Hyper-IgD syndrome

Hyper IgD syndrome is the less severe form of a metabolic disorder known as mevalonate kinase deficiency. It is considered an auto-inflammatory disease, with recurrent episodic or chronic unexplained inflammation, characterized by periodic episodes of fever, and other symptoms such as joint pain, swollen lymph nodes, skin rash, headaches, and abdominal pain. Most episodes last several days and occur periodically throughout life. The frequency of episodes and their severity vary greatly from person to person.These attacks can occur spontaneously or be triggered by vaccinations, infections, and/or emotional or physical stress. Growth and development is usually not affected.

Signs and symptoms

HIDS is one of a number of periodic fever syndromes. It is characterised by attacks of fever, arthralgia, skin lesions including cyclical mouth ulcers, and diarrhea. Laboratory features include an acute phase response (elevated CRP and ESR) and markedly elevated IgD (and often IgA), although cases with normal IgD have been described. It has mainly been described in the Netherlands and France, although the international registry includes a number of cases from other countries.

The differential diagnosis includes fever of unknown origin, familial Mediterranean fever (FMF) and familial Hibernian fever (or TNFα reception associated periodic syndrome/TRAPS).

Cause

Hyper IgD syndrome is caused by mutations in the gene MVK which encodes the enzyme mevalonate kinase. The mutations lead to a partial deficiency of the enzyme mevalonate kinase. This enzyme converts a substance called mevalonic acid into mevalonate-5-phosphate.

This conversion is the second step in a pathway that makes cholesterol. The cholesterol is later converted into steroid hormones and bile acids. Steroid hormones are needed for normal development and reproduction, and bile acids are used to digest fats. The enzyme also helps to produce other substances that are necessary for certain cellular functions, such as cell growth, cell maturation (differentiation), formation of the cell's structural framework (the cytoskeleton), gene activity (expression), and protein production and modification. A role for this pathway in inflammation is still not completely understood, but it is known that the mevalonate pathway produces cholesterol, and also some compounds which are necessary for the proper interaction and function of these proteins.

Inheritance

Autosomal recessive inheritance, a 25% chance

Hyper IgD syndrome is inherited in an autosomal recessive manner, which means both copies of the gene in each cell have mutations. The parents of a person with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition

Pathophysiology

It is not known how mevalonate kinase mutations cause the febrile episodes, although it is presumed that other products of the cholesterol biosynthesis pathway, the prenylation chains (geranylgeraniol and farnesol) might play a role.

Diagnosis

Mevalonate kinase deficiency causes an accumulation of mevalonic acid in the urine, resulting from insufficient activity of the enzyme mevalonate kinase

Mevalonate pathway

Classified as an inborn error of metabolism, mevalonate kinase deficiency usually results in developmental delay, hypotonia, anemia, hepatosplenomegaly, various dysmorphic features, mental retardation, an overall failure to thrive and several other features.

Treatment

Canakinumab has been approved for treatment of HIDS and has shown to be effective. The immunosuppressant drugs etanercept and anakinra have also shown to be effective. Statin drugs might decrease the level of mevalonate and are presently being investigated. A recent single case report highlighted bisphosphonates as a potential therapeutic option.

Prognosis

Life expectancy is not shortened except for in rare cases where severe infections or kidney amyloidosis occur. Between fever attacks, patients are generally free of symptoms. Attacks do continue throughout the lifetime, although there may be a slight decrease following adolescence. A long-term follow-up study showed that the frequency of the attacks decreases over time, but about half of the patients more than 20 years of age still experience six or more attacks per year, with quite an impact on quality of life.

NIH genetic and rare disease info

• NIH info on Hyper-IgD syndrome

Hyper-IgD syndrome is a rare disease.