DeCure for Microcephalic osteodysplastic primordial dwarfism type I

Microcephalic osteodysplastic primordial dwarfism type I (MOPD I) is a rare autosomal recessive developmental disorder. Fewer than 100 cases of primordial dwarfism had been reported worldwide as of 2014, with MOPD I comprising fewer than 30 of those cases. The condition is characterised by extreme intrauterine growth retardation, severe microcephaly, central nervous system abnormalities, dysmorphic facial features, skin abnormalities, skeletal changes, limb deformations, and early death. In 2011, biallelic mutations in the RNU4ATAC gene, which encodes U4atac small nuclear RNA, were identified as the cause of MOPD I in 17 cases, including 15 previously unreported cases. This made MOPD I the first disease known to be associated with a defect in small nuclear RNAs.

Clinical and radiological overlap between MOPD types I and III has been noted, and the classification of patients within the spectrum remains difficult. A 1989 report described a male infant whose manifestations most closely resembled those of a patient with features overlapping both types I and III, and the distinctive neuropathology was documented. A 2000 study presented two children of consanguineous Arabic parents who showed typical growth retardation, microcephalus, facial anomalies, and osteodysplastic deformities including hip dysplasia, but notably no signs of mental retardation, which is in discrepancy with Seckel syndrome. That study concluded that MOPD has greater clinical and radiological heterogeneity than previously assumed, and that whether their cases represent a variant or a new subtype could only be resolved by further cases. The exact pathogenesis was then unknown, but autosomal recessive inheritance was considered the most probable cause.

A 2014 case reported a neonate with MOPD I who also had a subpulmonic ventricular septal defect and a dorsal interhemispheric cyst in the brain, associations not previously described. A 2023 study focused on MOPD type II, which is caused by biallelic loss-of-function variants in the PCNT gene, and is the most common form of primordial dwarfism. In three MOPD II patients, whole-exome and transcriptome sequencing revealed downregulation of key growth factors including IGF1R, IGF2R, and RAF1, and identified a shortlist of genes with deleterious rare variants. That study suggested that next-generation sequencing could be applied to characterise the complex genotypic background of MOPDII, but did not address MOPD I.

What is still missing is a larger cohort of molecularly confirmed MOPD I cases to clarify the full clinical spectrum and genotype-phenotype correlations. No targeted therapy or clinical trial has been reported for MOPD I. The pathogenesis linking RNU4ATAC mutations to the specific skeletal and neurological features remains poorly understood, and no animal model or functional study in the abstracts directly addresses this. Funding for natural history studies and for the development of cellular or animal models is lacking, and no stratification of patients by mutation type or modifier genes has been attempted.