X–linked lymphoproliferative syndrome

CLINICAL DESCRIPTION OF THE SYNDROME

In 1974, two groups independently reported an apparent genetic disorder affecting males with a rapidly fatal course following Epstein–Barr virus (EBV) infection [1,2]. While the case for X-linkage was strong, in neither report was there evidence of the disease in two or more generations. Many of the cases involved paediatric patients. In one report, six male maternal cousins out of 18 who were born in one generation, but none of their sisters, died of this disease [2]. The disease was characterized by a proliferation of lymphocytes and histiocytes, variable hepatic abnormalities and alterations in serum immunoglobulins ranging from agammaglobulinaemia to polyclonal hypergammaglobulinaemia. Two of the cousins who were half brothers from separate fathers had lymphomas of the ileum and central nervous system. It was proposed to call the disease X-linked recessive progressive combined variable immunodeficiency or Duncan's disease, after the family's surname. In another report [3], there were three males, two brothers and a maternal cousin, who had a febrile illness with cervical adenopathy, hepatosplenomegaly and a fulminant course during which one of the patients died. Antibody titres to EBV were elevated in one patient during the acute illness and in the remaining two patients after the acute illness. Agammaglobulinaemia developed in the two survivors. Peripheral blood lymphocytes had many of the morphologic features of cells found in infectious mononucleosis but also raised the possibility of a lymphoproliferative disorder. It was speculated that a cytotoxic effect of EBV on B cells or an abnormal T cell response to transformation of B cells by EBV might lead to B cell dysfunction and agammaglobulinaemia. In the ensuing years the disease became known as X-linked lymphoproliferative disease or syndrome (XLP). An XLP registry was established in 1978 and now has over 270 patients registered from over 80 families [4,5]. Fatal infectious mononucleosis occurs in roughly 50% of cases and many of the surviving patients have hypogammaglobulinaemia (30%), and 25% develop lymphoma. Of patients with hypogammaglobulinaemia, about one-third have the hypogammaglobulinaemia prior to EBV infection. In the fulminant fatal form, the onset is typically before the age of 5 years but older cases are well described. There is an impressive polyclonal T cell and B cell proliferation, which infiltrates many organs leading to fulminant hepatitis and bone marrow failure with a haemophagocytic component. The pathogenesis of the disease remains obscure.

Discovery of the gene altered in XLP (see below) has permitted a better analysis of the relationship between the three major phenotypes and EBV. There are some doubts on a critical role for EBV in the syndrome because EBV genomic fragments have not been detectable in the tissues of several patients with lymphomas or dysgammaglobulinaemia (Table 1).

Table 1.

X-linked proliferative syndrome (XLP) (Duncan's disease)

Prevalent phenotypes
1. Fatal infectious mononucleosis (approx. 50%)
Following infection with EBV, patients mount a vigorous, uncontrolled polyclonal expansion of T and B cells.
Destruction of the liver and bone marrow seems to stem from  the uncontrolled T cell responses.
2. B cell lymphomas (approx. 25%)
Most of the lymphomas are extranodal non-Hodgkin's lymphomas,  usually of the Burkitt type, and most involve the ileocaecal  region of the intestine
3. Dysgammaglobulinaemia (approx. 30%)
(Acquired) agammaglobulinaemia and other abnormalities in  immunoglobulin synthesis (similar to CVID)

DISCOVERY OF THE XLP GENE

In 1998, two groups, using very different approaches, identified the gene that is altered in XLP (6,7). Coffey et al. employed a genetic mapping approach of the XLP locus using XLP patient X chromosome deletions as a reference. Their positional cloning studies built on previous genetic linkage evidence showing that the XLP locus was situated on the long arm of the X chromosome in Xq24-25. Additionally, an interstitial deletion of Xq25 was visible in one XLP patient along with his sister and mother. They constructed YAC contigs, exploiting the fact that certain X chromosome markers were absent in XLP-affected males. Marker data from the smallest of these reported deletions were used as a reference for mapping the locus. YAC contigs were sequenced, and as markers were added to this contig map, sequence data from an affected patient were used as a reference to define the proximal and distal boundaries of the XLP locus. Using exon trapping, four genes were identified in this locus; two were outside the region of deletion in the reference patient's DNA, excluding them as potential XLP genes. Of the other two, one SH2 domain-containing gene was found to lie within a region of double recombination in the original Duncan kindred. This gene, termed SH2D1A, was shown by reverse transcriptase-polymerase chain reaction (RT-PCR) sequencing analysis of its four exons to be mutated in nine of 16 unrelated XLP patients analysed.

Sayos et al. (7) identified the XLP gene using a functional/biochemical approach as an SH2 domain-containing signalling molecule, which binds the T/B surface receptor SLAM (CD150). SLAM is a costimulator of T and B cell activation, and as a self-ligand I also appears to be an adhesion molecule (Fig. 1). Using the cytoplasmic tail of SLAM as bait in the yeast two-hybrid system and a human T cell clone library, a small 128 amino acid SH2 domain molecule with a short tail of 26 amino acids was identified. The SH2 domain of this molecule termed SAP (SLAM-associated protein) binds to SLAM as shown in Figs 1 and 2.

