1 results for Ameratunga, Rohan

  • The molecular pathology of the hyper immunoglobulin M syndrome

    Ameratunga, Rohan (2000)

    Doctoral thesis
    The University of Auckland Library

    The Hyper Immunoglobulin M syndrome (HIM) is a rare primary immune deficiency disorder characterised by an inability to produce immunoglobulin isotypes other than IgM and IgD. The B cell-surface antigen CD40, and its conjugate T cell counter-structure, CD40 ligand (CD154), mediate immunoglobulin isotype switching to IgE and IgG4 in the presence of cytokines such as interleukin 4. Patients with the X-linked form of the hyper IgM syndrome (XHIM) have mutations in the CD40 ligand gene which is the molecular basis for impaired immunoglobulin isotype switching in XHIM. This thesis is a tale of two HIM patients, one with XHIM (patient H1) and the other (patient M1) with a novel immune defect which has yet to be fully characterised. Analysis of Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) products of the CD40 ligand gene from patient H1 revealed two mutant transcripts, one missing exon 4 and the other missing both exons 3 and 4. In addition, the cells of patient H1 also produce small quantities of wild type cD40 ligand transcripts. Molecular analysis has shown that this complex splicing defect is precipitated by a point mutation at the invariant 3’ splice acceptor site with a g→c substitution at position -1 (tcacGTA) of intron 3. RT-PCR studies have shown that the cells of the mother of patient H1 produce both wild type and deleted CD40 ligand transcripts, consistent with her carrier state. Genomic sequencing of intron 3 has shown that she is heterozygous for the point mutation (tcacgEGTA), confirming her carrier status. Analysis of other family members, including the sister and the maternal grandmother, has indicated that they have not inherited the mutation. Patients with XHIM are predisposed to Pneumocystis carinii pneumonitis and to Cryptosporidiosis. Studies of T cell function in patient H1 has identified a defect in antigenspecific proliferation which provides an explanation for the susceptibility of XHIM patients to opportunistic infections. In contrast to patient H1, the cells of patient M1 have wild type CD40 ligand sequence and express normal amounts of cell surface CD40 ligand indicating that he does not have XHIM. Flow cytometry and in vitro antibody synthesis studies have shown that the B cells of this patient are able to undergo immunoglobulin isotype switching both in vivo and in vitro. Unlike the mild T cell defect shown in XHIM patients, a severe T cell defect is present in this patient who also has impaired CD40-induced B cell function. The impaired CD40 function may be secondary to the T cell defect. The demonstration of wild type CD40 ligand transcripts in the patient with a splicing defect may explain the milder phenotype seen in some XHIM patients. The identification of the T cell defect should result in re-classification of XHIM as a combined immune deficiency disorder The characterisation of patient M1 extends the phenotypic spectrum of the hyper IgM syndrome to include a severe T cell defect in the context of normal CD40 ligand structure and function.

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