Chronic granulomatous disease


Chronic granulomatous disease is a diverse group of hereditary diseases in which certain cells of the immune system have difficulty forming the reactive oxygen compounds used to kill certain ingested pathogens. This leads to the formation of granulomata in many organs. CGD affects about 1 in 200,000 people in the United States, with about 20 new cases diagnosed each year.
This condition was first discovered in 1950 in a series of 4 boys from Minnesota, and in 1957 it was named "a fatal granulomatosus of childhood" in a publication describing their disease. The underlying cellular mechanism that causes chronic granulomatous disease was discovered in 1967, and research since that time has further elucidated the molecular mechanisms underlying the disease. Bernard Babior made key contributions in linking the defect of superoxide production of white blood cells, to the cause of the disease. In 1986, the X-linked form of CGD was the first disease for which positional cloning was used to identify the underlying genetic mutation.

Symptoms

Classically, patients with chronic granulomatous disease will suffer from recurrent bouts of infection due to the decreased capacity of their immune system to fight off disease-causing organisms. The recurrent infections they acquire are specific and are, in decreasing order of frequency:
Most people with CGD are diagnosed in childhood, usually before age 5. Early diagnosis is important since these people can be placed on antibiotics to ward off infections before they occur. Small groups of CGD patients may also be affected by McLeod syndrome because of the proximity of the two genes on the same X-chromosome.

Atypical infections

People with CGD are sometimes infected with organisms that usually do not cause disease in people with normal immune systems. Among the most common organisms that cause disease in CGD patients are:
Patients with CGD can usually resist infections of catalase-negative bacteria but are susceptible to catalase-positive bacteria. Catalase is an enzyme that catalyzes the breakdown of hydrogen peroxide in many organisms. In infections caused by organisms that lack catalase, the host with CGD is successfully able to "borrow" hydrogen peroxide being made by the organism and use it to fight off the infection. In infections by organisms that have catalase, this "borrowing mechanism" is unsuccessful because the enzyme catalase first breaks down any hydrogen peroxide that would be borrowed from the organism. Therefore in the CGD patient, hydrogen peroxide cannot be used to make oxygen radicals to fight infection, leaving the patient vulnerable to infection by catalase-positive bacteria.

Genetics

Most cases of chronic granulomatous disease are transmitted as a mutation on the X chromosome and are thus called an "X-linked trait". The affected gene on the X chromosome codes for the gp91 protein p91-PHOX. CGD can also be transmitted in an autosomal recessive fashion which affect other PHOX proteins. The type of mutation that causes both types of CGD are varied and may be deletions, frame-shift, nonsense, and missense.
A low level of NADPH, the cofactor required for superoxide synthesis, can lead to CGD. This has been reported in women who are homozygous for the genetic defect causing glucose-6-phosphate dehydrogenase deficiency, which is characterised by reduced NADPH levels.

Pathophysiology

Phagocytes require an enzyme to produce reactive oxygen species to destroy bacteria after they are ingested, a process known as the respiratory burst. This enzyme is termed "phagocyte NADPH oxidase". This enzyme oxidizes NADPH and reduces molecular oxygen to produce superoxide anions, a reactive oxygen species. Superoxide is then disproportionated into peroxide and molecular oxygen by superoxide dismutase. Finally, peroxide is used by myeloperoxidase to oxidize chloride ions into hypochlorite, which is toxic to bacteria. Thus, NADPH oxidase is critical for phagocyte killing of bacteria through reactive oxygen species.
Defects in one of the four essential subunits of phagocyte NADPH oxidase can all cause CGD of varying severity, dependent on the defect. There are over 410 known possible defects in the PHOX enzyme complex that can lead to chronic granulomatous disease.

