2.1. Mevalonate kinase deficiency

At least 80 mutations in the MVK gene have been found to cause mevalonate kinase deficiency. There are two types of mevalonate kinase deficiency, which are differentiated by the severity of the signs and symptoms. The less severe of the two types is called hyperimmunoglobulinemia D syndrome (HIDS) and the more severe type is called mevalonic aciduria (MVA). Most of the MVK gene mutations that cause mevalonate kinase deficiency lead to changes in single protein building blocks (amino acids) in the mevalonate kinase enzyme. One mutation that replaces the amino acid valine with the amino acid isoleucine at position 337 in the enzyme (written as Val337Ile or V337I) is found in approximately 80 percent of people with HIDS. The V337I mutation has never been found in people with MVA.

Most MVK gene mutations lead to the production of a mevalonate kinase enzyme that is unstable and folded into an incorrect 3-dimensional shape, leading to a reduction of mevalonate kinase enzyme activity. The severity of the enzyme shortage (deficiency) determines the severity of the condition. People who have approximately 1 to 20 percent of normal mevalonate kinase activity typically develop HIDS. Individuals who have less than 1 percent of normal enzyme activity usually develop MVA. Despite this shortage of mevalonate kinase activity, people with mevalonate kinase deficiency typically have normal production of cholesterol, steroid hormones, and bile acids.

Some researchers believe the features may be due to a buildup of mevalonic acid, the substance that mevalonate kinase normally acts on. Other researchers think that a shortage of the substances produced from mevalonic acid, such as those substances necessary for certain cellular functions, causes the fever episodes and other features of this condition. The exact mechanism that causes inflammatory reactions such as fevers, skin rashes, elevated immune system proteins, and many other features of mevalonate kinase deficiency is unclear.