A synonymous NPR2 variant causes acromesomelic dysplasia through aberrant pre-mRNA splicing.

Precise regulation of pre-mRNA splicing is essential for normal development, and its disruption represents an important but frequently underrecognized mechanism of human disease. The C-type natriuretic peptide (CNP) receptor NPR2 is a critical regulator of growth plate chondrocyte proliferation and differentiation, and loss-of-function variants in NPR2 cause acromesomelic dysplasia, Maroteaux type (AMDM). Here, we identify a homozygous synonymous NPR2 variant (NM_003995.4:c.2484C > T) in an individual with AMDM and demonstrate its pathogenic mechanism at the RNA level. Although predicted to be silent at the protein level, in silico analysis suggested splice donor gain. Functional analysis using patient-derived leukocyte RNA revealed aberrant splicing leading to partial exon truncation, frameshift, and premature termination of NPR2 which is predicted to trigger nonsense-mediated mRNA decay given its position upstream of multiple downstream exon-exon junctions. Heterozygous family members expressed both normal and aberrant transcripts, whereas the affected individual showed exclusive expression of the aberrant isoform, consistent with a dosage-dependent loss-of-function mechanism. These findings establish aberrant splicing induced by a synonymous variant as a disease-causing mechanism affecting a core developmental signaling pathway. Our study highlights the importance of transcript-level functional analysis in the interpretation of rare variants and underscores the central role of precise RNA processing in skeletal development and human disease.