Monozygotic triplets with juvenile-onset autoimmunity and 18p microdeletion involving PTPRM

Abstract

Abnormal gene dosage from copy number variants has been associated with susceptibility to autoimmune disease. This includes 18p deletion syndrome, a chromosomal disorder with an estimated prevalence of 1 in 50,000 characterized by intellectual disability, facial dysmorphology, and brain abnormalities. The underlying causes for autoimmune manifestations associated with 18p deletions, however, remain unknown. Our objective was to investigate a distinctive case involving monozygotic triplets concordant for developmental delay, white matter abnormalities, and autoimmunity, specifically juvenile-onset Graves' thyroiditis. By chromosomal microarray analysis and whole genome sequencing, we found the triplets to carry a de novo interstitial 5.9 Mb deletion of chromosome 18p11.31p11.21 spanning 19 protein-coding genes. We conducted a literature review to pinpoint genes affected by the deletion that could be associated with immune dysregulation and identified PTPRM as a potential candidate. Through dephosphorylation, PTPRM serves as a negative regulator of STAT3, a key factor in the generation of Th17 cells and the onset of specific autoimmune manifestations. We hypothesized that PTPRM hemizygosity results in increased STAT3 activation. We therefore performed assays investigating PTPRM expression, STAT3 phosphorylation, Th1/Th2/Th17 cell fractions, Treg cells, and overall immunophenotype, and in support of the hypothesis, our investigations showed an increase in cells with phosphorylated STAT3 and higher levels of Th17 cells in the triplets. We propose that PTPRM hemizygosity can serve as a contributing factor to autoimmune susceptibility in 18p deletion syndrome. If confirmed in unrelated 18p/PTPRM deletion patients, this susceptibility could potentially be treated by targeted inhibition of IL-17.

Keywords: 18p deletion; PTPRM; STAT3 transcription factor; Th17 cells; autoimmunity; cytogenetics.

FIGURE 1

Interstitial 18p microdeletion identified in monozygotic triplets with juvenile-onset autoimmunity, intellectual disability, and white matter lesions. (A) Monozygotic triplets, 37 years old [left to right: triplet A–C]. (B) Brain MRI of triplet A. Axial and sagittal FLAIR (1.5 T) showing multiple small, primarily subcortical white matter lesions, as frequently reported in 18p deletion syndrome patients. All triplets had these lesions, which were found stable over time. (C) Pedigree of the family. The deletion was not present in the healthy parents. (D) Visualization of the 18p11.31p11.21 microdeletion in CytoGenomics.

FIGURE 2

Functional immunological analyses. (A–D) STAT3 phosphorylation was examined in peripheral lymphocytes by flow cytometry. (A) Example plot showing the gating strategy for lymphocyte isolation. (B) STAT3 phosphorylation signal from lymphocytes without prior stimulation for healthy blood donors (blue), parents (grey), and triplets (red). (C) As in previous panel but with prior IL-6 stimulation. (D) The fractions of lymphocytes with IL6-induced STAT3 phosphorylation in controls (healthy blood donors and parents) and triplets. The columns are the geometric means of the two groups. The fold-difference of the means was estimated with 95% confidence intervals (CIs) using bootstrap sampling distributions. (E–H) The fractions of Th17 cells were determined by flow cytometry. (E) Plot showing the gating strategy for identifying CD8 negative T-cells as surrogates of T-helper cells. (F) Plot of CD8⁻ T-cells IL17 content without stimulation. (G) As in previous panel but with stimulation. (H) The fraction of Th17 cells in controls (healthy blood donors and parents) and triplets. The columns are the geometric means of the two groups. The fold-difference of the means was estimated with 95% CIs using bootstrap sampling distributions. As an in-house reference, the central 90%-prediction interval of the Th17 cell measurements from 51 healthy blood donors was [0.21%; 1.7%].