Getting a Precise Diagnosis

Because Cernunnos-XLF deficiency is an ultra-rare condition, the journey to a diagnosis often involves several layers of testing. Doctors look for a combination of physical signs, immune system blood markers, and a definitive genetic match ^[1][2].

The Definitive Diagnostic Tests

A diagnosis is considered “definitive” only when genetic testing identifies pathogenic or likely pathogenic biallelic variants in the NHEJ1 gene, and parental segregation testing confirms these variants are on opposite copies of the gene (in trans) ^[1][2]. Variants of uncertain significance (VUS) do not establish a definitive diagnosis.

• Genetic Sequencing: Most families receive a diagnosis through Whole Exome Sequencing (WES) or a comprehensive Inborn Errors of Immunity (IEI) panel. It is important to know that some standard “SCID panels” or “Bone Marrow Failure panels” may not include the NHEJ1 gene, which can sometimes lead to a delayed diagnosis ^[2][3].
• Radiosensitivity Assays: Because the XLF protein is part of the DNA repair process, a child’s cells are abnormally sensitive to radiation. In a lab, specialists may test skin cells (fibroblasts) by exposing them to radiation and measuring how well they survive or repair the damage ^[1][4]. This functional assay supports a DNA-repair defect, but abnormal results can overlap with other disorders and must be interpreted alongside the sequencing ^[5].

Reading the Immune Lab Reports

Your child’s blood work provides a “snapshot” of how their immune system is currently performing. These results must be interpreted using age-based reference ranges, as a baby’s counts differ significantly from an adult’s.

1. Lymphocyte Subsets (Flow Cytometry)

This test counts different types of white blood cells.

• T-cells (CD3, CD4, CD8): Often very low (lymphopenia). Even if counts are near normal, these cells may lack the “naïve” markers indicating they are new and ready to fight ^[6][7].
• B-cells (CD19, CD20): Often low, though some children have normal B-cell counts despite the deficiency ^[8].
• NK Cells (CD16, CD56): These “natural killer” cells are frequently in the normal range, which is a helpful distinguishing clue for doctors ^[1][2].

2. Immunoglobulins (Antibodies)

This measures protective proteins. Note that if your child receives IVIG, IgG levels will reflect the donor medicine, not their own production.

• IgG and IgA: Often low (hypogammaglobulinemia) ^[9][1].
• IgM: Interestingly, IgM levels can be normal or even high in some children with Cernunnos-XLF, even if other antibodies are missing ^[7][8].

3. TREC Levels

If your child was diagnosed via newborn screening, you may hear about TRECs (T-cell Receptor Excision Circles). These are tiny pieces of DNA waste created when the body makes new T-cells. Low TREC levels indicate few recent cells migrating from the thymus, though this is not necessarily proof the thymus itself is failing entirely, nor is it specific to Cernunnos-XLF ^[9].

Identifying Look-Alike Conditions

Cernunnos-XLF is part of a family of DNA repair disorders. Because their clinical symptoms can overlap, genetic testing is the only way to definitively tell them apart ^[4][10].

Completeness Checklist

If you are still in the process of diagnosis, ensure your team has considered these steps:

• [ ] Full Genetic Panel: Includes NHEJ1 and look-alike genes (LIG4, DCLRE1C, NBN, ATM).
• [ ] Parental Testing: Segregation testing to confirm the variants are in trans ^[1].
• [ ] Immune Phenotyping: Flow cytometry for CD3, CD4, CD8, CD19, and CD56 cells.
• [ ] Antibody Levels: Quantitative IgG, IgA, IgM, and IgE testing (interpreted with age ranges).
• [ ] Functional Check: Mitogen stimulation testing (how well T-cells respond when challenged).