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

TLR4 deficiency does not alter glaucomatous progression in a mouse model of chronic glaucoma

Glaucoma is a leading cause of irreversible blindness worldwide. Toll-like receptor 4 (TLR4) is a pattern-recognition transmembrane receptor that induces neuroinflammatory processes in response to injury. Tlr4 is highly expressed in ocular tissues and is known to modulate inflammatory processes in both anterior and posterior segment tissues. TLR4 activation can lead to mitochondrial dysfunction and metabolic deficits in inflammatory disorders. Due to its effects on inflammation and metabolism, TLR4 is a candidate to participate in glaucoma pathogenesis. It has been suggested as a therapeutic target based on studies using acute models, such as experimentally raising IOP to ischemia-inducing levels. Nevertheless, its role in chronic glaucoma needs further evaluation. In the current study, we investigated the role of TLR4 in an inherited mouse model of chronic glaucoma, DBA/2J. To do this, we analyzed the effect of Tlr4 knockout (Tlr4 -/-) on glaucoma-associated phenotypes in DBA/2J mice. Our studies found no significant differences in intraocular pressure, iris disease, or glaucomatous progression in Tlr4 -/- compared to Tlr4 +/+ DBA/2J mice. These data do not identify a role for TLR4 in this chronic glaucoma, but further research is warranted to understand its role in other glaucoma models and different genetic contexts.

Lack of Association between LOXL1 Variants and Pigment Dispersion Syndrome/Pigmentary Glaucoma: A Meta-Analysis

The phenotypic similarities between exfoliation syndrome (XFS)/exfoliation glaucoma (XFG) and pigment dispersion syndrome (PDS)/pigmentary glaucoma (PG), particularly their association with material deposition in the eye's anterior segment, have prompted investigations into genetic commonalities. This study focuses on the LOXL1 gene, conducting a comprehensive meta-analysis of three candidate gene association studies. We analyzed three single nucleotide polymorphisms (SNPs) of LOXL1: rs1048661, rs3825942, and rs2165241. Our results reveal nominal significance for the exonic SNPs rs1048661 and rs3825942 (p ≤ 0.01), but show no significant association for the intronic SNP rs2165241 (p = 0.83) with PDS/PG. There was homogeneity across study cohorts (I2 = 0), and sensitivity analyses and funnel plots confirmed a lower likelihood of bias in our findings. The lack of a statistically significant association between LOXL1 variants and PDS/PG at p < 0.05 was attributable to the insufficient statistical power of the pooled data, which ranged from 5% to 37% for the three SNPs. This study suggests no association between LOXL1 variants and PDS/PG. Further validation and exploration of XFS/XFG-associated genes in larger and more diverse cohorts would be helpful to determine the genetic correlation or distinctiveness between these conditions.

Genetic Basis of Pigment Dispersion Syndrome and Pigmentary Glaucoma: An Update and Functional Insights

Pigment Dispersion Syndrome (PDS) and Pigmentary Glaucoma (PG) comprise a spectrum of ocular disorders characterized by iris pigment dispersion and trabecular meshwork changes, resulting in increased intraocular pressure and potential glaucomatous optic neuropathy. This review summarizes recent progress in PDS/PG genetics including rare pathogenic protein coding alterations (PMEL) and susceptibility loci identified from genome-wide association studies (GSAP and GRM5/TYR). Areas for future research are also identified, especially the development of efficient model systems. While substantial strides have been made in understanding the genetics of PDS/PG, our review identifies key gaps and outlines the future directions necessary for further advancing this important field of ocular genetics.

Integrating non-mammalian model organisms in the diagnosis of rare genetic diseases in humans

Next-generation sequencing technology has rapidly accelerated the discovery of genetic variants of interest in individuals with rare diseases. However, showing that these variants are causative of the disease in question is complex and may require functional studies. Use of non-mammalian model organisms - mainly fruitflies (Drosophila melanogaster), nematode worms (Caenorhabditis elegans) and zebrafish (Danio rerio) - enables the rapid and cost-effective assessment of the effects of gene variants, which can then be validated in mammalian model organisms such as mice and in human cells. By probing mechanisms of gene action and identifying interacting genes and proteins in vivo, recent studies in these non-mammalian model organisms have facilitated the diagnosis of numerous genetic diseases and have enabled the screening and identification of therapeutic options for patients. Studies in non-mammalian model organisms have also shown that the biological processes underlying rare diseases can provide insight into more common mechanisms of disease and the biological functions of genes. Here, we discuss the opportunities afforded by non-mammalian model organisms, focusing on flies, worms and fish, and provide examples of their use in the diagnosis of rare genetic diseases.

