Introduction
Glaucoma is one of the most heritable of all human diseases (Wang et al., 2017) and exhibits both Mendelian (childhood onset) and complex (adult onset) inheritance (Wiggs and Pasquale, 2017). Affected patients typically develop elevated intraocular pressure (IOP) that causes irreversible degeneration of retinal ganglion cells and the optic nerve (Weinreb et al., 2016). Increased IOP in most patients is caused by impaired drainage of intraocular fluid from the eye through the trabecular meshwork and outflow pathways (Weinreb et al., 2014).
Patients with childhood-onset glaucoma, defined as diagnosis before age 40, are affected by severe disease characterized by very high IOP, and without treatment are likely to become blind over their lifetimes (Papadopoulos et al., 2020). Juvenile-onset primary open-angle glaucoma (JOAG) is one type of childhood glaucoma that typically exhibits dominant inheritance. MYOC mutations account for disease in 15-20% of JOAG families primarily with European Caucasian ancestry andMYOC does not appear to be a major cause of disease in other ethnic groups (Liu et al., 2021). To investigate the genetic etiology of JOAG in an ethnically diverse population, we identified and recruited 14 JOAG pedigrees from regions throughout the Philippines.
EFEMP1 (EGF-containing fibulin-like extracellular matrix protein 1) also known as Fibulin 3 is a member of the Fibulin family of extracellular matrix proteins characterized by tandem arrays of EGF (epidermal growth factor)-like domains and a C-terminal fibulin-type module (Figure 1B) (Kobayashi et al., 2007). Among the fibulin family members,EFEMP1 has higher ocular expression (Wagner et al., 2013) and is known to contribute to other ocular phenotypes. Common SNPs nearEFEMP1 on chromosome 2p16 have been associated with adult-onset glaucoma (POAG) (Gharahkhani et al., 2021) as well as IOP (Khawaja et al., 2018) and cup-to-disc ratio (CDR) an optic nerve quantitative trait related to POAG risk (Springelkamp et al., 2015). A low frequencyEFEMP1 variant p.Arg140Trp, present in 2 individuals in gnomAD has been identified in 5 affected individuals from an adult-onset POAG African-American family (Mackay et al., 2015) and a stop loss variant (Ter494Gluext*29) has been reported in 3 affected members of a Chinese POAG family (Liu et al., 2020). A single EFEMP1 missense allele (p.Arg345Trp) is known to cause Malattia Leventinese (MLVT) (also known as Doyne’s Honeycomb dystrophy (DHRD; MIM:126600), a rare autosomal dominant retinal degeneration characterized by extracellular deposits (drusen) beneath the retinal pigment epithelium (RPE) (Stone et al., 1999). Among the many patients affected by MLVT/DHRD, only p.Arg345Trp has been identified as a causative mutation. Recently it has been suggested that the Arg345 residue is a mutation hot spot and that founder effects are not likely to account for the predominance of this mutation in MLVT/DHRD patients (Vaclavik et al., 2020). Notably, wildtype EFEMP1 is a secreted protein, while the p.Arg345Trp mutations leads to protein misfolding and reduced secretion (Marmorstein et al., 2002).
Here, we report three independent families from the Philippines with novel EFEMP1 rare variants segregating with JOAG. Functional studies suggest that the disease-associated EFEMP1 variants cause protein misfolding and intracellular aggregation and retention. These results expand the spectrum of EFEMP1 associated disease to include childhood glaucoma.