Phosphoinositides (PIs) are a family of seven low abundance membrane lipids, each with distinct signaling functions. The phosphoinositide kinase PIKfyve generates phosphoinositide-3,5-bisphosphate (PI(3,5)P2) and PI5P. Emerging evidence implicates PIKfyve in key cellular processes, including autophagy, phagocytosis, endosomal trafficking, lysosomal maintenance, and melanosome formation. Complete loss of PIKfyve function is embryonic lethal in model organisms. In humans, heterozygous mutations in PIKFYVE are associated with Fleck corneal dystrophy and congenital cataracts. In this study, we investigate the role of PIKfyve in photoreceptors and the adjacent retinal pigment epithelium (RPE), host to dynamic endolysosomal pathways required for enduring the high oxidative stress environment, transporting metabolites and phototransduction components, and the breakdown of outer segment discs. To assess PIKfyve function in the retina and RPE in our zebrafish model, we employed CRISPR/Cas9-mediated gene editing and pharmacological inhibition using the specific PIKfyve inhibitor apilimod. Loss of PIKfyve activity leads to RPE expansion characterized by the accumulation of LC3- and LAMP1-positive vacuoles, along with defects in phagosome degradation and minor changes to melanosome biogenesis. Photoreceptors deprived of PIKfyve function develop a single large vacuole in the inner segment, while the OS remains largely intact over the timespan analyzed. Electroretinography (ERG) recordings revealed complete visual impairment in pikfyve crispant larvae and significantly reduced visual function in larvae treated with apilimod post embryogenesis. These findings highlight the critical role of PIKfyve in the development and homeostasis of the RPE and retina.

Fleck corneal dystrophy (FCD) is a rare autosomal dominant disease that affects exclusively the corneal stroma. The disease is caused by heterozygous variants in PIKFYVE, a gene encoding a lipid kinase involved in multiple cellular pathways, primarily participating in membrane dynamics and signaling. This report describes a familial case of FCD caused by a complete deletion of the PIKFYVE gene. A clinical ophthalmic examination was performed on the proband and family members. Genetic testing included next-generation sequencing (multigene panel), and chromosomal microarray analysis. A quantitative PCR assay was designed in order to segregate the deletion. A 19-year-old male, with no family or personal history of ocular disease, presented for evaluation due to an acute illness consisting of burning, foreign body sensation, and red eye. Slit lamp biomicroscopy revealed bilateral small pterygia and scattered bilateral white opacities in the corneal stroma, a very similar corneal phenotype was found in the 47-year-old father, who was asymptomatic. NGS detected a heterozygous deletion of the entire PIKFYVE coding sequence. CMA in DNA from the propositus indicated a 543 kb deletion in 2q33.3q34 spanning the entire PIKFYVE gene. The deletion was confirmed in the father. We add to the molecular spectrum of FCD by describing a familial case of a whole PIKFYVE gene deletion in affected subjects. Our findings support that normal expression of PIKFYVE is necessary for corneal keratocytes homeostasis and normal corneal appearance. We conclude that PIKFYVE haploinsufficiency is the molecular mechanism underlying this familial case of FCD. Corneal dystrophy (CD) is most recently defined as a collection of rare hereditary non-inflammatory disorders of abnormal deposition of substances in the cornea. CD was coined in 1890 by Arthur Groenouw and Hugo Biber, and the efforts of Ernst Fuchs, Wilhelm Uhthoff, and Yoshiharu Yoshida solidified the foundation of the understanding of these diseases. As proposed in 2015 by the International Classification of Corneal Dystrophies (IC3D), CD is sub-classified by the anatomic location affected: epithelial/subepithelial, epithelial-stromal, stromal, and endothelial dystrophies. Discoveries and unique case studies continue to broaden our understanding and classification of these diseases; therefore, it is difficult to categorize every single dystrophy solely into these four major labels.   