Secondary glaucoma: Toward interventions based on molecular underpinnings

Glaucoma is a heterogeneous group of progressive diseases that leads to irreversible blindness. Secondary glaucoma refers to glaucoma caused by a known underlying condition. Pseudoexfoliation and pigment dispersion syndromes are common causes of secondary glaucoma. Their respective deposits may obstruct the trabecular meshwork, leading to aqueous humor outflow resistance, ocular hypertension, and optic neuropathy. There are no disease-specific interventions available for either. Pseudoexfoliation syndrome is characterized by fibrillar deposits (pseudoexfoliative material) on anterior segment structures. Over a decade of multiomics analyses taken together with the current knowledge on pseudoexfoliative glaucoma warrant a re-think of mechanistic possibilities. We propose that the presence of nucleation centers (e.g., vitamin D binding protein), crosslinking enzymes (e.g., transglutaminase 2), aberrant extracellular matrix, flawed endocytosis, and abnormal aqueous-blood barrier contribute to the formation of proteolytically resistant pseudoexfoliative material. Pigment dispersion syndrome is characterized by abnormal iridolenticular contact that disrupts iris pigment epithelium and liberates melanin granules. Iris melanogenesis is aberrant in this condition. Cytotoxic melanogenesis intermediates leak out of melanosomes and cause iris melanocyte and pigment epithelium cell death. Targeting melanogenesis can likely decrease the risk of pigmentary glaucoma. Skin and melanoma research provides insights into potential therapeutics. We propose that specific prostanoid agonists and fenofibrates may reduce melanogenesis by inhibiting cholesterol internalization and de novo synthesis. Additionally, melatonin is a potent melanogenesis suppressor, antioxidant, and hypotensive agent, rendering it a valuable agent for pigmentary glaucoma. In pseudoexfoliative glaucoma, where environmental insults drive pseudoexfoliative material formation, melatonin's antioxidant and hypotensive properties may offer adjunct therapeutic benefits. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology.

Production of light-coloured, low heat-absorbing Holstein Friesian cattle by precise embryo-mediated genome editing

CONTEXT

Genome editing enables the introduction of beneficial sequence variants into the genomes of animals with high genetic merit in a single generation. This can be achieved by introducing variants into primary cells followed by producing a live animal from these cells by somatic cell nuclear transfer cloning. The latter step is associated with low efficiencies and developmental problems due to incorrect reprogramming of the donor cells, causing animal welfare concerns. Direct editing of fertilised one-cell embryos could circumvent this issue and might better integrate with genetic improvement strategies implemented by the industry.

METHODS

In vitro fertilised zygotes were injected with TALEN editors and repair template to introduce a known coat colour dilution mutation in the PMEL gene. Embryo biopsies of injected embryos were screened by polymerase chain reaction and sequencing for intended biallelic edits before transferring verified embryos into recipients for development to term. Calves were genotyped and their coats scanned with visible and hyperspectral cameras to assess thermal energy absorption.

KEY RESULTS

Multiple non-mosaic calves with precision edited genotypes were produced, including calves from high genetic merit parents. Compared to controls, the edited calves showed a strong coat colour dilution which was associated with lower thermal energy absorbance.

CONCLUSIONS

Although biopsy screening was not absolutely accurate, non-mosaic, precisely edited calves can be readily produced by embryo-mediated editing. The lighter coat colouring caused by the PMEL mutation can lower radiative heat gain which might help to reduce heat stress.

IMPLICATIONS

The study validates putative causative sequence variants to rapidly adapt grazing cattle to changing environmental conditions.

Observation of pH-Dependent Residual Structure in the Pmel17 Repeat Domain and the Implication for Its Amyloid Formation

The varying conformational states of amyloid-forming protein monomers can determine their fibrillation outcome. In this study, we utilize solution NMR and the paramagnetic relaxation enhancement (PRE) effect to observe monomer properties of the repeat domain (RPT) from a human functional amyloid, premelanosomal protein, Pmel17. After excision from the full-length protein, RPT can self-assemble into amyloid fibrils, functioning as a scaffold for melanin deposition. Here, we report possible conformational states of the short RPT (sRPT) isoform, which has been demonstrated to be a fibrillation nucleator. NMR experiments were performed to determine conformational differences in sRPT by comparing aggregation-prone vs nonaggregating solution conditions. We observed significant chemical shift perturbations localized to residues near the C-terminus, demonstrating that the local chemical environment of the amyloid core region is highly sensitive to changes in pH. Next, we introduced cysteine point mutations for the covalent attachment of PRE ligands to sRPT to facilitate the observation of intramolecular interactions. We also utilized solvent PRE molecules with opposing charges to measure changes in the electrostatic potential of sRPT in different pH environments. These observed PRE effects offer insight into initial molecular events that might promote intermolecular interactions, which can trigger fibrillation. Taken together, our results show that sRPT monomers adopt a conformation inconsistent with a fully random coil at neutral pH and undergo conformational changes at lower pH values. These observations highlight regulatory mechanisms via organelle-associated pH conditions that can affect the fibrillation activity of proteins like RPT.

