMVK, GAD1, and ACAT1 that reprogram glutamine metabolism to synthesize a potent CD8+ T cell inhibitor 4-Ac-GABA. Blocking 4-Ac-GABA signaling in CD8+ T cells, particularly when combined with immune checkpoint inhibition, potentially represents a new and potent form of immunotherapy.