Low IgE is a useful tool to identify STAT3 gain-of-function mutations

To the Editor,

Germline gain-of-function mutations (GoF) in STAT3 were recently identified as the cause of early-onset autoimmune disease (MIM: 615952). This rare monogenic autoimmune disorder commonly encompasses autoimmune haematological disorders (identified in 70% of patients), recurrent infections (60%) and autoimmune enteropathies (50%)^1–3. The phenotype observed contrasts to that of patients with loss-of-function (LoF) STAT3 mutations causing hyper-IgE (Job) syndrome, typically characterised by elevated immunoglobulin E (IgE), recurrent infections and eczema⁴. Identifying patients with STAT3 GoF mutations informs families and clinicians of prognosis and facilitates personalised treatment.

There is substantial clinical variability between patients with GoF STAT3 mutations and also overlap in phenotype with other monogenic autoimmune disorders such as IPEX syndrome (immunodysregulation, polyendocrinopathy, enteropathy, X-linked; MIM: 304790) resulting from hemizygous loss-of-function FOXP3 mutations, and also early-onset polygenic autoimmune disease. Distinguishing the specific genetic etiology using clinical characteristics alone is not possible and genetic testing, often performed sequentially until a pathogenic mutation is found, is essential for accurate diagnosis.

Genetic testing for STAT3 is expensive and available only in a limited number of specialist centers. An inexpensive and widely available test to identify patients most likely to harbor GoF STAT3 mutations is desirable as it will prevent inappropriate genetic testing. We assessed whether IgE would be a useful tool to aid in the identification of patients suitable for STAT3 sequencing.

Total serum IgE concentration was measured by using the Immuno-CAP 1000 instrument (Phadia) for both healthy reference samples (n = 1510) and STAT3 patient samples (n = 6). Calibration was performed with the 2nd International Reference Preparation 75/502 of Human Serum IgE from the World Health Organization⁵. Statistical analysis was performed in Stata®; specificity, sensitivity and binomial confidence intervals were calculated. The age of the patients with STAT3 GoF mutations ranged from 1-9 years, and the median time from the first clinical symptom to IgE testing was four years.

Immunological assessment of individuals with STAT3 GoF mutations identified total serum IgE below the lower reference limit of age-matched controls in all six cases, (<2 kU/L, range 0.7-2 kU/L; See Figure 1) ¹. Using 2 kU/L as a cut off for the assay for identification against all age groups gave a sensitivity and specificity of 100% (95% CI: 54.1 – 100) and 97.2% (95% CI: 96.2-97.9), respectively.

Figure 1.

Boxplot of serum IgE concentrations in healthy controls and patients with STAT3 GoF mutations. P value determined using the Mann-Whitney test.

We have demonstrated that serum IgE, an inexpensive and widely available diagnostic test, is likely to be very useful to facilitate identifying patients suitable for STAT3 sequencing, and a 2 kU/L cut-off gives high sensitivity and specificity. The role we propose for IgE measurement is in patients with multi-system early-childhood autoimmune disease in whom STAT3 GoF mutations are being considered. This is likely to be particularly helpful in distinguishing from multiple autoimmunity resulting from hemizygous FOXP3 mutations as more than 90% of patients with IPEX syndrome have increased serum IgE. In this scenario raised IgE would be helpful to rule out STAT3 GoF mutations, reducing total genetic testing expenditure. In addition, despite recent advances in sequencing technologies, identifying pathogenic mutations from benign variants remains a challenge. Measuring IgE could be a tool to aid molecular geneticists in classifying STAT3 variants found by DNA sequencing; serum IgE concentration below 2 kU/L supports pathogenic GoF.

We have assessed IgE in a small number of patients with GoF STAT3 mutations (n=6) and assessment in further patients is required to increase confidence in the diagnostic utility, but it is notable that all six patients had IgE concentrations below the 1^st centile of the reference population. Two previous case series of patients with STAT3 GoF mutations measured IgE in a total of nine additional individuals (four of those previously reported are included in this study^{1, 2}). One described two additional individuals with serum IgE below 2 kU/L², supporting our findings. The other reported more variability in IgE than we have observed (0.1-58.5 mg/dL; n=7)³, but we were unable to ascertain reference interval, testing platform or comparable units to our assay for these individuals and therefore, unable to make a direct comparison to our findings³. A potential limitation of the diagnostic accuracy of IgE to identify patients with STAT3 GOF mutations is that an allergic response or parasitic infection may increase serum IgE. Given that allergies are relatively common (for example allergic rhinitis is present in >20% of the European population) this is an important consideration as this may decrease the performance of the test in these patients. In conclusion, total serum IgE concentration is an inexpensive and robust tool for determining which patients with multiple early-onset autoimmunity are likely to have a STAT3 GoF mutation. In combination with the identification of clinical features of early-onset autoimmune disease IgE concentrations could be assayed locally to facilitate appropriate, cost-efficient and rapid genetic testing. Further work is warranted to determine IgE in other subtypes of monogenic autoimmunity.