Biogenesis of fibrils requires C-mannosylation of PMEL

Premelanosome protein (PMEL), a melanocyte-specific glycoprotein, has an essential role in melanosome maturation, assembling amyloid fibrils for melanin deposition. PMEL undergoes several post-translational modifications, including N- and O-glycosylations, which are associated with proper melanosome development. C-mannosylation is a rare type of protein glycosylation at a tryptophan residue that might regulate the secretion and localization of proteins. PMEL has one putative C-mannosylation site in its core amyloid fragment (CAF); however, there is no report focusing on C-mannosylation of PMEL. To investigate this, we expressed recombinant PMEL in SK-MEL-28 human melanoma cells and purified the protein. Mass spectrometry analyses demonstrated that human PMEL is C-mannosylated at multiple tryptophan residues in its CAF and N-terminal fragment (NTF). In addition to the W153 or W156 residue (CAF), which lies in the consensus sequence for C-mannosylation, the W104 residue (NTF) was C-mannosylated without the consensus sequence. To determine the effects of the modifications, we deleted the PMEL gene by using CRISPR/Cas9 technology and re-expressed wild-type or C-mannosylation-defective mutants of PMEL, in which the C-mannosylated tryptophan was replaced with a phenylalanine residue (WF mutation), in SK-MEL-28 cells. Importantly, fibril-containing melanosomes were significantly decreased in W104F mutant PMEL-re-expressing cells compared with wild-type PMEL, observed using transmission electron microscopy. Furthermore, western blot and immunofluorescence analysis suggested that the W104F mutation may cause mild endoplasmic reticulumretention, possibly associated with early misfolding, and lysosomal misaggregation, thus reducing functional fibril formation. Our results demonstrate that C-mannosylation of PMEL is required for proper melanosome development by regulating PMEL-derived fibril formation.

Pigment dispersion syndrome and pigmentary glaucoma: overview and racial disparities

Pigment dispersion syndrome (PDS) and pigmentary glaucoma (PG) are two stages within the same ophthalmic disease spectrum, which are known to be affected by race. The prevalence of PDS is underestimated, largely due to its minor clinical symptoms. Although the prevalence of PG is low, the visual impairment associated with PG is extremely severe. The prevalence of PDS-PG is four or more times higher in Caucasians than in Blacks or Asians, and the "classic" PDS in Caucasians has long been used as a benchmark diagnostic criterion. Following extensive research focused on African Americans and Asians, the standard for diagnosing PDS-PG was refined. At the same time, the pathogenesis of PDS is not the same in different races. Hence, the effectiveness of preventive treatment and the need for treatment may not be equivalent in different races. The rate of conversion of PDS to PG is nearly 1/3 in Caucasians and higher in blacks and Asians, requiring more aggressive treatment and monitoring. We systematically searched a PubMed database from inception to March 2022 to provide an overview of research progress in various aspects of PDS-PG. Specifically, this paper considers the effects of race on disease prevalence, clinical manifestation, diagnostic criteria, disease mechanism, hereditary traits, treatment, and prevention to provide an accurate and comprehensive guide for the diagnosis and treatment of PDS-PG in various races.

PMEL is mutated in oculocutaneous albinism

Oculocutaneous albinism (OCA) is a group of Mendelian disorders characterized by hypopigmentation of skin, hair and pigmented ocular structures. While much of the genetic heterogeneity of OCA has been resolved, many patients still lack a molecular diagnosis following exome sequencing. Here, we report a consanguineous family in which the index patient presented with OCA and Hirschsprung disease but tested negative for known genetic causes of OCA. Instead, he was found to have a homozygous presumptive loss of function variant in PMEL. PMEL encodes a scaffolding protein that is essential for the normal maturation of melanosomes and normal deposition of the melanin pigment therein. Numerous PMEL vertebrate ortholog mutants have been reported and all were characterized by conspicuous pigmentary abnormalities. We suggest that the patient we report is the first human equivalent of PMEL loss of function.

OCA7 is a melanosome membrane protein that defines pigmentation by regulating early stages of melanosome biogenesis

Mutations in C10orf11 (oculocutaneous albinism type 7 [OCA7]) cause OCA, a disorder that presents with hypopigmentation in skin, eyes, and hair. The OCA7 pathophysiology is unknown, and there is virtually no information on the OCA7 protein and its cellular function. Here, we discover that OCA7 localizes to the limiting membrane of melanosomes, the specialized pigment cell organelles where melanin is synthesized. We demonstrate that OCA7 is recruited through interaction with a canonical effector-binding surface of melanosome proteins Rab32 and Rab38. Using newly generated OCA7-KO MNT1 cells, we show OCA7 regulates overall melanin levels in a melanocyte autonomous manner by controlling melanosome maturation. Importantly, we found that OCA7 regulates premelanosome protein (PMEL) processing, impacting fibrillation and the striations that define transition from melanosome stage I to stage II. Furthermore, the melanosome lumen of OCA7-KO cells displays lower pH than control cells. Together, our results reveal that OCA7 regulates pigmentation through two well-established determinants of melanosome biogenesis and function, PMEL processing, and organelle pH.

