Non-Synonymous variants in premelanosome protein (PMEL) cause ocular pigment dispersion and pigmentary glaucoma

Abstract

Pigmentary glaucoma (PG) is a common glaucoma subtype that results from release of pigment from the iris, called pigment dispersion syndrome (PDS), and its deposition throughout the anterior chamber of the eye. Although PG has a substantial heritable component, no causative genes have yet been identified. We used whole exome sequencing of two independent pedigrees to identify two premelanosome protein (PMEL) variants associated with heritable PDS/PG. PMEL encodes a key component of the melanosome, the organelle essential for melanin synthesis, storage and transport. Targeted screening of PMEL in three independent cohorts (n = 394) identified seven additional PDS/PG-associated non-synonymous variants. Five of the nine variants exhibited defective processing of the PMEL protein. In addition, analysis of PDS/PG-associated PMEL variants expressed in HeLa cells revealed structural changes to pseudomelanosomes indicating altered amyloid fibril formation in five of the nine variants. Introduction of 11-base pair deletions to the homologous pmela in zebrafish by the clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 method caused profound pigmentation defects and enlarged anterior segments in the eye, further supporting PMEL's role in ocular pigmentation and function. Taken together, these data support a model in which missense PMEL variants represent dominant negative mutations that impair the ability of PMEL to form functional amyloid fibrils. While PMEL mutations have previously been shown to cause pigmentation and ocular defects in animals, this research is the first report of mutations in PMEL causing human disease.

A copy number variant is associated with a spectrum of pigmentation patterns in the rock pigeon (Columba livia)

“…For example, PMEL mutations are linked to pigmentary glaucoma in humans [85]. More strikingly, Pmel mutations in horse, dog, and zebrafish result in both epidermal pigmentation phenotypes and eye defects, similar to Almond pigeons [44,[85][86][87][88][89][90]. The merle coat pattern in dogs is associated with a transposon insertion in an intron of PMEL, resulting in a non-functional PMEL protein and a phenotype that is remarkably similar to the Almond phenotype in pigeons [44,87].…”

“…Melanosome matrix defects can also have pleiotropic effects beyond the skin and its appendages. For example, PMEL mutations are linked to pigmentary glaucoma in humans [85]. More strikingly, Pmel mutations in horse, dog, and zebrafish result in both epidermal pigmentation phenotypes and eye defects, similar to Almond pigeons [44,[85][86][87][88][89][90].…”

Modulating functional amyloid formation via alternative splicing of the premelanosomal protein PMEL17

“…Exposure of pre-formed RPT fibrils to cytosolic conditions (pH ≥7) leads to rapid dissolution in vitro, which is proposed to protect against cytotoxicity in vivo should fibrils escape the melanosome (9,10,17). Additionally, 6 of 9 missense PMEL mutations identified in heritable pigmentary glaucoma localize to the RPT domain and disrupt fibril formation in vivo (22).…”

The Almond plumage color pattern is associated with eye pigmentation defects in the domestic pigeon (Columba livia)

“…Another potential route to eye structural defects is the “leaking” of toxic pigment cell components. In humans, pigmentary glaucoma (PG) and pigment dispersal syndrome (PDS) affect anterior eye structures and are sometimes associated with mutations in PMEL [60], PMEL is critical for melanosome maturation and physically interacts with MLANA [38], a protein that is encoded in the Almond genomic candidate region of pigeons [36]. In cases of PDS, pigment disperses from the iris to surrounding tissues and induces other defects, such as increased ocular pressure and corneal endothelial pigment deposition [61].…”

“…Pmel mutant animals have varying degrees of epidermal pigmentation changes and eye defects, and their phenotypes are often qualitatively similar to Almond pigeons. In zebrafish, loss of function in one pmel paralog ( pmela ) leads to a reduction in pigment in the body and eye, and also enlarged anterior eye segments, likely due to increased intraocular pressure [60]. Similarly, some homozygous Almond pigeons also appear to have enlarged anterior segments [30, 69, 70, personal observations].…”