Primary immunodeficiencies – molecular basis and gene therapy Alain Fischer, Geneviève de Saint Basile, James P. DiSanto, Marina Cavazzana-Calvo INSERM U 429, Hôpital Necker-Enfants Malades, Université René Descartes, 149 rue de Sèvres, Paris Cedex 15, France Introduction cations that consist not only in infections but also in allergic and autoimmune manifestations as well Primary immunodeficiencies (PID) consist in ap- as in cancer (particularly lymphomas). proximately 80 distinct diseases that disturb de- velopment and/or function of the immune system. It is estimated that about 1 in 5000 individuals is born with or will develop a symptomatic primary Classification of primary immunodeficiencies immunodeficiency. It is usual to classify PID in four groups: i) predominant T-cell deficiency, ii) Significant advances in the molecular understand- predominant B-cell deficiency, iii) deficiencies of ing of several PID have occurred during the last the phagocyte cell system and iv) complement de- three years; affected genes from 9 PID have been ficiencies. They represent 20, 70, 10 and ~1% of identified within this period of time. A list of PID the overall group of PID, respectively. Several PID for which a molecular basis is now understood is can induce life threatening and recurrent compli- given in tables I, II and III. Table I: T-cell immunodeficiencies. Disease Phenotype Gene product X-linked SCID Absence of T and NK cells IL-2Rγ (γc) Adenosine deaminase (ADA) deficiency Reduction in T, B and NK cells ADA Purine nucleoside phosphorylase Progressively reduced T cell counts PNP (PNP) deficiency HLA class II deficiency (group A) Low CD4 T-cell counts, defective ClassIItrans-activator antibody response (CIITA) TAP 2 deficiency Reduced class I expression, low NK activity TAP 2 (peptide transporter) ZAP 70 deficiency Low CD8 T-cell counts, defective activation ZAP 70 tyrosine kinase of CD4 T-cells CD3 ε deficiency Defective T-cell activation CD3 ε CD3 γ deficiency Low CD8 T-cell counts, defective T-cell activation CD3 γ Wiskott-Aldrich syndrome (WAS) Progressive lymphocytopenia, defective antibody WASP (nuclear protein) response to polysaccharides, thrombocytopenia “Hyper IgM syndrome” Defective Ig heavy chain switch, defective handling CD40 ligand of mucosal opportunistic microorganisms Annales Nestlé 1996;54:25-34 25 Alain Fischer, Geneviève de Saint Basile, James P. DiSanto, Marina Cavazzana-Calvo T-cell immunodeficiencies mRNA translation [8]. Surprisingly, some residual T-cell differentiation occurs as observed in mice The most severe form of PID consists in severe in which the c gene has been inactivated [9], sug- combined immunodeficiencies (SCID) charac- γ terised by a profound block in T-cell differentia- gesting a partially effective supplementary path- tion and thus by a virtual absence of mature T- way for proliferation of early T-cell precursors. cells. SCID are leading to severe infections occurring in the first year of life causing death in Adenosine deaminase deficiency the absence of treatment by allogeneic bone mar- Adenosine deaminase (ADA) deficiency was the row transplantation. Frequency is estimated to be first identified PID. Accumulation of deoxyadeno- about 1 in 75 000 live births [1]. sine results in excessive concentration of deoxy- ATP in lymphocyte precursors. The latter product X-linked SCID blocks DNA synthesis by inhibiting synthesis of Two molecular defects have been identified that the other deoxynucleotides. The consequences of can cause SCID in human beings. The most fre- this block are limited to lymphocytes because quent form of SCID, i.e. X-linked SCID (about 50 5’nucleotidase reduces deoxy-ATP level in other to 60% of cases of SCID), is characterised by an cell types [10]. The residual ADA enzymatic ac- absence of mature T- and natural killer (NK) cells tivity determines PID severity, i.e., between full while B-cells are present in normal or elevated blown expression (alymphocytosis) to a mild im- numbers. The thymus is profoundly depleted of munodeficiency with late onset symptoms. A thymocytes. The X-linked SCID locus was as- residual activity of 10% of normal ADA activity is signed to Xq1.3-2.1. It was then recognised that sufficient to avoid any expression of immunodefi- the gene encoding the γ chain of the interleukin-2 ciency. ADA deficiency can be treated by weekly (IL-2) receptor, renamed γc, maps to the same re- intramuscular injection of bovine ADA coupled to gion of the X chromosome than the X-linked SCID polyethyleneglycol (PEG) or by bone marrow locus and was found mutated in patients with X- transplant. PEG-ADA is quite stable in vivo. The linked SCID [2]. A variety of mutations affecting enzyme is active and enables some T- and B-cell all parts of the gene has now been described [3, 4]. differentiation and function. About 40 patients Interestingly, it is not primarily because the high have been treated world-wide for a maximum fol- affinity IL-2 