Compound Heterozygous Variants of the CPAMD8 Gene Co-Segregating in Two Chinese Pedigrees With Pigment Dispersion Syndrome/Pigmentary Glaucoma

The molecular mechanisms underlying the pathogenesis of pigment dispersion syndrome and pigmentary glaucoma remain unclear. In pedigree-based studies, familial aggregation and recurrences in relatives suggest a strong genetic basis for pigmentary glaucoma. In this study, we aimed to identify the genetic background of two Chinese pedigrees with pigmentary glaucoma. All members of these two pedigrees who enrolled in the study underwent a comprehensive ophthalmologic examination, and genomic DNA was extracted from peripheral venous blood samples. Whole-exome sequencing and candidate gene verifications were performed to identify the disease-causing variants; in addition, screening of the CPAMD8 gene was performed on 38 patients of sporadic pigmentary glaucoma. Changes in the structure and function of abnormal proteins caused by gene variants were analyzed with a bioinformatics assessment. Pigmentary glaucoma was identified in a total of five patients from the two pedigrees, as were compound heterozygous variants of the CPAMD8 gene. No signs of pigmentary glaucoma were found in carriers of monoallelic CPAMD8 variant/variants. All four variants were inherited in an autosomal recessive mode. In addition to the 38 patients of sporadic pigmentary glaucoma, 13 variants of the CPAMD8 gene were identified in 11 patients. This study reported a possible association between CPAMD8 variants and pigment dispersion syndrome/pigmentary glaucoma.

From Bench to Bed: The Current Genome Editing Therapies for Glaucoma

Glaucoma is a group of optic neuropathies featured by degeneration of retinal ganglion cells and loss of their axons in the optic nerve. The only currently approved therapies focus on lowering intraocular pressure with medication and surgery. Over the previous few decades, technological advances and research progress regarding pathogenesis has brought glaucomatous gene therapy to the forefront. In this review, we discuss the three current genome editing methods and potential disease mechanisms of glaucoma. We further summarize different genome editing strategies that are being developed to target a number of glaucoma-related genes and pathways from four aspects including strategies to lower intraocular pressure, neuroprotection, RGC and optic nerve neuro-regeneration, and other strategies. In summary, genome therapy is a promising therapy for treating patients with glaucoma and has great potential to be widely applied in clinical practice.

Genome-Wide Association Study Identifies Two Common Loci Associated with Pigment Dispersion Syndrome/Pigmentary Glaucoma and Implicates Myopia in its Development

PURPOSE

To identify genetic variants associated with pigment dispersion syndrome (PDS) and pigmentary glaucoma (PG) in unrelated patients and to further understand the genetic and potentially causal relationships between PDS and associated risk factors.

DESIGN

A 2-stage genome-wide association meta-analysis with replication and subsequent in silico analyses including Mendelian randomization.

PARTICIPANTS

A total of 574 cases with PG or PDS and 52 627 controls of European descent.

METHODS

Genome-wide association analyses were performed in 4 cohorts and meta-analyzed in 3 stages: (1) a discovery meta-analysis was performed in 3 cohorts, (2) replication was performed in the fourth cohort, and (3) all 4 cohorts were meta-analyzed to increase statistical power. Two-sample Mendelian randomization was used to determine whether refractive error and intraocular pressure exert causal effects over PDS.

MAIN OUTCOME MEASURES

The association of genetic variants with PDS and whether myopia exerts causal effects over PDS.

RESULTS

Significant association was present at 2 novel loci for PDS/PG. These loci and follow-up analyses implicate the genes gamma secretase activator protein (GSAP) (lead single nucleotide polymorphism [SNP]: rs9641220, P = 6.0×10-10) and glutamate metabotropic receptor 5 (GRM5)/TYR (lead SNP: rs661177, P = 3.9×10-9) as important factors in disease risk. Mendelian randomization showed significant evidence that negative refractive error (myopia) exerts a direct causal effect over PDS (P = 8.86×10-7).

CONCLUSIONS

Common SNPs relating to the GSAP and GRM5/TYR genes are associated risk factors for the development of PDS and PG. Although myopia is a known risk factor, this study uses genetic data to demonstrate that myopia is, in part, a cause of PDS and PG.