Diagnosis

When chronic granulomatous disease is suspected, neutrophil-function testing should be carried out, and positive findings should be confirmed by genotyping. The p47phox mutation is due to a pseudogene conversion, hence it may not be detectable by standard sequencing; in these cases, an immunoblot or gene dose determination may be needed to confirm p47phox deficiency.
Infections caused by the pathogens commonly associated with CGD should prompt functional or genetic screening; neonatal or early postnatal screening of potentially affected children is essential with a family history of CGD.
Neutrophil function tests: These include nitroblue tetrazolium reduction test, dihydrorhodamine 123 test, direct measurement of superoxide production, cytochrome c reduction assay, and chemiluminescence. DHR test is usually preferred because it is easy to use, objective, and it is able to distinguish between X-linked and autosomal forms of CGD; furthermore, it allows to detect gp91phox carriers.
Genetic testing: Once CGD has been diagnosed based on abnormal neutrophil function tests, genetic testing should go next. As mentioned above, p47phox defect is ussually difficult to identify genetically because it is caused by pseudogene conversion and may be missed in typical sequencing studies; in this case, immunoblotting or flow cytometry can show absence of protein.
Prenatal testing: It is particularly useful when a family member has already been diagnosed with CGD. This test may be performed by analysis of NADPH oxidase activity of neutrophils from fetal blood. Samples from amniotic fluid or chorionic villi provides an earlier and more reliable diagnosis for families at risk.

Classification

Chronic granulomatous disease is the name for a genetically heterogeneous group of immunodeficiencies. The core defect is a failure of phagocytic cells to kill organisms that they have engulfed because of defects in a system of enzymes that produce free radicals and other toxic small molecules. There are several types, including:
Management of chronic granulomatous disease revolves around two goals: 1) diagnose the disease early so that antibiotic prophylaxis can be given to keep an infection from occurring, and 2) educate the patient about his or her condition so that prompt treatment can be given if an infection occurs.

Antibiotics

Physicians often prescribe the antibiotic trimethoprim-sulfamethoxazole to prevent bacterial infections. This drug also has the benefit of sparing the normal bacteria of the digestive tract. Fungal infection is commonly prevented with itraconazole, although a newer drug of the same type called voriconazole may be more effective. The use of this drug for this purpose is still under scientific investigation.

Immunomodulation

, in the form of interferon gamma-1b is approved by the Food and Drug Administration for the prevention of infection in CGD. It has been shown to reduce infections in CGD patients by 70% and to decrease their severity. Although its exact mechanism is still not entirely understood, it has the ability to give CGD patients more immune function and therefore, greater ability to fight off infections. This therapy has been standard treatment for CGD for several years.

Hematopoietic stem cell transplantation (HSCT)

Hematopoietic stem cell transplantation from a matched donor is curative although not without significant risk.

Prognosis

There are currently no studies detailing the long term outcome of chronic granulomatous disease with modern treatment. Without treatment, children often die in the first decade of life. The increased severity of X-linked CGD results in a decreased survival rate of patients, as 20% of X-linked patients die of CGD-related causes by the age of 10, whereas 20% of autosomal recessive patients die by the age of 35.
Recent experience from centers specializing in the care of patients with CGD suggests that the current mortality has fallen to under 3% and 1% respectively.
CGD was initially termed "fatal granulomatous disease of childhood" because patients rarely survived past their first decade in the time before routine use of prophylactic antimicrobial agents. The average patient now survives at least 40 years.

Epidemiology

CGD affects about 1 in 200,000 people in the United States, with about 20 new cases diagnosed each year.
Chronic granulomatous disease affects all people of all races, however, there is limited information on prevalence outside of the United States. One survey in Sweden reported an incidence of 1 in 220,000 people, while a larger review of studies in Europe suggested a lower rate: 1 in 250,000 people.

History

This condition was first described in 1954 by Janeway, who reported five cases of the disease in children. In 1957 it was further characterized as "a fatal granulomatosus of childhood". The underlying cellular mechanism that causes chronic granulomatous disease was discovered in 1967, and research since that time has further elucidated the molecular mechanisms underlying the disease. Use of antibiotic prophylaxis, surgical abscess drainage, and vaccination led to the term "fatal" being dropped from the name of the disease as children survived into adulthood.

Research

Gene therapy is currently being studied as a possible treatment for chronic granulomatous disease. CGD is well-suited for gene therapy since it is caused by a mutation in single gene which only affects one body system. Viruses have been used to deliver a normal gp91 gene to rats with a mutation in this gene, and subsequently the phagocytes in these rats were able to produce oxygen radicals.
In 2006, two human patients with X-linked chronic granulomatous disease underwent gene therapy and blood cell precursor stem cell transplantation to their bone marrow. Both patients recovered from their CGD, clearing pre-existing infections and demonstrating increased oxidase activity in their neutrophils. However, long-term complications and efficacy of this therapy were unknown.
In 2012, a 16-year-old boy with CGD was treated at the Great Ormond Street Hospital, London with an experimental gene therapy which temporarily reversed the CGD and allowed him to overcome a life-threatening lung disease.