High-resolution mapping of probes near the X-linked lymphoproliferative disease (XLP) locus

X-linked lymphoproliferative disease: a progressive immunodeficiency

Our understanding of the X-linked lymphoproliferative syndrome (XLP) has advanced significantly in the last two years. The gene that is altered in the condition (SAP/SH2D1A) has been cloned and its protein crystal structure solved. At least two sets of target molecules for this small SH2 domain-containing protein have been identified: A family of hematopoietic cell surface receptors, i.e. the SLAM family, and a second molecule, which is a phosphorylated adapter. A SAP-like protein, EAT-2, has also been found to interact with this family of surface receptors. Several lines of evidence, including structural studies and analyses of missense mutations in XLP patients, support the notion that SAP/SH2D1A is a natural inhibitor of SH2-domain-dependent interactions with members of the SLAM family. However, details of its role in signaling mechanisms are yet to be unravelled. Further analyses of the SAP/SH2D1A gene in XLP patients have made it clear that the development of dys-gammaglobulinemia and B cell lymphoma can occur without evidence of prior EBV infection. Moreover, preliminary results of virus infections of a mouse in which the SAP/SH2D1A gene has been disrupted suggest that EBV infection is not per se critical for the development of XLP phenotypes. It appears therefore that the SAP/SH2D1A gene controls signaling via the SLAM family of surface receptors and thus may play a fundamental role in T cell and APC interactions during viral infections.

High-resolution mapping of the X-linked lymphoproliferative syndrome region by FISH on combed DNA

X-linked lymphoproliferative syndrome is an inherited immunodeficiency for which the responsible gene is currently unknown. Several megabase-sized deleted regions mapping to Xq25 have been identified in XLP patients, and more recently a 130-kb deletion has been reported (Lamartine et al., 1996; Lanyi et al., 1996). To establish a physical map of this deleted region and to identify the XLP gene, two cosmid contigs were established (Lamartine et al., 1996). However, the physical map of this region is still uncompleted and controversial and three points remain unsolved: (1) the centromeric-telomeric orientation of the whole region, (2) the relative orientation of the two contigs, and (3) the size of the gap between the two contigs. To provide a definitive answer to these questions, high-resolution mapping by fluorescence in situ hybridization on combed DNA and molecular approaches were combined to establish the physical map of the XLP region over 600 kb. Our results identified a gap of 150 kb between the two contigs, established the relative orientation of one contig to the other, and determine the centromeric-telomeric orientation of the whole region. Our results show that the order of the marker over this region is: cen.1D10T7-DF83-DXS982.tel.

A review of fluorescence in situ hybridization (FISH): current status and future prospects

Fluorescence in situ hybridization (FISH) is a powerful technique for detecting DNA or RNA sequences in cells, tissues and tumors. This molecular cytogenetic technique enables the localization of specific DNA sequences within interphase chromatin and metaphase chromosomes and the identification of both structural and numerical chromosome changes. FISH is quickly becoming one of the most extensively used cytochemical staining techniques owing to its sensitivity and versatility, and with the improvement of current technology and cost effectiveness, its use will surely continue to expand. Here we review the wide variety of current applications and future prospects of FISH technology.

Inactivating mutations in an SH2 domain-encoding gene in X-linked lymphoproliferative syndrome