CRISPR knockouts of pmela and pmelb engineered a golden tilapia by regulating relative pigment cell abundance

Premelanosome protein (pmel) is a key gene for melanogenesis. Mutations in this gene are responsible for white plumage in chicken, but its role in pigmentation of fish remains to be demonstrated. In this study we found that most fishes have two pmel genes arising from the teleost-specific whole genome duplication. Both pmela and pmelb were expressed at high levels in the eyes and skin of Nile tilapia. We mutated both genes in tilapia using CRISPR/Cas9. Homozygous mutation of pmela resulted in yellowish body color with weak vertical bars and a hypo-pigmented retinal pigment epithelium (RPE) due to significantly reduced number and size of melanophores. In contrast, we observed an increased number and size of xanthophores in mutants compared to wild-type fish. Homozygous mutation of pmelb resulted in a similar, but milder phenotype than pmela-/- mutants. Double mutation of pmela and pmelb resulted in loss of additional melanophores compared to the pmela-/- mutants, and also an increase in the number and size of xanthophores, producing a golden body color. The RPE pigmentation of pmela-/-;pmelb-/- was similar to pmela-/- mutants, with much less pigmentation than pmelb-/- mutants and wild-type fish. Taken together, our results indicate that, while both pmel genes are important for the formation of body color in tilapia, pmela plays a more important role than pmelb. To our knowledge, this is the first report on mutation of pmelb or both pmela;pmelb in fish. Studies on these mutants suggest new strategies for breeding golden tilapia, and also provide a new model for studies of pmel function in vertebrates.

The genetics of glaucoma: Disease associations, personalised risk assessment and therapeutic opportunities-A review

Glaucoma refers to a heterogenous group of disorders characterised by progressive loss of retinal ganglion cells and associated visual field loss. Both early-onset and adult-onset forms of the disease have a strong genetic component. Here, we summarise the known genetic associations for various forms of glaucoma and the possible functional roles for these genes in disease pathogenesis. We also discuss efforts to translate genetic knowledge into clinical practice, including gene-based tests for disease diagnosis and risk-stratification as well as gene-based therapies.

Consensus Recommendation for Mouse Models of Ocular Hypertension to Study Aqueous Humor Outflow and Its Mechanisms

Due to their similarities in anatomy, physiology, and pharmacology to humans, mice are a valuable model system to study the generation and mechanisms modulating conventional outflow resistance and thus intraocular pressure. In addition, mouse models are critical for understanding the complex nature of conventional outflow homeostasis and dysfunction that results in ocular hypertension. In this review, we describe a set of minimum acceptable standards for developing, characterizing, and utilizing mouse models of open-angle ocular hypertension. We expect that this set of standard practices will increase scientific rigor when using mouse models and will better enable researchers to replicate and build upon previous findings.

Zebrafish Model in Ophthalmology to Study Disease Mechanism and Drug Discovery

Visual impairment and blindness are common and seriously affect people’s work and quality of life in the world. Therefore, the effective therapies for eye diseases are of high priority. Zebrafish (Danio rerio) is an alternative vertebrate model as a useful tool for the mechanism elucidation and drug discovery of various eye disorders, such as cataracts, glaucoma, diabetic retinopathy, age-related macular degeneration, photoreceptor degeneration, etc. The genetic and embryonic accessibility of zebrafish in combination with a behavioral assessment of visual function has made it a very popular model in ophthalmology. Zebrafish has also been widely used in ocular drug discovery, such as the screening of new anti-angiogenic compounds or neuroprotective drugs, and the oculotoxicity test. In this review, we summarized the applications of zebrafish as the models of eye disorders to study disease mechanism and investigate novel drug treatments.

Exome-based investigation of the genetic basis of human pigmentary glaucoma

Background

Glaucoma is a leading cause of visual disability and blindness. Release of iris pigment within the eye, pigment dispersion syndrome (PDS), can lead to one type of glaucoma known as pigmentary glaucoma. PDS has a genetic component, however, the genes involved with this condition are largely unknown. We sought to discover genes that cause PDS by testing cohorts of patients and controls for mutations using a tiered analysis of exome data.

Results

Our primary analysis evaluated melanosome-related genes that cause dispersion of iris pigment in mice (TYRP1, GPNMB, LYST, DCT, and MITF). We identified rare mutations, but they were not statistically enriched in PDS patients. Our secondary analyses examined PMEL (previously linked with PDS), MRAP, and 19 other genes. Four MRAP mutations were identified in PDS cases but not in controls (p = 0.016). Immunohistochemical analysis of human donor eyes revealed abundant MRAP protein in the iris, the source of pigment in PDS. However, analysis of MRAP in additional cohorts (415 cases and 1645 controls) did not support an association with PDS. We also did not confirm a link between PMEL and PDS in our cohorts due to lack of reported mutations and similar frequency of the variants in PDS patients as in control subjects.

