1. DNA Ligase IV Deficiency is a rare form of autosomal recessive radiosensitive SCID that results in a T-B-NK+ phenotype.
2. Additional clinical features include microcephaly, bird-like facies (sloping forehead, receding chin/micrognathia, long nose), IUGR, growth retardation, and developmental delay. Some patients do not have any growth or developmental delay. Patients also have an increased risk of developing leukemia and lymphoma.
3. DNA Ligase IV is a protein involved in the repair of DNA double-strand breaks by the nonhomologous end-joining (NHEJ) -– this repair pathway is required for T and B cell V(D)J recombination.
4. The immunologic phenotype can range from T-B-NK+ SCID to milder degrees of lymphopenia and hypogammaglobulinemia. Pancytopenia from marrow failure has been reported in some patients.
5. There is currently no commercially available genetic testing for DNA Ligase IV deficiency. The presence of a T-B-NK+ phenotype and radiation sensitivity may suggest this diagnosis. Other clinical features (microcephaly, IUGR, growth delay, bird-like facies) are additional useful clues.
6. 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 (rotavirus, MMR, Varicella, BCG)
-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).
7. Successful stem cell transplantation for DNA Ligase IV deficiency using reduced intensity conditioning has been reported. However, the long-term outcome with this therapy remains unclear.
DNA Ligase IV Deficiency is a rare form of autosomal recessive radiosensitive SCID that results in a T-B-NK+ phenotype. Additional clinical features include microcephaly, bird-like facies (sloping forehead, receding chin/micrognathia, long nose), IUGR, growth retardation, and developmental delay. Some patients have no growth or developmental delay. Patients have an increased risk of developing leukemia and lymphoma.
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 are generated. Effective V(D)J recombination is essential for proper T and B cell development and requires intact DNA repair machinery.
DNA Ligase IV is a component of the protein complex that mediates repair of DNA double-strand breaks by nonhomologous end-joining (NHEJ). It is encoded for by the LIG4 gene. Other proteins involved in the NHEJ pathway include DNA-PKcs, Ku70, Ku80, Artemis, XRCC4, and Cernunnos. DNA Ligase IV forms of complex with XRCC4 and Cernunnos and is involved in the ligation of blunt DNA ends, compatible DNA overhangs, and incompatible short DNA overhangs. Mutations in Cernunnos, Artemis and DNA-PKcs may also result in radiosensitive SCID.
The immunologic phenotype can range from T-B-NK+ SCID to milder degrees of lymphopenia and hypogammaglobulinemia. Pancytopenia from marrow failure has been reported in some patients.
There is currently no commercially available genetic testing for DNA Ligase IV deficiency (LIG4). The presence of a T-B-NK+ phenotype and radiation sensitivity may suggest this diagnosis. Other clinical features (microcephaly, IUGR, growth delay, bird-like facies) are additional useful clues.
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)
-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 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: Radiosensitivity Assay
The presence of a T-B-NK+ phenotype and key clinical features (microcephaly, IUGR, growth delay, bird-like facies) should prompt testing for radiation sensitivity.
-This test is available through the UCLA diagnostic molecular pathology laboratory.
-In addition to DNA Ligase IV deficiency, this assay would be expected to be abnormal in other radiosensitive immune deficiencies (A-T, Nijmegen breakage syndrome, Cernunnos, etc.)
Step 3: 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.
-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.
The definitive diagnosis for DNA Ligase IV deficiency can be made by demonstrating the presence of mutations in the LIG4 gene. This testing is available commercially.
For patients who have mild combined immunodeficiency, supportive measures (immunoglobulin replacement, antibiotic prophylaxis, etc.) may be reasonable. However, for patients with a T-B-NK+ SCID phenotype, HSCT should be considered with reduced intensity conditioning.
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)
Diagnostic Resources (LAB ORDER FORMS)
1. UCLA RADIOSENSITIVITY ASSAY
2. The following tests resources are accessible on the SCID overview diagnostic resources page:
1. Gennery 2006
Immunodeficiency syndromes associated with defective DNA double-strand break
2. Gruhn 2007
Successful matched HSCT in a patient with DNA Ligase IV deficiency
DNA Ligase IV deficiency identified in patients with immunodeficiency and delay
4. Slatter 2010
PID associated with DNA repair (review)
5. Unal 2009
A novel mutation in a family with DNA Ligase IV deficiency
Radiosensitive T-B-NK+ SCID in DNA Ligase IV deficiency