Genetics and pathophysiology of MCOPS12
Pathogenic dominant mutations in the RARB gene cause syndromic microphthalmia 12 (MCOPS12), also referred to as RARB-related disorder (RARB-RD). The condition is characterized by developmental eye anomalies, hypotonia, and early-onset progressive movement disorders (dystonia and/or spasticity).
Several dozen pathogenic variants are described in literature, with the recurrent p.R387C variant accounting for ~30% of cases. In vitro transfection studies classified these variants as either gain-of-function (GOF), increasing retinoic acid-induced transcriptional activity, or loss-of-function (LOF), exhibiting dominant-negative effects (Caron et al. 2023).
Movement disorders such as dystonia are generally thought to arise from dysfunction within the striatum, a key component of the basal ganglia involved in motor control, decision-making, and reward processing. The striatum is composed predominantly of medium spiny neurons (MSNs), which account for more than 90% of its neuronal population and are divided into two main subtypes based on dopamine receptor expression: D1 receptor–expressing (D1R) and D2 receptor–expressing (D2R) neurons.
RARB is highly expressed in the striatum, where it plays a critical role both in the development of D1R MSNs during embryogenesis and in neuroprotection of D2R MSNs in adulthood (Ciancia et al. 2022).
Generation and comprehensive analysis of four patient-specific Rarb mouse models yielded several key findings (Zinter et al. 2026):
(Rarb-/-) (RarbR387C/+) (RarbL402P/+) (Rarb+/-)
01
Behavioral phenotype
Behavioral analyses of RarbR387C/+ and RarbL402P/+ mice exhibitprogressive motor deficits, hyperactivity and cognitive impairment, closely recapitulating the clinical phenotypes observed in MCOPS12 patients carrying these variants.
02
Selective D2R MSN loss
At the cellular level, the striatum shows a selective loss of D2R medium spiny neurons (MSNs), while D1R MSN populations remain preserved.
03
Dominant-negative transcriptomic signature
Transcriptomic analysis in the striatum reveals that both models predominantly downregulate putative RARB target genes and display a transcriptional signature more pronounced than that ofRarb knockout (Rarb-/-) mice, supporting a dominant-negative mechanism in vivo. Notably, both the transcriptional alterations and the selective vulnerability of D2R MSNs overlap with findings in Huntington's disease mouse models, suggesting shared pathways of striatal dysfunction between Huntington's disease and MCOPS12.
04
Rarb⁺ᐟ⁻ → ASO therapy rationale
In contrast, Rarb+/- mice show no detectable phenotypes, consistent with observations in individuals carrying only one functional RARB allele. This supports the concept that allele-specific knockdown of mutant RARB — such as via an ASO therapy — represents a promising therapeutic strategy for MCOPS12.