receptor (IL-2R) is lacking as a conse- low-up of 8 years. The treatment’s efficacy varies quence of the absence of the γc chain that T-cell between patients and although PEG-ADA treat- differentiation is blocked. Indeed, PID patients ment does not achieve full correction of the im- with selective IL-2 deficiency, or mice unable to munodeficiency it appears to be more effective in make IL-2 because the IL-2 gene has been inacti- milder forms of the ADA deficiency. This treat- vated, have mature T-cells indicating that T-cell ment nonetheless allows most patients to survive differentiation can occur in the absence of IL-2/ in good condition and is rather safe until a cura- IL-2R interaction. The γc chain has been found to tive therapy becomes available. be a member of several cytokine receptors, i.e. Other forms of SCID, mainly characterised by IL-2, IL-4, IL-7, IL-9 and IL-15 [5]. The SCID phe- both defective T- and B-cell differentiation, have notype thus results from the summation of these been described. They possibly correspond to de- defects among which the IL-7 receptor anomaly fects in the machinery necessary for T-cell recep- may play a major role in blocking early T (NK)- tor (TCR) and immunoglobulin gene rearrange- cell precursor proliferation. ments as observed in the murine SCID model; Attenuated phenotypes have been described. among the defects studied are those in rag-1 and They are characterised by oligoclonal T-cell dif- rag-2 proteins necessary for the initiation of the ferentiation, found in patients with either reduced recombination process [11]. γc expression [6] or with mutation of γc in the binding site of the Jak3 tyrosine kinase [7]. An X- Atypical SCID linked SCID has been described in a dog pedigree; A number of inherited T-cell immunodeficiencies, the γc chain is deficient because of an early stop in sometimes called “atypical SCID” or “combined 26 Annales Nestlé 1996;54:25-34 Primary immunodeficiencies – molecular basis and gene therapy immunodeficiencies” (CID), remain ill defined 50% of normal, respectively) and partially defec- although some T-cell differentiation occurs. tive signal transduction [14, 15]. These deficits are characterised by defective T- cell activation or effector function, although some ZAP deficiency T-cell differentiation does occur. Very interesting Recognition by the TCR of peptides associated findings were recently reported which led to the with HLA molecules induces a cascade of signals, molecular characterisation of diseases described the first of which involves a series of tyrosine ki- below. nases: lck, fyn and ZAP 70. A ZAP 70 deficiency has been demonstrated in several immunodefi- HLA class II deficiency cient patients. Absence of detectable ZAP 70 acti- Some patients present with the defective expres- vity blocks signal transduction in T-cells empha- sion of HLA class II molecules caused by the ab- sizing its crucial role in the initiation of T-cell normal transcription of otherwise structurally nor- activation. It also leads to a lack of CD8 T-cells mal genes. Defective expression of HLA class II presumably because ZAP 70 cannot be replaced molecules leads to a reduction (but not a disap- satisfactorily by a similar kinase, syk, in thymo- pearance) of the CD4+ T-cell pool as well as lack cytes following class I (CD8 dependent) restricted of HLA class II restricted T-cell responses and activation [16, 17]. defective antibody responses. Complementation analysis revealed three distinct groups of HLA Wiskott-Aldrich syndrome class II deficiency. In two, the promoter region of The Wiskott-Aldrich syndrome (WAS) is charac- HLA class II genes is not associated with DNA terized by the association of complex features, binding proteins in intact cells, thus the DNA bind- i.e. progressive T- and B-cell deficiency with de- ing proteins that interact with the so-called X1 fective antibody responses to polysaccharides, box of the promoter may be missing. In the third allergic and autoimmune manifestations, and (group A), the promoter is normally occupied; but quantitative and qualitative (small volume) a “class II transactivator” (CIITA) nuclear protein platelet anomalies. The WAS locus was assigned is defective in these patients. The gene was cloned to Xp11.23. The WAS gene was identified by by complementation. Its product, which is γ- in- positional cloning. It encodes a nuclear protein of terferon inducible, is necessary for class II expres- unknown function that is expressed by lympho- sion. Patients from group A exhibit mutations of cytes and megakaryocytes [18]. the CIITA encoding gene [12]. “Hyper IgM syndrome” TAP 2 deficiency The hyper IgM syndrome (HIGM-1) is primarily a TAP 2 is required as an heterodimer with TAP 1 to B-cell deficiency. It is characterized by a defec- transport peptides from the cytosol to the endo- tive Ig switch to IgG, IgA and IgE, but IgM and IgD plasmic reticulum, where they interact with HLA are produced normally.