Conclusions

We did not detect a statistical enrichment of mutations in melanosome-related genes in human PDS patients and we found conflicting data about the likely pathogenicity of MRAP mutations. PDS may have a complex genetic basis that is not easily unraveled with exome analyses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07782-0.

Functional Domains and Evolutionary History of the PMEL and GPNMB Family Proteins

The ancient paralogs premelanosome protein (PMEL) and glycoprotein nonmetastatic melanoma protein B (GPNMB) have independently emerged as intriguing disease loci in recent years. Both proteins possess common functional domains and variants that cause a shared spectrum of overlapping phenotypes and disease associations: melanin-based pigmentation, cancer, neurodegenerative disease and glaucoma. Surprisingly, these proteins have yet to be shown to physically or genetically interact within the same cellular pathway. This juxtaposition inspired us to compare and contrast this family across a breadth of species to better understand the divergent evolutionary trajectories of two related, but distinct, genes. In this study, we investigated the evolutionary history of PMEL and GPNMB in clade-representative species and identified TMEM130 as the most ancient paralog of the family. By curating the functional domains in each paralog, we identified many commonalities dating back to the emergence of the gene family in basal metazoans. PMEL and GPNMB have gained functional domains since their divergence from TMEM130, including the core amyloid fragment (CAF) that is critical for the amyloid potential of PMEL. Additionally, the PMEL gene has acquired the enigmatic repeat domain (RPT), composed of a variable number of imperfect tandem repeats; this domain acts in an accessory role to control amyloid formation. Our analyses revealed the vast variability in sequence, length and repeat number in homologous RPT domains between craniates, even within the same taxonomic class. We hope that these analyses inspire further investigation into a gene family that is remarkable from the evolutionary, pathological and cell biology perspectives.

Melanosome biogenesis in the pigmentation of mammalian skin

Melanins, the main pigments of the skin and hair in mammals, are synthesized within membrane-bound organelles of melanocytes called melanosomes. Melanosome structure and function are determined by a cohort of resident transmembrane proteins, many of which are expressed only in pigment cells, that localize specifically to melanosomes. Defects in the genes that encode melanosome-specific proteins or components of the machinery required for their transport in and out of melanosomes underlie various forms of ocular or oculocutaneous albinism, characterized by hypopigmentation of the hair, skin and eyes and by visual impairment. We review major components of melanosomes, including the enzymes that catalyze steps in melanin synthesis from tyrosine precursors, solute transporters that allow these enzymes to function, and structural proteins that underlie melanosome shape and melanin deposition. We then review the molecular mechanisms by which these components are biosynthetically delivered to newly forming melanosomes-many of which are shared by other cell types that generate cell type-specific lysosome-related organelles. We also highlight unanswered questions that need to be addressed by future investigation.

Functional amyloids in the human body

Amyloids have long been associated with a variety of human degenerative diseases. Discoveries indicate, however, that there are several amyloids that serve functional roles in the human body. These amyloids are involved in a variety of biological processes ranging from storage of peptide hormones to necroptosis of cells. Additionally, there are distinct differences between toxic amyloids and their functional counterparts including kinetics of assembly/disassembly and structural features. This digest article surveys the biological roles of functional amyloids found in the human body, key differences between functional and toxic amyloids, and potential therapeutic applications.

Identification of critical amino acid residues in the regulatory N-terminal domain of PMEL

The pigment cell-specific protein PMEL forms a functional amyloid matrix in melanosomes onto which the pigment melanin is deposited. The amyloid core consists of a short proteolytic fragment, which we have termed the core-amyloid fragment (CAF) and perhaps additional parts of the protein, such as the PKD domain. A highly O-glycosylated repeat (RPT) domain also derived from PMEL proteolysis associates with the amyloid and is necessary to establish the sheet-like morphology of the assemblies. Excluded from the aggregate is the regulatory N-terminus, which nevertheless must be linked in cis to the CAF in order to drive amyloid formation. The domain is then likely cleaved away immediately before, during, or immediately after the incorporation of a new CAF subunit into the nascent amyloid. We had previously identified a 21 amino acid long region, which mediates the regulatory activity of the N-terminus towards the CAF. However, many mutations in the respective segment caused misfolding and/or blocked PMEL export from the endoplasmic reticulum, leaving their phenotype hard to interpret. Here, we employ a saturating mutagenesis approach targeting the motif at single amino acid resolution. Our results confirm the critical nature of the PMEL N-terminal region and identify several residues essential for PMEL amyloidogenesis.