X-linked lymphoproliferative syndrome (XLP) is an inherited immunodeficiency characterized by increased susceptibility to Epstein-Barr virus (EBV). In affected males, primary EBV infection leads to the uncontrolled proliferation of virus-containing B cells and reactive cytotoxic T cells, often culminating in the development of high-grade lymphoma. The XLP gene has been mapped to chromosome band Xq25 through linkage analysis and the discovery of patients harboring large constitutional genomic deletions. We describe here the presence of small deletions and intragenic mutations that specifically disrupt a gene named DSHP in 6 of 10 unrelated patients with XLP. This gene encodes a predicted protein of 128 amino acids composing a single SH2 domain with extensive homology to the SH2 domain of SHIP, an inositol polyphosphate 5-phosphatase that functions as a negative regulator of lymphocyte activation. DSHP is expressed in transformed T cell lines and is induced following in vitro activation of peripheral blood T lymphocytes. Expression of DSHP is restricted in vivo to lymphoid tissues, and RNA in situ hybridization demonstrates DSHP expression in activated T and B cell regions of reactive lymph nodes and in both T and B cell neoplasms. These observations confirm the identity of DSHP as the gene responsible for XLP, and suggest a role in the regulation of lymphocyte activation and proliferation. Induction of DSHP may sustain the immune response by interfering with SHIP-mediated inhibition of lymphocyte activation, while its inactivation in XLP patients results in a selective immunodeficiency to EBV.

Host response to EBV infection in X-linked lymphoproliferative disease results from mutations in an SH2-domain encoding gene

X-linked lymphoproliferative syndrome (XLP or Duncan disease) is characterized by extreme sensitivity to Epstein-Barr virus (EBV), resulting in a complex phenotype manifested by severe or fatal infectious mononucleosis, acquired hypogammaglobulinemia and malignant lymphoma. We have identified a gene, SH2D1A, that is mutated in XLP patients and encodes a novel protein composed of a single SH2 domain. SH2D1A is expressed in many tissues involved in the immune system. The identification of SH2D1A will allow the determination of its mechanism of action as a possible regulator of the EBV-induced immune response.

4.5-Mb YAC STS contig at 50-kb resolution, spanning Xq25 deletions in two patients with lymphoproliferative syndrome

Sequence-tagged site (STS) content mapping in yeast artificial chromosomes (YACs) was used to cover the region deleted in two patients affected with X-linked lymphoproliferative disorder. The order of markers includes, centromere to telomere, DXS8009-DXS1206-DXS8078-DXS8044-DXS982- DXS6811-DXS8093-AFM240xblO- DXS75-DXS737-DXS100-DXS6-DXS1046-DXS803 8. The order of six major markers is confirmed by fluorescent in situ hybridization, and all the markers assigned by linkage mapping fall within a 1.6-cM interval. The contig comprises 90 clones containing 89 STSs, yielding a resolution of 50 kb; DNA in a gap just telomeric to DXS8044 has not been found in > 20 equivalents of YACs or bacterial clones. The two deletions were found to have centromeric breakpoints that lie close to DXS1206 and may be identical; the telomeric breakpoints are -150 kb apart, one falling between DXS737 and DXS100, the other between DXS100 and DXS1046. Several STSs near the breakpoints show weak amplification from more than one site; one gives products from three groups of YACs, and lie, respectively, within 50 kb of the centromeric and the two telomeric deletion borders. Such partially duplicated segments of DNA are candidates for involvement in the formation of the deletions.

Deletion of the entire NF1 gene detected by the FISH: four deletion patients associated with severe manifestations

Genetic analysis of NF1 has indicated a wide diversity of mutations, including chromosome rearrangements, deletions, insertions, duplications, and point mutations. Recently, five severely affected individuals have been found by Kayes et Al. [1994] to have deletions encompassing the entire gene. These deletions were detected by quantitative Southern analysis. To simplify deletion detection, we have employed fluorescence in situ hybridization (FISH) using intragenic probes. Thirteen unrelated individuals with NF1 have been studied. Among six with severe manifestations, four have been found to have deletions detected by probes cFF13, cFB5D, cP5, yA43A9, yA113D7 and yD8F4. All four deletions patients have severe developmental delay, minor and major anomalies (including one with bilateral iris colobomas), and multiple cutaneous neurofibromas or plexiform neurofibromas which were present before age 5 years. FISH provides a simple and rapid means of identification of NF1 gene deletions and will allow more rigorous testing of the hypothesis that such deletions are associated with severe manifestations.