1. RAG1 or RAG2 deficiency causes a form of autosomal recessive T-B-NK+ SCID. It accounts for approximately 4% of all SCID cases.
2. The enormous diversity of specific immunoglobulins/B cell receptors and T cell receptors is generated through a process of V(D)J recombination. Random somatic recombination of genetic segments leads to the formation of a vast array of specific antigen receptors.
3. The RAG1 and RAG2 proteins are responsible for mediating DNA cleavage at the TCR and immunoglobulin heavy and light chain loci to initiate V(D)J recombination.
4. Defects in RAG1 or RAG2 cause impaired V(D)J recombination and this leads to defective expression of the pre-TCR and pre-BCR, a critical event in the development of T cells and B cells. As a result, patients present with a T-B-NK+ phenotype. Patients with RAG1 and RAG2 mutations do not exhibit increased radiosensitivity.
5. Presentation occurs in infancy with lymphopenia, FTT, diarrhea, candidiasis, and Pneumocystis jiroveci pneumonia.
6. Hypomorphic or leaky RAG1/RAG2 defects that allow for partial function of the protein can give rise to an atypical form of SCID known as Omenn syndrome. This type of SCID is characterized by proliferation of oligoclonal T cells, severe erythroderma and desquamation, splenomegaly, eosinophilia, and elevated IgE.
7. Laboratory evaluation reveals lymphopenia (ALC <2800 cells/mm3) with markedly reduced T cell and B cell numbers but normal NK cell numbers. T cell proliferation in response to mitogens and specific antigens is reduced. Quantitative immunoglobulins are very low and specific antibody responses are absent.
8. In addition to treatment of acute infections, the following immediate management steps must be implemented:
-Avoid all live viral vaccines
-Only irradiated, CMV negative blood products should be used (to prevent GVHD and infections)
-Pneumocystis jiroveci prophylaxis with trimethoprim-sulfamethoxazole
-IVIG replacement therapy
-Start HLA-typing for the patient and any siblings for possible hematopoietic stem cell transplantation (HSCT).
9. Even with supportive therapies, patients with SCID will not survive without a HSCT. Patients transplanted before 3 months of age have a greater survival while patients who are transplanted later and have suffered end organ damage from infections have a much lower success rate. Overall, patients with T-B- SCID have less successful HSCT outcomes than patients with T-B+ SCID.
The adaptive immune system is able to recognize a vast array of foreign antigens through specific B cell (Ig) receptors and T cell receptors. The antigen recognition regions of these receptors are composed of variable (V), diversity (D), and joining (J) gene segments. A process of somatic recombination known as V(D)J recombination occurs at the TCR and immunoglobulin heavy/light chain loci to produce in excess of 10 to the 14th power receptor combinations.
The first step in V(D)J recombination involves the recognition of recombination signal sequences (RSS) by RAG-1 and RAG-2 proteins and the introduction of double-strand breaks. Next, the DNA damage is recognized by the DNA repair machinery and the breaks are repaired.
Defects in RAG1 or RAG2 cause impaired V(D)J recombination and this leads to defective expression of the pre-TCR and pre-BCR, a critical event in the development of T cells and B cells. RAG1 or RAG2 knockout mice lack both mature T cells and B cells. Humans with these mutations develop T-B-NK+ SCID.
Step 1: Immune Evaluation
-CBC with Differential
-Lymphocyte subset enumeration by flow cytometry (CD3, CD4, CD8, CD19, CD16/56)
-Naïve (CD45RA) and memory (CD45RO) T-cell enumeration by flow cytometry
-T-cell proliferation to Mitogens (PHA)
-IgG, IgA, IgM levels
-Specific Antibody levels (if older than 6 months)
-Chest X ray
-The absolute lymphocyte count (ALC) should be calculated from the CBC (WBC multiplied by the lymphocyte percentage). As with other types of SCID, lymphopenia is common.
-T cell and B cell numbers are markedly reduced while NK cell numbers are normal. However, total T cell numbers can be normal in the SCID variant Omenn syndrome.
-Very low naïve (CD45RA) T cell numbers can be a useful clue for lack of thymic output. Maternally engrafted T cells and oligoclonally expanded patient T cells have a memory (CD45RO) rather than a naïve phenotype and proliferate poorly in response to mitogens.
-Extremely low T-cell proliferation to mitogens can be seen (<10% of control).
-Immunoglobulin levels before 6 months of age may reflect transplacentally aquired maternal IgG). However, immunoglobulin levels can be low prior to 6 months in SCID due to accelerated consumption from recurrent infections. Specific antibody responses to vaccines are poor.
-A chest X-ray may reveal absent thymic tissue.
Step 2: Additional Immune Evaluation
The following tests may provide additional support for a diagnosis of SCID and can be helpful in certain clinical situations but not necessarily required.
-TCR Gene Rearrangement PCR (TCR Spectratyping)
-Maternal Engraftment Study
-TRECs (T-cell receptor excision circles) are loops of DNA excised during TCR rearrangement in the thymus. Because TRECs are not replicated with cell division, they are gradually diluted as T-cells become activated and expand. Thus, naïve T-cells that are recent thymic emigrants have high TREC numbers. SCID patients typically have very low TREC numbers.
-TCR gene rearrangement is useful for identifying oligoclonally expanded T-cells. This can be seen in maternal engraftment as well as Omenns syndrome.
-Maternal T cells can occasionally undergo clonal expansion in patients with SCID. Assessing for the presence of maternal cells in circulation (maternal engraftment) is useful because it can affect the selection of a stem cell donor and it may necessitate immunosuppression prior to transplantation.
Step 3: Genetic Testing
-Genetic testing for RAG1 and RAG2 is available through Correlagen diagnostics (see resources section)
Pending the completion of an immunologic evaluation for suspected CD3 complex deficiency SCID, it is critical to initiate certain measures to prevent life-threatening complications for patients. The following precautions should be implemented immediately:
1. Avoid all live viral vaccines (rotavirus, varicella, MMR, BCG)
Severe vaccine strain disease can occur if SCID patients receive these vaccines.
2. Only irradiated, CMV negative blood products should be used
Leukocytes from non-irradiated blood can cause graft versus host disease and CMV can cause
3. Pneumocystis jiroveci prophylaxis with trimethoprim-sulfamethoxazole
4-6mg/kg/day of Trimethoprim component divided twice daily 3 days per week
4. IVIG replacement therapy
5. High resolution HLA-typing for the patient and any siblings
For possible Hematopoietic Stem Cell Transplantation (HSCT)
SPECIFIC RAG1/RAG2 TESTING:
1. Correlagen - RAG1/RAG2 gene sequencing
The following tests resources are accessible on the SCID overview diagnostic resources page: (click here to link)
1. Lymphocyte Subsets by Flow Cytometry for T-cell (CD3, CD4, CD8), B-cell
(CD19), and NK cell (CD16/56).
2. Naïve (CD45 RA) and Memory (CD45 RO) T cells by Flow Cytometry
3. T-cell proliferation to Mitogens and Specific Antigens (candida, tetanus)
4. TREC (T-cell receptor excision circle) Analysis
5. T-cell Receptor Gene Rearrangement (TCR Spectratyping)
1. Niehius 2010
The phenotypic range of RAG1/RAG2 mutations
2. Antoine 2003
HSCT for primary immune deficiency - European experience 1968-1999. Mortality is higher for T-B- SCID.