Autoimmune diseases are defined by adaptive immune system dysfunction, with lymphocytes attacking self-antigens. In contrast, autoinflammatory diseases arise from antigen-independent hyperactivation of innate immunity. Mevalonate kinase deficiency (MKD) is a monogenic, autosomal recessive, autoinflammatory disease resulting from biallelic pathogenic variants in the mevalonate kinase (MVK) gene. The downstream effects include defective protein prenylation, inflammasome activation, and hypersecretion of pro-inflammatory cytokines such as interleukin-1 (IL-1) (1). MKD encompasses a spectrum of phenotypes partially correlated with the MVK variants and residual enzymatic activity (2). Severely affected patients typically retain undetectable to 0.5% mevalonate kinase activity, often presenting with constitutive mevalonic aciduria, prominent neurological and ophthalmic involvement, and chronic systemic inflammation (3). Classically, milder forms preserve 1% to 20%–28% enzymatic activity (4) and exhibit recurrent febrile episodes with aphtosis, abdominal pain, diarrhea, arthromyalgia, painful lymphadenopathies, maculopapular rash, and other manifestations (3, 5). Attacks last for 3–7 days and are often triggered by fatigue, physical exertion, infections, or vaccination. In mild MKD, urinary mevalonic acid is usually detectable only during attacks. Atypical MKD phenotypes are increasingly reported, and the spectrum of MKD phenotypes continues to expand within the medical literature (3). We recently reported the first case of MKD-related inflammation causing ischemic stroke (IS) (6).

We now report the second known MKD patient with an inflammatory flare complicated by IS. This patient harbors the common pathogenic c.1129G>A (V377I) variant in addition to a novel c.1049A>C (Q350P) variant. Mevalonate kinase activity in lymphocytes was measured to confirm the pathogenicity of the novel variant. By computational modeling, we analyzed the structural consequences of the Q350P substitution on mevalonate kinase. The amino acid sequence of mevalonate kinase (2R3V) was obtained from the Protein Data Bank (7) and encoded into AlphaFold3 (8). The predicted structure of mevalonate kinase was visualized in PyMOL (https://www.pymol.org) to identify the structural GHMP domain and compared to the known X-ray crystallography of mevalonate kinase (9). The glutamine 350 residue was localized, and all amino acid residues within 5 Å were identified. The amino acid sequence of mevalonate kinase was modified and substituted with a proline residue at position 350. The resulting sequence was encoded into AlphaFold3 and visualized with PyMOL. The tertiary structure of mevalonate kinase was compared between the wild-type and the novel Q350P variant.

The patient displayed trans compound heterozygosity for the known pathogenic V377I variant and the novel Q350P variant in the MVK gene. The Q350P substitution is located within a highly conserved kinase domain during evolution C-terminal GHMP kinase domain formed of residues 227-374 (4). The GHMP domain, named after four metabolic enzymes participating in essential phosphorylation reactions (with M standing for mevalonate kinase), is necessary for ATP and substrate binding (10). The Q350P substitution involves replacing a polar amino acid (glutamine) with a non-polar amino acid (proline). Proline substitutions are known to alter the protein tertiary structure and thermal stability due to their cyclic structure and lack of amide hydrogen to participate in peptide backbone hydrogen bonding (11). By computational modeling, we demonstrated that Q350P substitution alters mevalonate kinase tertiary structure, especially in its highly conserved kinase domain, which is essential for its function ( Figure 3 ). The Q350P variant is therefore likely pathogenic, expanding the spectrum of MKD disorders.

Consistent with MKD, this patient presented with macular rash, lymphadenopathies, and increased inflammatory blood markers, spontaneous clinical improvement occurred, and laboratory evidence of inflammation persisted until anti-interleukin-1β antibody treatment was initiated ( Figure 2 ). The favorable clinical response to canakinumab supports a central role for innate immune dysregulation, possibly through subthreshold activation of the nucleotide-binding domain leucine-rich repeat receptor family pyrin domain containing 3 (NLRP3) inflammasome and endothelial dysfunction in the presence of partial mevalonate kinase activity (12). Like this patient, 20% of MKD patients have normal immunoglobulin D levels, and recent studies have shown relatively low sensitivity and specificity of hyperimmunoglobulinemia D for MKD (3). His presentation in adulthood and the absence of prior repeated inflammatory crises are atypical for MKD, probably reflecting his residual mevalonate kinase activity, which ranges between normal and typically pathogenic levels. Atypical forms of MKD with prominent skin, liver, inflammatory bowel, and cardiorespiratory disease or amyloidosis were described in association with variable levels of residual mevalonate activity, resulting in delayed diagnosis (3). Adults with biallelic pathogenic mutations presenting late in life without recurrent inflammation or with no-disease phenotypes were also previously reported (13). Indeed approximately 300 MKD cases have been reported worldwide. MKD seems considerably rarer than expected from population studies of allele frequencies (https://gnomad.broadinstitute.org), suggesting an underestimation of its prevalence due to a delay between symptom onset and diagnosis (14), lack of awareness in the medical community, and milder presentations reflecting low penetrance (15).