Fig. 1.

Interactions between cell surface molecules take place at the interface between an antigen-presenting cell (e.g. Epstein–Barr virus (EBV)-infected B cell) and an activated T cell.

Fig. 2.

SLAM-associated protein (SAP) is a small cytoplasmic protein that binds to SLAM in a resting T cell and to phosphorylated SLAM in an activated T cell.

Cytogenetic studies using a genomic clone of SAP encoding all four exons revealed that the SAP gene was localized in band A5.1 of the mouse X chromosome (8). This region is syntenous with Xq25 in humans, the previously reported locus of XLP. Analysis of the four exons of the SAP gene in three XLP patients revealed one patient with a point mutation in the intron between exons 1 and 2, which affected the splicing events of the SAP mRNA. Thus, most of the SAP mRNA molecules in this patient did not contain exon 2. Because elimination of exon 2 affects the reading frame of the downstream SAP protein, only the first exon was left intact. In two of the other patients all four exons of SAP were deleted. SAP was therefore identified as the product of the XLP gene, its DNA sequence being identical to that of SH2D1A.

THE XLP GENE PRODUCT SAP BINDS TO SLAM IN A UNIQUE FASHION

The SAP crystal structure model fully explains at the atomic level some unique features of binding. SAP has a characteristic SH2 domain's fold, which includes a central β sheet with α helices packed against either side (9). However, exclusively (i) the phosphorylated and non-phosphorylated SLAM peptides bind in the same manner across the surface of the domain, and (ii) the SLAM N-terminal residues at positions pY-1, pY-2 and pY-3 intercalate with residues in strand αA and βD of the SAP SH2 domain. This N-terminal interaction probably provides the additional binding energy that explains the extremely high affinity of SAP/SLAM binding.

To date, various types of SAP mutations have been identified in XLP patients: (i) micro/macro-deletion; (ii) mutations interfering with mRNA transcription or splicing; (iii) missense mutations leading to premature stop codons or amino acid substitutions. Examination of XLP patients' missense mutations (6) suggests that the full complement of binding interactions observed in the crystal structure model are critical for SAP functions. No correlation could be made between the type of mutation and the specific disease phenotype (Table 1).

A MODEL FOR THE FUNCTION OF SAP

Based on its simple structure comprising an SH2 domain with a short N-terminal tail and its high affinity for the cytoplasmic tail of SLAM, SAP has the characteristics of a natural blocker of events involving its binding site. Indeed, SAP was shown to block the binding to SLAM of another SH2-containing signalling protein, the tyrosine phosphatase SHP-2 (Fig. 3). The SLAM/SAP complex provides the cell with a series of signals (A) to interfere either positively or negatively with the signal 1 of T cell activation through the T cell receptor for antigen (see Fig. 1).

Fig. 3.

SLAM-associated protein (SAP) can block recruitment of the tyrosine phosphatase SHP-2.

If SAP is not available, SHP-2 binds to the phosphorylated form of SLAM and becomes enzymatically active, thus generating a second set of signals (B) (Fig. 4). The notion that these alternate states can exist in an activated T cell is supported by the observation that the SAP mRNA is down-regulated immediately upon T cell activation (9), whilst the SLAM gene is up-regulated. This is schematically indicated in Fig. 5.

Fig. 4.

The tyrosine phosphatase SHP-2 is activated upon binding to Phospho-SLAM via its SH2 domains.

Fig. 5.

Expression of the SLAM-associated protein (SAP) and SLAM genes during in vitro T cell activation.

These observations do not exclude the possibility that SAP could block interactions with other SH2 domain-containing proteins and/or interfere with other types of protein–protein interactions. This simple biochemical model does not explain the complete series of events that lead to the apparent uncontrolled T cell proliferation in XLP. However, the notion that the absence of a natural blocker leaves a major set of proliferation signals unchecked is in general agreement with the clinical observations in XLP patients.

DIAGNOSIS

A Pan-American Group (PAGID) and the European Society for Immunodeficiencies (ESID) [10] has published diagnostic criteria for XLP. Definitive diagnostic criteria include a male patient with lymphoma/Hodgkin's disease, fatal EBV infection, immunodeficiency, aplastic anaemia, or lymphohistiocytic disorder and who has a mutation in SH2D1A.

Using antibodies to SAP, Western blot techniques are now available to screen peripheral blood mononuclear cells from suspected XLP patients for SAP protein [11]. Although there are only limited data, it is anticipated that this will identify about 90% of cases. Mutation screening can then be undertaken in those with absent protein, and in the rare cases with SAP protein production who have a typical family history of XLP. It is important to identify other affected males and female carriers in the family; the latter can then be offered prenatal diagnosis or arrangements made to screen male infants at birth.

