Opportunities for genetic errors during reproduction are sofrequent that one wonders how any of us turn out healthy. Althoughmany of these errors are inconsequential, and some can beneficiallyincrease human diversity, germ-line mutations underlie risksfor thousands of diseases. Genetic diseases, however, need notbe inherited through the germ line: somatic mutations can arisein discrete cell lineages early in embryonic development orduring postnatal life. The implications of somatic mutationsare best known in the field of oncology. As predicted by RobertWeinberg two decades ago, and elegantly elucidated with regardto colon cancer by Bert Vogelstein, many cancers arise fromsingle cells through the accretion of somatic mutations thatafford growth and survival advantages. This insight from thefield of cancer is now being extended to nonmalignant diseases.In a recent Journal article, Schwartz discussed mutations arisingin hematopoietic cells in paroxysmal nocturnal hematuria,1 andin this issue of the Journal, Holzelova and colleagues (pages1409–1418) teach us that somatic mutations can also causean autoimmune disease.
In 1967, Canale and Smith described children who had chronic,nonmalignant hypersplenism, lymphadenopathy, and autoimmunity.A clue to the pathogenesis of this condition, which is now calledthe autoimmune lymphoproliferative syndrome (ALPS), was therecognition that two patients with the syndrome had elevatednumbers of lymphocytes bearing / T-cell receptors but lackingCD4 or CD8 surface determinants.2 These lymphocytes were thuscalled double-negative T cells. This finding in ALPS was reminiscentof the phenotype of MRL lpr/lpr mice, whose T cells also failto express the cell-surface receptor Fas, a mediator of programmedcell death, or apoptosis. In 1995, two groups described eightpatients with ALPS, including two siblings, in whom lymphoproliferation,autoimmunity, and abundant double-negative T cells were associatedwith germ-line mutations in the gene that encodes Fas.3,4 Morethan 200 families with hereditary ALPS have now been described(http://research.nhgri.nih.gov/alps/).
In contrast to the homozygous recessive murine lpr mutationthat causes a loss of Fas expression, most humans with ALPShave heterozygous mutations that encode abnormal Fas proteins.These proteins exhibit dominant interference with normal Fasmolecules in trimeric apoptosis-signaling complexes. BecauseFas-mediated apoptosis is a prominent mechanism for eliminatinglymphocytes that have been activated by antigen (see Figure),lymphocytes with Fas defects escape apoptosis and accumulate,causing lymphadenopathy and splenomegaly. It is believed thatsome of the excess, aging lymphocytes lose their CD4 or CD8coreceptors and become double-negative T cells. Other lymphocytesmay bind to host antigens and cause autoimmune hemolytic anemia,neutropenia, thrombocytopenia, and other autoimmune conditions.Criteria to establish a diagnosis of ALPS include chronic nonmalignantsplenomegaly or lymphadenopathy, expansion of /+ double-negativeT cells, and defective lymphocyte apoptosis (demonstrable invitro). Overt autoimmunity occurs in most patients. InheritedFas defects greatly increase the risk of lymphomas of diversehistologic types,5 as further illustrated by the report of Clementiet al. in this issue of the Journal (pages 1419–1424).
Figure. Survival Advantage and Effects of Lymphocytes with Somatic Fas Mutations.
Normal lymphocyte homeostasis depends on maintaining a balance between the expansion of naive cells and their elimination by apoptosis, with a small minority of the cells that are generated after stimulation persisting as memory lymphocytes. Somatic mutations in a fraction of naive cells (brown) lead to their persistence as double-negative T cells, premalignant cells, and autoreactive cells that can mediate autoimmune responses.
Studies of additional patients have revealed that the case criteriafor ALPS were met in only about half of those who had lymphoproliferationand autoimmunity; among the patients who did meet the criteria,one fourth lacked Fas mutations in DNA from whole blood. Wenow know that ALPS is also rarely associated with mutationsin the Fas ligand or in caspases 8 and 10, which transduce thedeath signal triggered by Fas. Our classification system (seeTable) categorizes cases in which no mutation has been identifiedas ALPS type III. In our series of 178 cases studied at theNational Institutes of Health, 42 were classified as ALPS typeIII.
Table. Categorization of ALPS Cases According to Underlying Gene Defects.
Holzelova et al. provide new insights into causation in somepatients with this enigmatic type of ALPS. Pursuing the hypothesisthat double-negative T cells in patients with ALPS accumulatebecause they resist apoptosis, these investigators searchedfor somatic mutations of Fas in isolated double-negative T cellsfrom patients with type III ALPS. Not only were Fas mutationsdetected in this durable cell population, but they were of theintracellular, dominant-interfering type associated with themost highly penetrant cases of familial ALPS. The origin ofmutations was traced back through the T-cell lineage and, inone case, as far as CD34+ hematopoietic progenitor cells, butthe mutations were absent in epithelial cells from the mouthor skin and hence were somatic rather than germ-line mutations.
These observations raise many questions. How does ALPS developin patients in whom somatic mutations are predominantly limitedto a minor, nonproliferating population of double-negative Tcells? If mutant B cells and single-positive T cells (i.e.,CD4+ or CD8+ T cells) cause the disease, why aren't these cellsreadily detected in the blood? Are they lurking in the enlargedlymph nodes and spleen, where they presumably mediate the autoimmunedestruction of blood elements? In fact, a lymph node from oneof the patients studied by Holzelova et al. (Patient 4) showedthe typical paracortical lymphoid hyperplasia of ALPS.
Are the autoantibodies in ALPS a primary result of long-livedautoreactive B cells with defective Fas? Not necessarily. Asshown by Uri Lopatin and our colleagues, double-negative T cellsin ALPS greatly overproduce interleukin-10, an important B-cellstimulator and driver of type 2 T-helper responses. Thus, superannuateddouble-negative T cells could promote the proliferation of nonmutatedlymphocytes, thereby indirectly leading to autoimmune disease.
Are patients in whom ALPS is caused by somatic mutations atrisk for blood-cell cancers? The findings of Holzelova et al.suggest that such patients, like those with inherited Fas defects,are at risk for cancers originating in Fas-defective hematopoieticcells. A substantial proportion of sporadic lymphomas and othercancers in previously healthy subjects bear somatic mutationsin Fas.
Do somatic mutations in apoptosis pathways underlie more commonautoimmune diseases, such as lupus and rheumatoid arthritis?After the discovery of ALPS, investigators looked in vain forgerm-line mutations in apoptosis genes, but lymphocytes at thesites of injury should now be reexamined for somatic mutations.Indeed, the powerful combination of somatic mutation and naturalselection for increased survival may explain the evolution ofdiverse human diseases whose origins are currently unknown.
Source Information
From the Genetics and Molecular Biology Branch, National Human Genome Research Institute (J.M.P), and the Laboratory of Clinical Infectious Diseases, National Institute of Allergy and Infectious Diseases (S.E.S.), National Institutes of Health, Bethesda, Md.
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T-cell receptors but lacking CD4 or CD8 surface determinants.2 These lymphocytes were thus called double-negative T cells. This finding in ALPS was reminiscent of the phenotype of MRL lpr/lpr mice, whose T cells also fail to express the cell-surface receptor Fas, a mediator of programmed cell death, or apoptosis. In 1995, two groups described eight patients with ALPS, including two siblings, in whom lymphoproliferation, autoimmunity, and abundant double-negative T cells were associated with germ-line mutations in the gene that encodes Fas.3,4 More than 200 families with hereditary ALPS have now been described (http://research.nhgri.nih.gov/alps/). 
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