Residual enzyme activity was found to be 55% in this patient. Typically, MKD patients present a residual mevalonate kinase activity of anywhere between undetectable levels and as high as 20%–28% (5). In the recent years, mevalonate kinase-associated disorders have been increasingly recognized as a spectrum of disease rather than a limited set of distinct phenotypes ( Figure 4 ) (3). Since MKD is known to be a very rare and underdiagnosed condition, we do not hold enough data to discard patients with subnormal but higher than 20%–28% enzyme activity. Heterozygous carriers for MKD (V377I) have been reported to be symptomatic and respond well to canakinumab (16). The patient that we report exhibited a strong response to anti-IL1 canakinumab, both clinically and through blood markers of inflammation, supporting an autoinflammatory disease diagnosis and a mevalonate kinase spectrum disorder. We hypothesize that patients with 20%–60% of residual mevalonate kinase activity may have an attenuated phenotype. This could lead to reduced intensity or frequency of systemic auto-inflammatory episodes and periodic fever and late occurrence of auto-inflammatory-related events, such as the case presented here. Mouse models show a spectrum of protein prenylation and inflammation based on the residual enzymatic activity, which should be between 50% and 75% (4). Symptomatic individuals with intermediate enzymatic activity of known autoinflammatory genes are reported in other autoinflammatory diseases, including familial Mediterranean fever and adenosine deaminase deficiency 2 (17, 18). Our patient shows compound heterozygosity with a known pathogenic variant (V377I) and a novel missense Q350P variant. The pathogenicity of this novel variant remains to be fully understood, but we hypothesize that the change in the tertiary structure of the protein and its thermal stability support its pathogenicity (4, 15).

The investigation found no common cause of IS. Lacunar IS due to arteriolosclerosis is unlikely in the absence of chronic ischemic or hemorrhagic changes on brain MRI. The investigation documented inflammation, but positron emission tomography and skin biopsy showed no evidence of vasculitis. Moreover, the patient’s favorable outcome in the absence of immunosuppressant treatment does not support vasculitis. We believe that our patient had an IS due to MKD-related inflammation and small artery occlusion. Abnormal activation of sophisticated interlinks between innate immunity and hemostasis is involved in thromboembolic complications of autoinflammatory diseases (19). We hypothesize that our patient’s risk factors for arteriosclerosis, including cigarette and cannabis smoking and dyslipidemia, and actual atherosclerotic plaques contributed to the systemic prothrombotic environment of thromboinflammation. Unfortunately, the biomarkers of endothelial dysfunction (e.g., vascular cell adhesion molecule-1, intercellular adhesion molecule 1) were not measured. The presence of cryofibrinogen and monoclonal gammopathy raises doubt about the IS etiology, and there is a potential for overlapping of different causes. Whereas essential cryofibrinogenemia is frequently asymptomatic, symptomatic patients typically present in the fifth decade with cold intolerance, skin ulcers affecting exposed surfaces, livedo, and thrombosis (20, 21).However, in the patient that we report here, there were no dermatological manifestations or livedo, and the blood tests did not find cryofibrinogen at IS presentation and on a subsequent measurement ( Figure 2 ). Cryofibrinogen was variably detected from 3 months post-stroke until canakinumab treatment. Skin biopsy revealed nonspecific changes, without plugging of deep and superficial blood vessels by cryofibrinogen-containing thrombi (20, 21). Cryofibrinogenemia in this patient likely resulted from autoinflammation rather than being the cause of IS and skin ulcers (22). While skin biopsy findings could be consistent with PG, occurring in association with the documented monoclonal gammopathy of unknown significance, it can also represent a manifestation of MKD or its treatment with anti-IL1 agents. Histopathology of skin biopsies in MKD is varied and under-reported to date (23). This patient had monoclonal gammopathy but no evidence of multiple myeloma. Monoclonal gammopathy may increase the risk of thrombosis. However, monoclonal gammopathy of thrombotic significance is generally not associated with inflammation, and the absence of recurrent thromboembolic events when no treatment targeting this condition has been initiated does not support this diagnosis (24).

Our patient is the second reported case with IS attributed to MKD. The only other patient with IS secondary to MKD was reported from our center (6). Unlike the current patient, this first case suffered from recurrent systemic inflammatory attacks from a young age. In both cases, the IS resulted from arteriolar occlusion during an attack. IS due to small artery occlusion is likewise reported in other autoinflammatory diseases, including familial Mediterranean fever and adenosine deaminase 2 deficiency (17, 18). To our knowledge, no other MKD patients presenting with IS in adulthood have been previously reported. A survey of 50 patients with MKD identified a single asymptomatic adult. This 40-year-old patient was a homozygous carrier of the pathogenic variant c.1129G>A (V377I) and had a mevalonate kinase activity level of 9.7% (25). This supports the finding that pre-symptomatic individuals may develop clinical manifestations of MKD late in life.

This patient’s late clinical presentation and systemic inflammation documented by blood tests, their response to anti-interleukin-1β treatment, the intermediate level of residual activity of mevalonate kinase, and its altered tertiary structure support that the novel c.1049A>C (Q350P) variant may contribute to an intermediate phenotype when combined in trans with another pathogenic MVK mutation. Consistent with the expanding spectrum of MKD, patients retaining 20%–60% residual mevalonate kinase activity may have milder inflammatory symptoms and a late onset compared to classic MKD. The identification of two MKD patients from a single center in 5 years suggests underestimation or underreporting of this potential cause of IS. Improved recognition of MKD among IS patients could lead to proper treatment with anti-interleukin-1β inhibiting agents and better outcomes. Larger epidemiological studies and multicenter efforts are needed to better define the clinical, biochemical, and genetic spectrum of MKD.

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