EXPLORING PHENOTYPE AND THERAPY FOR RHIZOMELIC CHONDRODYSPLASIA PUNCTATA

Abstract

Rhizomelic Chondrodysplasia Punctata (RCDP) is a heterogeneous group of peroxisomal disorders resulting from genetic defects in plasmalogen (PL) biosynthesis. RCDP is mainly caused by impairment in the PTS2 specific importing system (PEX7) that leads to deficiency in AGPS, a key peroxisomal enzyme in the PL biosynthetic pathway. Less commonly, RCDP can originate from defects in the peroxisomal enzymes required for PL synthesis (GNPAT, AGPS, FAR1) as well as the PEX7 co-transporter, PEX5. PL deficiency is the hallmark of RCDP pathology and the degree of disease severity can be determined by the residual amount of PL levels. In patients with severe (classic) RCDP, erythrocyte or fibroblast PL levels are almost undetectable however these levels are considerably higher in patients with mild (nonclassic) forms of the disease. Only a few mild RCDP patients with limited information about their clinical features and disease progression were previously reported. In order to understand the natural course of mild (nonclassic) disease, we established specific criteria to distinguish mild RCDP and systematically described the clinical, biochemical and molecular profiles of 16 individuals with mild (nonclassic) RCDP. Our results showed that individuals with mild RCDP commonly presented with cataracts, development delays and joint contractures. Neurobehavioral issues were prominent and could affect learning abilities. Skeletal defects and joint contractures had a significant impact on quality of life in older individuals. All patients had residual PL levels that were higher than classic RCDP, and up to 43% of average controls. We have also identified several common, and 4 novel, PEX7 or GNPAT hypomorphic alleles associated with this mild RCDP phenotype. In the nervous system, PL lipids have a critical role in myelin formation and physiological function. Primary PL deficiency due to RCDP significantly affects different aspects of brain function leading to neurological impairment. In our natural history study of mild (nonclassic) RCDP we showed that neurobehavioral issues were common and prominent in these individuals. To further understand the consequences of PL deficiency caused by RCDP on brain structure and function, we generated and characterized a series of Pex7 deficient mouse models with graded PL levels (mild, intermediate and severe) to reflect the spectrum of disease severity in human RCDP. Our main goal for this study was to establish effective clinical endpoints that could be used to test therapeutic intervention. Our results revealed that the genotype severity of the Pex7 models correlated with the levels of peroxisome metabolites affected: PL, C26:0 lysophosphatidylcholine and phytanic acid. A strong correlation was also observed between the genotype severity and survival rates, growth, brain myelin content and extent of loss of cerebellar Purkinje cells (PC) in Pex7 deficient mice. Behavioral assessment of all Pex7 deficient mice showed comparable hyperactivity in the open field environment. This hyperactive phenotype correlated with reduced levels of brain neurotransmitters in our Pex7 deficient mouse series. To date there is no definitive or disease-modifying therapy for RCDP. Therefore, we studied the efficacy of an oral synthetic PL compound (PPI-1040) to improve PL levels and ameliorate the hyperactivity phenotype of Pex7 deficient mice using the intermediate model. Following a 4-week oral administration of PPI-1040 to Pex7 deficient mice, PL levels were significantly increased in blood and in some peripheral tissues. While we did not measure any improvement in PL levels in brain tissue, PPI-1040 treatment normalized the hyperactive behavior observed in the Pex7 deficient mice. Thus, PPI-1040 represents a novel therapeutic option for augmenting PL levels in patients with RCDP, and potentially individuals with secon