SUMMARY
1. Mutations in the gene for Artemis (DCLRE1C) cause a rare form of autosomal recessive radiosensitive SCID which results in a T-B-NK+ phenotype. This type of SCID was originally described in Athabascan-speaking native Americans.
2. Similar to other types of SCID, presentation occurs in infancy with lymphopenia, FTT, diarrhea, candidiasis, and Pneumocystis jiroveci pneumonia. Patients with an Omenn syndrome phenotype have also been described. T and B cell numbers are markedly decreased but NK cell numbers are normal. Patients have an increased risk of developing lymphomas.
3. Artemis is protein in the pathway that mediates repair of DNA double-strand breaks (dsb) by nonhomologous end-joining (NHEJ) – this process is required for T cell and B cell V(D)J recombination. Artemis interacts with DNA-PKcs and plays a role in opening of the DNA hairpin that is formed following double-stranded breaks introduced by RAG.
4. The diagnosis of Artemis deficiency is suggested by a T-B-NK+ SCID phenotype and radiosensitivity. The definitive diagnosis is made by sequencing of the DCLRE1C gene.
5. In addition to treatment of acute infections, the following immediate management steps must be implemented for patients with a SCID phenotype:
- 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).
6. 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. Excessive exposure to radiation in the form of imaging studies should be avoided unless absolutely necessary.
OVERVIEW
Artemis deficiency is a rare form of autosomal recessive radiosensitive SCID that results in a T-B-NK+ phenotype. It is caused by mutations in the DCLRE1C gene. This type of SCID was originally described in Athabascan-speaking native Americans –- fibroblasts from these patients exhibit increased cellular sensitivity to ionizing radiation.
Similar to other types of SCID, presentation occurs in infancy with lymphopenia, FTT, diarrhea, candidiasis, and Pneumocystis jiroveci pneumonia. T and B cell numbers are markedly decreased but NK cell numbers are normal. Patients have an increased risk of developing lymphomas.
Artemis is a protein in the pathway that mediates repair of DNA double-strand breaks by nonhomologous end-joining (NHEJ). NHEJ is mediated by a number of proteins including DNA-PKcs, Ku70, Ku80, Artemis, XRCC4, DNA Ligase 4, and Cernunnos. Artemis interacts with DNA-PKcs and plays a role in opening of the DNA hairpin that is formed following double-stranded breaks introduced by RAG. Mutations in other proteins involved in NHEJ (ex. DNA Ligase IV, Cernunnos, DNA-PKcs) also result in radiosensitive SCID.
DNA double-strand breaks arise from DNA damage induced by ionizing radiation and during V(D)J recombination – the process by which an enormous diversity of specific immunoglobulins/B cell receptors and T cell receptors is generated. Effective V(D)J recombination is essential for proper T and B cell development but not NK cell development.
EVALUATION
Artemis deficiency should be considered in male or female patients presenting with a SCID-like phenotype (FTT, diarrhea, thrush, Pneumocystis jiroveci pneumonia, severe respiratory virus infections) along with low absolute lymphocyte counts.
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 responses to vaccine antigens (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). SCID presents with an ALC less than 2800 cells/mm3 in 95% of cases.
-Low T cell and B cell numbers are present while NK cell numbers are normal.
-Very low naïve (CD45RA) T-cell numbers can be a useful clue for lack of thymic output. In cases of maternal T cell engraftment the circulating T cells have a predominantly memory (CD45RO) phenotype and have poor proliferation in response to mitogens.
-Extremely low T cell proliferation to mitogens (<10% of control) is seen in patients with a SCID phenotype. The large blood volume required to perform mitogen proliferation is often an issue with small infants. Performing the proliferation assay with one stimulus (PHA) is acceptable and requires less blood.
-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.
-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.
-TREC Analysis
-TCR Gene Rearrangement PCR (TCR Spectratyping)
-Maternal Engraftment Study
-Radiosensitivity Assay
-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.
-Maternal T cells can occasionally undergo clonal expansion in patients with SCID. Maternal T cells typically are CD45RO+ and proliferate poorly to mitogen stimulation. An evaluation 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.
-A radiosensitivity assay is commercially available through the UCLA diagnostic molecular pathology laboratory. This assay would be expected to be abnormal in other radiosensitive immune deficiencies (A-T, Nijmegen breakage syndrome, DNA Ligase IV deficiency, Cernunnos etc.)
MANAGEMENT
Pending the completion of an immunologic evaluation, it is critical to initiate certain measures to prevent life-threatening complications for patients with SCID. 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 severe infections.
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)
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. Excessive exposure to radiation in the form of imaging studies should be avoided unless absolutely necessary.
RESOURCES
Diagnostic Resources
ARTEMIS SPECIFIC RESOURCES:
-UCLA RADIOSENSITIVITY ASSAY
-CORRELAGEN ARTEMIS SEQUENCING
-GENE DX ARTEMIS SEQUENCING
The following tests resources are accessible on the SCID overview diagnostic resources page:
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)
Literature Resources
1. Slatter 2010
PID associated with DNA repair
2. Ege 2005
Artemis
3. Li 2002
Artemis mutations cause Athabascan SCID
4. Moshous 2001
Artemis mutations in radiosensitive SCID patients