Histiocytosis-lymphadenopathy plus syndrome (also known as SLC29A3 spectrum disorder) is a group of conditions with overlapping signs and symptoms that affect many parts of the body. This group of disorders includes H syndrome, pigmented hypertrichosis with insulin-dependent diabetes mellitus (PHID), Faisalabad histiocytosis, and familial Rosai-Dorfman disease (RDD). These conditions were once thought to be distinct disorders; however, because of the overlapping features and shared genetic cause, they are now considered to be part of the same disease spectrum. While some affected individuals have signs and symptoms characteristic of one of the conditions, others have a range of features from two or more of the conditions. The pattern of signs and symptoms can vary even within the same family.

A feature common to the disorders in this spectrum is histiocytosis, which is the overgrowth of immune system cells called histiocytes. The cells abnormally accumulate in one or more tissues in the body, which can lead to organ or tissue damage. The buildup often occurs in the lymph nodes, leading to swelling of the lymph nodes (lymphadenopathy). Other areas of cell accumulation can include the skin, kidneys, brain and spinal cord (central nervous system), or digestive tract.

This spectrum is known as histiocytosis-lymphadenopathy plus syndrome because the disorders that make up the spectrum can have additional signs and symptoms. A characteristic feature of H syndrome is abnormal patches of skin (lesions), typically on the lower body. These lesions are unusually dark (hyperpigmented) and have excessive hair growth (hypertrichosis). In addition, histiocytes accumulate at the site of the skin lesions. Other features of H syndrome include enlargement of the liver (hepatomegaly), heart abnormalities, hearing loss, reduced amounts of hormones that direct sexual development (hypogonadism), and short stature.

Like H syndrome, PHID causes patches of hyperpigmented skin with hypertrichosis. PHID is also characterized by the development of type 1 diabetes (also known as insulin-dependent diabetes mellitus), which usually begins in childhood. Type 1 diabetes occurs when the body does not produce enough of the hormone insulin, leading to dysregulation of levels of blood glucose, also called blood sugar.

Faisalabad histiocytosis typically causes lymphadenopathy and swelling of the eyelids due to accumulation of histiocytes. Affected individuals can also have joint deformities called contractures in their fingers or toes and hearing loss.

The most common feature of familial RDD is lymphadenopathy, usually affecting lymph nodes in the neck. Histiocytes can also accumulate in other parts of the body. (Familial RDD is one of several forms of RDD; the other forms are not considered part of histiocytosis-lymphadenopathy plus syndrome.)

Histiocytosis-lymphadenopathy plus syndrome is a group of conditions with overlapping signs and symptoms that affect many parts of the body. This group of disorders includes H syndrome, pigmented hypertrichosis with insulin-dependent diabetes mellitus (PHID), Faisalabad histiocytosis, and familial Rosai-Dorfman disease (also known as familial sinus histiocytosis with massive lymphadenopathy or FSHML). These conditions were once thought to be distinct disorders; however, because of the overlapping features and shared genetic cause, they are now considered to be part of the same disease spectrum. While some affected individuals have signs and symptoms characteristic of one of these conditions, others have a range of features from two or more of the conditions. The pattern of signs and symptoms can vary, even within the same family.

All of the conditions in the spectrum are characterized by histiocytosis, which is an overgrowth of immune system cells called histiocytes. These cells abnormally accumulate in one or more tissues in the body, which can lead to organ or tissue damage. The lymph nodes are commonly affected, leading to swelling of the lymph nodes (lymphadenopathy). Other areas of cell accumulation can include skin, kidneys, brain and spinal cord (central nervous system), or digestive tract. The spectrum is known as histiocytosis-lymphadenoapthy plus syndrome because the disorders that make up the spectrum can have additional signs and symptoms.

H syndrome is named for the collection of symptoms - all starting with the letter H - that are commonly present. These include hyperpigmented skin lesions with excessive hair growth (hypertrichosis) and histiocyte accumulation, enlargement of the liver or liver and spleen (hepatomegaly or hepatosplenomegaly), heart abnormalities, hearing loss, reduced amounts of hormones that direct sexual development (hypogonadism), and short stature (reduced height). In some cases, hyperglycemia/diabetes mellitus may also be present.

PHID is characterized by patches of hyperpigmented skin with hypertrichosis and the development of type 1 diabetes during childhood.

Faisalabad histiocytosis is characterized by lymphadenopathy and swelling of the eyelids due to the accumulation of histiocytes. Affected individuals may also have joint deformities (contractures) in their fingers or toes, and hearing loss.

Familial Rosai-Dorfman disease is characterized by lymphadenopathy, most often in the neck. Histiocytes can also accumulate in other parts of the body.

Histiocytosis-lymphadenopathy plus syndrome is caused by mutations in the SLC29A3 gene. The condition is inherited in an autosomal recessive pattern. Treatment is aimed at treating the symptoms present in each individual.

Histiocytosis-lymphadenopathy plus syndrome is caused by variants (also known as mutations) in the SLC29A3 gene, which provides instructions for making a protein called equilibrative nucleoside transporter 3 (ENT3). ENT3 belongs to a family of proteins that transport molecules called nucleosides in cells. With chemical modification, nucleosides become the building blocks of DNA, its chemical cousin RNA, and molecules such as ATP and GTP, which serve as energy sources in the cell. Molecules derived from nucleosides play an important role in many functions throughout the body.

ENT3 is found in cellular structures called lysosomes, which break down large molecules into smaller ones that can be reused by cells. Researchers believe that this protein transports nucleosides generated by the breakdown of DNA and RNA out of lysosomes into the cell so they can be reused. The protein is also thought to transport nucleosides into structures called mitochondria, which are the energy-producing centers of cells. In mitochondria, nucleosides are likely used in the formation or repair of DNA found in these structures, known as mitochondrial DNA.

The SLC29A3 gene variants involved in histiocytosis-lymphadenopathy plus syndrome reduce or eliminate the activity of the ENT3 protein. Researchers speculate that the resulting impairment of nucleoside transport leads to a buildup of nucleosides in lysosomes, which may be damaging to cell function. A lack of ENT3 activity may also lead to a reduction in the amount of nucleosides in mitochondria. This nucleoside shortage could impair cellular energy production, which would impact many body systems. It is unclear how the variants lead to histiocytosis and other features of the condition or why affected individuals can have different patterns of signs and symptoms.

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have variants. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Histiocytosis-lymphadenopathy plus syndrome is a rare disorder, affecting approximately 100 individuals worldwide.