Genetic Heritability of Pigmentary Glaucoma and Associations With Other Eye Phenotypes

Importance

Mechanisms behind pigmentary glaucoma, a form of early-onset glaucoma that may potentially lead to severe visual impairment or blindness, are poorly understood.

Objective

To calculate the single-nucleotide polymorphism (SNP) heritability of pigmentary glaucoma and identify genetic associations with the disease.

Design, Setting and Participants

This genome-wide association study included affected individuals from Germany and control participants from the United Kingdom. Genome-wide information was obtained for patients with pigmentary glaucoma and control participants free of glaucoma by using the Illumina Human Omni Express Exome 8v1-2 chip and genomic imputation. The SNP heritability of pigmentary glaucoma was estimated through a restricted maximum likelihood analysis. Associations between the genetic variants and pigmentary glaucoma obtained from age, sex, and principal component-adjusted logistic regression models were compared with those of SNPs previously associated with other eye phenotypes using Pearson product-moment correlations. Data were collected from November 2008 to January 2018, and analysis was completed between April 2018 and August 2019.

Main Outcomes and Measures

An estimate of SNP-explained heritability for pigmentary glaucoma; correlations of effect sizes between pigmentary glaucoma and iris pigmentation and myopia; and correlations of effect sizes between pigmentary glaucoma and other eye phenotypes.

Results

A total of 227 affected individuals (mean [SD] age, 58.7 [13.3] years) and 291 control participants (mean [SD] age, 80.2 [4.9] years) were included; all were of European ancestry. The SNP heritability of pigmentary glaucoma was 0.45 (SE, 0.22; P = 6.15 × 10-10). Twelve SNPs previously reported with genome-wide significant associations with eye pigmentation were associated with pigmentary glaucoma's SNP heritability (4.9% SNP heritability; 0.022; P = 6.0 × 10-4). Pigmentary glaucoma SNP effect sizes were correlated moderately for myopia (r, 0.42 [95% CI, 0.14-0.63]; P = 4.3 × 10-3) and more strongly with those for iris pigmentation (r = -0.69 [95% CI, -0.91 to -0.20]; P = .01), although this was nonsignificant per a strict adjusted significance threshold (P < .01).

Conclusions and Relevance

These findings support the conclusion that pigmentary glaucoma may have a genetic basis and be highly heritable. Variants associated with lighter eye color and myopia appear to be associated with increased risk of pigmentary glaucoma, but no shared genetic basis with primary open-angle glaucoma (or its quantitative endophenotype of cup-disc ratio) was observed.

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

The premelanosomal protein (PMEL17) forms functional amyloid fibrils involved in melanin biosynthesis. Multiple PMEL17 isoforms are produced, two of which arise from excision of a cryptic intron within the amyloid-forming repeat (RPT) domain, leading to long (lRPT) and short (sRPT) isoforms with 10 and 7 imperfect repeats, respectively. Both lRPT and sRPT isoforms undergo similar pH-dependent mechanisms of amyloid formation and fibril dissolution. Here, using human PMEL17, we tested the hypothesis that the minor, but more aggregation-prone, sRPT facilitates amyloid formation of lRPT. We observed that cross-seeding by sRPT fibrils accelerates the rate of lRPT aggregation, resulting in propagation of an sRPT-like twisted fibril morphology, unlike the rodlike structure that lRPT normally adopts. This templating was specific, as the reversed reaction inhibited sRPT fibril formation. Despite displaying ultrastructural differences, self- and cross-seeded lRPT fibrils had a similar β-sheet structured core, revealed by Raman spectroscopy, limited-proteolysis, and fibril disaggregation experiments, suggesting the fibril twist is modulated by N-terminal residues outside the amyloid core. Interestingly, bioinformatics analysis of PMEL17 homologs from other mammals uncovered that long and short RPT isoforms are conserved among members of this phylogenetic group. Collectively, our results indicate that the short isoform of RPT serves as a "nucleator" of PMEL17 functional amyloid formation, mirroring how bacterial functional amyloids assemble during biofilm formation. Whereas bacteria regulate amyloid assembly by using individual genes within the same operon, we propose that the modulation of functional amyloid formation in higher organisms can be accomplished through alternative splicing.

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

Rock pigeons (Columba livia) display an extraordinary array of pigment pattern variation. One such pattern, Almond, is characterized by a variegated patchwork of plumage colors that are distributed in an apparently random manner. Almond is a sex-linked, semi-dominant trait controlled by the classical Stipper (St) locus. Heterozygous males (Z^StZ⁺ sex chromosomes) and hemizygous Almond females (Z^StW) are favored by breeders for their attractive plumage. In contrast, homozygous Almond males (Z^StZ^St) develop severe eye defects and often lack plumage pigmentation, suggesting that higher dosage of the mutant allele is deleterious. To determine the molecular basis of Almond, we compared the genomes of Almond pigeons to non-Almond pigeons and identified a candidate St locus on the Z chromosome. We found a copy number variant (CNV) within the differentiated region that captures complete or partial coding sequences of four genes, including the melanosome maturation gene Mlana. We did not find fixed coding changes in genes within the CNV, but all genes are misexpressed in regenerating feather bud collar cells of Almond birds. Notably, six other alleles at the St locus are associated with depigmentation phenotypes, and all exhibit expansion of the same CNV. Structural variation at St is linked to diversity in plumage pigmentation and gene expression, and thus provides a potential mode of rapid phenotypic evolution in pigeons.

