Biology & Diagnosis: TNFAIP3, NF-κB, and Genetic Testing

Understanding the biology of A20 Haploinsufficiency (HA20) helps explain why the immune system is behaving the way it does. At its core, HA20 is a disorder of cellular signaling—the internal “messaging system” that regulates inflammation ^[1].

The Broken Brake: TNFAIP3 and NF-κB

Every cell in the body uses a major communication pathway called NF-κB (Nuclear Factor kappa B) ^[1]. Think of NF-κB as an “on switch” for inflammation. When the body detects a threat, like a virus or bacteria, NF-κB turns on genes that create the fever and swelling needed to fight the infection ^[2].

Under normal conditions, a protein called A20 (produced by the TNFAIP3 gene) acts to regulate this system ^[3]. A20 uses a process called deubiquitination to strip away chemical tags that keep the NF-κB switch in the “on” position ^[4][5].

In people with HA20, a pathogenic variant in the TNFAIP3 gene reduces or impairs this A20 regulation ^[6][7]. Without adequate A20 regulation:

• The NF-κB pathway may stay active for too long or trigger inappropriately ^[1].
• The immune system produces excessive inflammatory chemicals, such as IL-1β and TNF-α ^[3][2].
• This innate immune dysregulation contributes to the fevers, ulcers, and joint pain seen in HA20 ^[8].

Types of Genetic Variants

When you look at a genetic report, the laboratory will describe the specific change they found in the TNFAIP3 gene. Most established HA20 is caused by a heterozygous pathogenic variant, interpreted in a clinical context. HA20 can involve splice, in-frame, copy-number, and other loss-of-function changes in addition to nonsense, frameshift, and missense variants.

• Null Variants: These include nonsense or frameshift mutations that disrupt the protein’s production, often resulting in a protein that is too short to work or is destroyed by the cell ^[6][7].
• Missense Variants: A missense variant changes a single amino acid in the protein. Missense variants can be pathogenic or benign ^[4]. Because their effect isn’t always clear, they are sometimes labeled as a Variant of Uncertain Significance (VUS) ^[9].
  • Functional Assays: In research settings, scientists may perform “functional assays” to see if a specific missense variant impairs the protein’s ability to control inflammation. These assays provide supporting evidence but do not by themselves prove causation ^[4][9].

Essential Tests for a Definitive Diagnosis

Standard genetic sequencing catches most cases, but not all. A complete diagnostic workup should include:

1. Gene Sequencing: Using Next-Generation Sequencing (NGS) to look for sequence variants in the TNFAIP3 gene ^[10][11].
2. Copy-Number Analysis: If sequencing is normal but clinical suspicion is high, doctors use tests like MLPA or Chromosomal Microarray (CMA) to see if an entire section of the gene—or the whole gene—is missing (a deletion) ^[12][13].

Reading Your Genetic Report

A genetic report can be confusing, but a few key pieces of information are critical for care:

• Variant Nomenclature: This is the “address” of the mutation.
  • c. notation (e.g., c.759delC) describes the change in the DNA ^[14].
  • p. notation (e.g., p.Lys91*) describes the change in the protein ^[3].
• ACMG Classification: Laboratories follow the American College of Medical Genetics and Genomics (ACMG) guidelines, which use multiple types of evidence to classify variants ^[9]:
  • Pathogenic: Very strong evidence the variant causes disease, though penetrance and severity still vary among individuals.
  • Likely Pathogenic: Strong evidence it causes disease.
  • Variant of Uncertain Significance (VUS): Not enough evidence yet to classify as pathogenic or benign. A VUS is not diagnostic and must not be used to make predictive or management decisions for relatives. While testing family members (segregation testing) may help a lab reclassify a VUS eventually, the VUS alone does not confirm or rule out HA20.
  • Benign/Likely Benign: Evidence suggests it does not cause disease.