SUMMARY
1. Cernunnos 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 NHEJ1 gene. Eight patients with this disease have been reported to date.
2. Additional clinical features include microcephaly, bird-like facies (sloping forehead, receding chin/micrognathia, long nose), IUGR, growth retardation, and developmental delay.
3. Cernunnos is protein in the pathway that mediates repair of DNA double-strand breaks by nonhomologous end-joining (NHEJ) this process is required for T cell 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. All reported patients suffered from recurrent bacterial and opportunistic infections. Two patients were also reported to have autoimmune cytopenias. Malignancies have not been described to date.
5. There is currently no commercially available genetic testing for Cernunnos 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
- 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 unrelated stem cell transplantation for Cernunnos deficiency using reduced intensity conditioning has been reported. However, the long-term outcome with this therapy remains unclear.
OVERVIEW
Cernunnos 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 NHEJ1 gene. Additional clinical features include microcephaly, bird-like facies (sloping forehead, receding chin/micrognathia, long nose), IUGR, growth retardation, and developmental delay.
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. Thus, T and B cell numbers are decreased but NK cell number is normal.
Telangiectasias on the bulbar conjunctiva, ear pinna, and nose start to develop around 3 to 5 years of age. Thus, the onset of ataxia precedes the development of telangiectasia by a few years.
Cernunnos is a protein that helps to mediate 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. Cernunnos forms a complex with XRCC4 and DNA Ligase 4 to catalyze the ligation of DNA ends. Mutations in other proteins involved in NHEJ (ex. DNA Ligase IV, Artemis, DNA-PKcs) also result in radiosensitive SCID.
The immunologic phenotype can range from T-B-NK+ SCID to milder degrees of lymphopenia and hypogammaglobulinemia. All reported patients suffered from recurrent bacterial and opportunistic infections. Two patients were also reported to have autoimmune cytopenias.
EVALUATION
There is currently no commercially available genetic testing for Cernunnos 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.
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- ell 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). 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: 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.
- Radiosensitivity Assay
-This test is available through the UCLA diagnostic molecular pathology laboratory.
- In addition to Cernunnos deficiency, this assay would be expected to be abnormal in other radiosensitive immune deficiencies (A-T, Nijmegen breakage syndrome, DNA Ligase IV deficiency, etc.)
Step 3: Additional Immune Evaluation
- TREC Analysis
- 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 or Omenn syndrome.
-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.
Step 4: Gene Sequencing
The definitive diagnosis for Cernunnos deficiency can be made by demonstrating the presence of mutations in the NHEJ1 gene. This testing is available commercially.
MANAGEMENT
For patients who have mild combined immunodeficiency, supportive measures (immunoglobulin replacement, PJP 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 life-threatening 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)
RESOURCES
Diagnostic Resources
1. UCLA RADIOSENSITIVITY ASSAY
2. The following tests resources are accessible on the SCID overview diagnostic resources page:
-Lymphocyte Subsets by Flow Cytometry for T-cell (CD3, CD4, CD8), B-cell (CD19), and NK cell (CD16/56).
-Naïve (CD45 RA) and Memory (CD45 RO) T cells by Flow Cytometry
-T-cell proliferation to Mitogens and Specific Antigens (candida, tetanus)
-TREC (T-cell receptor excision circle) Analysis
-T-cell Receptor Gene Rearrangement (TCR Spectratyping)
Literature Resources
1. Buck 2006
Cernunnos deficiency - 5 patients with immunodeficiency and microcephaly
2. Faraci 2009
Successful unrelated HSCT in a patient with Cernunnos deficiency
3. Gennery 2006
Immunodeficiency syndromes associated with defective DNA double-strand break
4. Slatter 2010
PID associated with DNA repair (review)