The genetic changes responsible for different animal color patterns are poorly understood, due in part to a paucity of research organisms that are both genetically tractable and phenotypically diverse. Domestic pigeons (Columba livia) have been artificially selected for many traits, including an enormous variety of color patterns that are variable both within and among different breeds of this single species. We investigated the genetic basis of a sex-linked color pattern in pigeons called Almond that is characterized by a sprinkled pattern of plumage pigmentation. Pigeons with one copy of the Almond allele have desirable color pattern; however, male pigeons with two copies of the Almond mutation have severely depleted pigmentation and congenital eye defects. By comparing the genomes of Almond and non-Almond pigeons, we discovered that Almond pigeons have extra copies of a chromosome region that contains a gene that is critical for the formation of pigment granules. We also found that different numbers of copies of this region are associated with varying degrees of pigment reduction. The Almond phenotype in pigeons bears a remarkable resemblance to Merle coat color mutants in dogs, and our new results from pigeons suggest that similar genetic mechanisms underlie these traits in both species. Our work highlights the role of gene copy number variation as a potential driver of rapid phenotypic evolution.

The Canadian Rare Diseases Models and Mechanisms (RDMM) Network: Connecting Understudied Genes to Model Organisms

Advances in genomics have transformed our ability to identify the genetic causes of rare diseases (RDs), yet we have a limited understanding of the mechanistic roles of most genes in health and disease. When a novel RD gene is first discovered, there is minimal insight into its biological function, the pathogenic mechanisms of disease-causing variants, and how therapy might be approached. To address this gap, the Canadian Rare Diseases Models and Mechanisms (RDMM) Network was established to connect clinicians discovering new disease genes with Canadian scientists able to study equivalent genes and pathways in model organisms (MOs). The Network is built around a registry of more than 500 Canadian MO scientists, representing expertise for over 7,500 human genes. RDMM uses a committee process to identify and evaluate clinician-MO scientist collaborations and approve 25,000 Canadian dollars in catalyst funding. To date, we have made 85 clinician-MO scientist connections and funded 105 projects. These collaborations help confirm variant pathogenicity and unravel the molecular mechanisms of RD, and also test novel therapies and lead to long-term collaborations. To expand the impact and reach of this model, we made the RDMM Registry open-source, portable, and customizable, and we freely share our committee structures and processes. We are currently working with emerging networks in Europe, Australia, and Japan to link international RDMM networks and registries and enable matches across borders. We will continue to create meaningful collaborations, generate knowledge, and advance RD research locally and globally for the benefit of patients and families living with RD.

The Heritability of Pigment Dispersion Syndrome and Pigmentary Glaucoma

PURPOSE

Pigment dispersion syndrome (PDS) and pigmentary glaucoma (PG) are presumed to be inherited in an autosomal dominant manner. We examine relatives of patients with PDS and PG in order to determine the heritability of these diseases.

DESIGN

This was a prospective, cross-sectional study.

METHODS

One hundred and one patients with PDS were prospectively recruited over 11 months. Four of the patients had PDS without ocular hypertension or glaucoma, 6 had PDS and ocular hypertension, and 91 had PG. Criteria for PDS were 2 of 3 signs: Krukenberg spindle, midperipheral iris transillumination defects, and/or heavy trabecular meshwork pigmentation. Criteria for PG were PDS and 2 of 3 signs: intraocular pressure >21 mm Hg, glaucomatous optic nerve damage, and/or glaucomatous visual field loss. Ninety-nine first-degree relatives living within a 100-mile radius of Iowa City, Iowa were examined in the clinic to determine the probability of familial transmission.

RESULTS

A total of 10 of 99 (10.10%) first-degree relatives were diagnosed with PDS (1 with PDS alone, 2 with PDS and ocular hypertension, and 7 with PG). Seven families with ≥2 affected members were identified. The majority of affected family members (8/10) showed moderate to heavy trabecular meshwork pigmentation and either Krukenberg spindle or transillumination defects.

CONCLUSIONS

Most of the cases of PDS in our study were sporadic, and the risk to first-degree relatives is lower than previously reported. However, there are families with apparent autosomal dominant inheritance of PDS in which the risk to relatives may be high.