DIFFERENTIAL DIAGNOSIS

The frequency of XLP among patients with a presumed diagnosis of X-linked agammaglobulinaemia (Bruton's agammaglobulinaemia) or common variable immunodeficiency (CVID) is currently being examined. Since CVID patients have an increased incidence of lymphoreticular neoplasia [12], some may actually have XLP.

Disorders such as Langerhans cell histiocytosis (LCH, Histiocytosis X), non-LCH histiocytic disorders and malignant histiocytic disorders all need to be considered in the differential diagnosis of XLP. Familial haemophagocytic lymphohistiocytosis is due to defects in the perforin gene at 10q21-22 [13]. This is an autosomal recessive disorder and the most common symptoms are fever, hepatosplenomegaly, skin rash and lymphadenopathy. Various cytopenias, hypertriglyceridaemia, hypofibrinogenaemia and abnormal liver function tests are common. There is defective natural killer (NK) function and frequently a hypercytokinaemia, including elevated levels of IL-1R antagonist, sIL-2Rα, IL-6, interferon-gamma (IFN-γ), tumour necrosis factor-alpha (TNF-α) and neopterin.

Secondary haemophagocytic histiocytosis or infection-associated haemophagocytic syndromes should also be considered. The clinical picture is one of high fever, hepatosplenomegaly and progressive pancytopenia. Abnormal laboratory values include elevated hepatic transaminases, lactic dehydrogenase, bilirubin and triglycerides with low fibrinogen levels, high levels of cytokines including IFN-γ, TNF-α, macrophage colony-stimulating factor (M-CSF), sIL-2Rα, IL-1 and IL-6. Triggering organisms include EBV in 121 of 163 cases; other causes include infection with herpes virus 6, cytomegalovirus, adenovirus, parvovirus and varicella-zoster virus. The disorder has also been seen with bacterial, fungal and parasitic infections. Some patients probably have the familial form of the disease.

Malignancy-associated haemophagocytic syndrome occurs in patients with acute lymphoblastic leukaemia and various tumours before or during treatment. In non-Hodgkin's lymphomas with related haemophagocytic syndromes, the lymphoma is often masked. In adults, the angiocentric immunoproliferative type of lymphoma and the CD30⁺ large cell anaplastic lymphomas are most commonly associated with this complication and must be considered in the differential diagnosis.

Malignant histiocytosis, typically associated with Ki-1 or CD30⁺ lymphomas with a translocation involving chromosomes 2 and 5, is typically associated with a proliferation of large, atypical clear histiocyte-like cells with activated macrophages and lymphocytes.

Lymphomatoid granulomatosis is an EBV-related lymphoproliferative disorder, which clinically may vary from a benign lymphoid proliferation to a frank lymphoma. It generally occurs in adults between the fourth to sixth decades and affects males more commonly than females (3:1). The lung is the most frequently involved organ and patients present with fever, malaise and weight loss.

Finally, erythrophagocytic T cell lymphoma, which has been described in two males in the third decade of life presenting with hepatosplenomegaly, jaundice, fever and weight loss, needs to be considered. In these cases, the hepatosplenomegaly was the result of infiltrating T cells which had erythrophagocytic properties [14].

CLINICAL MANAGEMENT

The EBV-associated manifestations of the disease can potentially be prevented by regular immunoglobulin therapy containing antibodies to EBV, although this has not always been successful [4]. The usual practice is to give regular intravenous immunoglobulin (IVIG) to EBV⁻ infants and children who have been genotypically confirmed as XLP during screening of family members of a diagnosed male patient. In patients with hypogammaglobulinaemia, immunoglobulin is usually indicated to prevent recurrent infections. Surveillance must be maintained for the development of lymphomas, particularly of the bowel.

TREATMENT

Allogeneic bone marrow transplantation (BMT) from an MHC-matched sibling following conditioning with etoposide, busulfan and cyclophosphamide has been successful in curing the disease [15], as has BMT from matched-unrelated donors (MUD) [16,17]. If an MHC-matched sibling donor is not available, the disease should be treated with some form of etoposide-containing regimen while the search for a MUD bone marrow donor is underway. Because of the overall poor prognosis, many paediatricians are now recommending BMT in infants diagnosed during family screening before signs of the disease appear. However, some affected individuals have remarkably benign disease, and more information is needed on the frequency of life-threatening complications so that families can be adequately counselled on the risks and benefits of BMT.

INFORMATION FOR PHYSICIANS, PATIENTS and NURSES

Scientific databases

The European Society for Immunodeficiency (ESID) database.

Mutation registry for X-linked lymphoproliferative syndrome (XLP) is located at http://www.uta.fi/imt/bioinfo/SH2D1Abase/

The Pan-American Group for Immunodeficiencies (PAGID) is located at http://http://www.clinimmsoc.org/pagid/

Patient support groups

The Immune Deficiency Foundation is located at http://http://www.primaryimmune.org/

The Jeffrey Modell Foundation is located at http://http://www.jmfworld.com/jmfworld.html

The International Patient Organization for Primary Immunodeficiencies is located at http://http://www.ipopi.org/