Developmental epileptic encephalopathy with hypomyelination and brain atrophy associated with PTPN23 variants affecting the assembly of UsnRNPs

PTPN23 encodes a ubiquitously expressed non-receptor type, catalytically inactive protein-tyrosine Phosphatase found in all cells including neurons. Recently, we have identified PTPN23 in a cellular screen for the systematic identification of novel regulators of survival motor neuron (SMN) function in the assembly of splicing factors (Uridine-rich small nuclear ribonucleoproteins, UsnRNPs). Based on three families, recessive PTPN23 variants have been associated with human disease tentatively, without functional studies. Here, we describe a pediatric proband with severe developmental delay, epilepsy, cortical blindness, hypomyelination and brain atrophy on MRI. Whole exome sequencing and family study showed two novel PTPN23 variants, c.1902C>G (p.(Asn634Lys)) and c.2974delC (p.(Leu992Tyrfs*168)), in compound heterozygous state, which are predicted in silico to be damaging. When studying patient's fibroblasts we found similar expression of SMN but a dramatic reduction of cells displaying SMN accumulation in Cajal bodies (CB). SMN strongly accumulated in CB in more than 50% of unrelated control cell fibroblasts as well as in fibroblasts from the parent carrying only the c.2974delC (p.(Leu992Tyrfs*168)) variant (predicted to cause loss-of-function). In contrast, only 22% of cells showed respective SMN accumulations in patient fibroblasts (p = 1.9-2.5 × 10^-7) while showing a higher level of nucleoplasmic SMN. Furthermore, the remaining accumulations in patient cells displayed weaker SMN signals than control or heterozygous wt/c.2974delC (p.(Leu992Tyrfs*168)) fibroblasts. Our report provides the first description of the clinical phenotype of recessive PTPN23 variants with pathogenicity substantiated by a functional study.