The objective of this article is to present an overview of the evaluation and management for the most prominent and understood variants of CD. Highlights of these dystrophies will be discussed. However, further in-depth discussion on these dystrophies will be in separate StatPearls articles.  Patients with CD can be asymptomatic, but if symptoms occur, they typically experience bilateral visual acuity loss, typically in the form of irregular astigmatism. Depending on the corneal layer affected, patients may also manifest with photophobia, dry eyes, corneal edema, and recurrent corneal erosions, especially with epithelial-based CD, which causes considerable pain. Symptoms can begin at any age, depending on the diagnosis. Treatment can range from conservative measures to corneal transplantation.   CD is a significant but rare ocular disease. The genetic component of this disease is important for patients to understand, especially for affected patients involved with family planning. As we begin to understand genetics in greater detail, better evaluation and treatments for CD will come to fruition.  The objective of this article is to present an overview of the general evaluation and management for the most prominent and understood variants of CD. Highlights of these dystrophies will be covered, but the author intends to elaborate on these dystrophies separately in other StatPearls articles. The variants of CD based on their new anatomic classifications in IC3D are:  Epithelial and subepithelial dystrophies  : Epithelial basement membrane corneal dystrophy (EBMCD), also previously known as map-finger-dot dystrophy, Cogan microcystic dystrophy, and anterior basement membrane dystrophy. . Epithelial recurrent erosion dystrophies (EREDs) which includes Franceschetti corneal dystrophy, dystrophia smolandiensis, and dystrophia helsinglandica . Subepithelial mucinous corneal dystrophy (SMCD) . Meesmann corneal dystrophy (MECD) also known as juvenile epithelial corneal dystrophy . Lisch epithelial corneal dystrophy (LECD) . Gelatinous drop-like corneal dystrophy (GDLD) . Epithelial-Stromal Dystrophies (still included under epithelial and subepithelial dystrophies) : Lattice corneal dystrophy (LCD), with its subtypes: type I (TGFBI mutation) and type II (familial amyloidosis Finnish type), including LCD variants . Granular corneal dystrophy (GCD), types I and II (Avellino-type)  . Reis-Bückler’s corneal dystrophy (RBCD) . Thiel-Behnke corneal dystrophy (honeycomb dystrophy) (TBCD) . Stromal dystrophies: Macular corneal dystrophy (MCD)  . Schnyder corneal dystrophy (SCD)  . Congenital stromal corneal dystrophy (CSCD)  . Fleck corneal dystrophy (FCD)  . Posterior amorphous corneal dystrophy (PACD)  . Pre-Descemet corneal dystrophy (PDCD)  . Central cloudy dystrophy of francois (CCDF) . Endothelial Corneal Dystrophies: Fuchs endothelial corneal dystrophy (FECD)  . Posterior polymorphous corneal dystrophy (PPCD)  . Congenital hereditary endothelial dystrophy (CHED)  . X-linked endothelial corneal dystrophy (XECD) .

To report the identification of a novel frameshift mutation and copy number variation (CNV) in PIKFYVE in two probands with fleck corneal dystrophy (FCD). Slit-lamp examination was performed to identify characteristic features of FCD. After genomic DNA was collected, PCR amplification and automated sequencing of all 41 exons of PIKFYVE was performed. Using genomic DNA, quantitative PCR (qPCR) was performed to detect CNVs within PIKFYVE. In the first FCD proband, numerous panstromal punctate opacities were observed in each of the proband's corneas, consistent with the diagnosis of FCD. Screening of PIKFYVE demonstrated a novel heterozygous frameshift mutation in exon 19, c.3151dupA, which is predicted to encode for a truncated PIKFYVE protein, p.(Asp1052Argfs*18). This variant was identified in an affected sister but not in the proband's unaffected mother or brother or 200 control chromosomes. The second FCD proband presented with bilateral, discrete, punctate, grayish-white stromal opacities. Exonic screening of PIKFYVE revealed no causative variant. However, CNV analysis demonstrated the hemizygous deletion of exons 15 and 16. We report a novel heterozygous frameshift mutation (c.3151dupA) and a CNV in PIKFYVE, representing the first CNV and the fifth frameshift mutation associated with FCD.