Inherited metabolic disorders involving the eye: a clinico-biochemical perspective

Syndromic Retinitis Pigmentosa: A 15-Patient Study

Retinitis pigmentosa is a group of genetically determined retinal dystrophies characterized by primary photoreceptor apoptosis and can occur in isolated or syndromic conditions. This study reviewed the clinical data of 15 patients with syndromic retinitis pigmentosa from a Rare Disease Reference Center in Brazil and the results of their next-generation sequencing tests. Five males and ten females participated, with the mean ages for ocular disease onset, fundoscopic diagnosis, and molecular evaluation being 9, 19, and 29 years, respectively. Bardet-Biedl syndrome (n = 5) and Usher syndrome (n = 3) were the most frequent diagnoses, followed by other rare conditions. Among the patients, fourteen completed molecular studies, with three negative results and eleven revealing findings in known genes, including novel variants in MKKS (c.432_435del, p.Phe144Leufs*14), USH2A (c.(7301+1_7302-1)_(9369+1_9370-1)del), and CEP250 (c.5383dup, p.Glu1795Glyfs*13, and c.5050del, p.Asp1684Thrfs*9). Except for Kearn-Sayre, all presented an autosomal recessive inheritance pattern with 64% homozygosity results. The long gap between symptom onset and diagnosis highlights the diagnostic challenges faced by the patients. This study reaffirms the clinical heterogeneity of syndromic retinitis pigmentosa and underscores the pivotal role of molecular analysis in advancing our understanding of these diseases.

Optical coherence tomography in patients with Wilson's disease

BACKGROUND

Morphological changes of retina in patients with Wilson's disease (WD) can be found by optical coherence tomography (OCT), and such changes had significant differences between neurological forms (NWD) and hepatic forms (HWD) of WD. The aim of this study was to evaluate the relationship between morphological parameters of retina and brain magnetic resonance imaging (MRI) lesions, course of disease, type of disease, and sexuality in WD.

METHODS

A total of 46 WD patients and 40 health controls (HC) were recruited in this study. A total of 42 WD patients were divided into different groups according to clinical manifestations, course of disease, sexuality, and brain MRI lesions. We employed the Global Assessment Scale to assess neurological severity of WD patients. All WD patients and HC underwent retinal OCT to assess the thickness of inner limiting membrane (ILM) layer to retinal pigment epithelium layer and inner retina layer (ILM to inner plexiform layer, ILM-IPL).

RESULTS

Compared to HWD, NWD had thinner superior parafovea zone (108.07 ± 6.89 vs. 114.40 ± 5.54 μm, p < .01), temporal parafovea zone (97.17 ± 6.65 vs. 103.60 ± 4.53 μm, p < .01), inferior parafovea zone (108.114 ± 7.65 vs. 114.93 ± 5.84 μm, p < .01), and nasal parafovea zone (105.53 ± 8.01 vs. 112.10 ± 5.44 μm, p < .01) in inner retina layer. Course of disease influenced the retina thickness. Male patients had thinner inner retina layer compared to female patients.

CONCLUSION

Our results demonstrated that WD had thinner inner retina layer compared to HC, and NWD had thinner inner retina layer compared to HWD. We speculated the thickness of inner retina layer may be a potential useful biomarker for NWD.

Cross-sectional observational analysis of the genetic referral practices across pediatric ophthalmology outpatient departments in an urban setting

Purpose

To analyze the genetic referral practices of pediatric ophthalmologists in an urban setting.

Methods

(1) The first limb of the study: cross-sectional, observational study among children visiting the outpatient department of pediatric ophthalmology across five centers in Mumbai. All pediatric patients were screened separately by pediatric ophthalmologists and a clinical geneticist for their ophthalmic and systemic complaints. Children were marked for referral to genetics (RTG) by both the specialists based on identification of distinctive features (red flag) and were requested to meet a local geneticist. (2a) Twenty-three months later, patients who had been marked for RTG were contacted telephonically to follow-up if they had met the geneticist. (2b) Additionally, the last 20 proformas from each center were checked retrospectively to note the RTG marked by the ophthalmologist alone.

Results

(1) In the first aspect of the study, 126 patients (male: female = 1.2:1) were included. Forty-nine (38.3%) patients were referred for genetic evaluation, of which three (6.1%), 31 (63.26%), and 15 (30.6%) cases were referred by the ophthalmologist alone, geneticist alone, and by both the specialists, respectively. Glaucoma (100%), nystagmus (86%), and leukocoria (83%) were the most prominent ocular diagnoses in cases referred for genetic evaluation. Facial dysmorphism (55.1%) and neurodevelopmental delays (51%) were among the most common systemic red flags found in patients referred to genetics. (2a) Twenty-three months later, on contacting the 49 patients marked for RTG, only one family had met the geneticist. (2b) Retrospective evaluation of 100 proformas: only three patients were marked for RTG by ophthalmologist alone.

Conclusion

This study found that the genetic referrals by pediatric ophthalmologist were far lesser than those by geneticist. The study highlights an area of knowledge gap among pediatric ophthalmologists, prompting a need for heightened awareness in this area.

Clinical and biochemical footprints of inherited metabolic disorders. VII. Ocular phenotypes

Ocular manifestations are observed in approximately one third of all inherited metabolic disorders (IMDs). Although ocular involvement is not life-threatening, it can result in severe vision loss, thereby leading to an additional burden for the patient. Retinal degeneration with or without optic atrophy is the most frequent phenotype, followed by oculomotor problems, involvement of the cornea and lens, and refractive errors. These phenotypes can provide valuable clues that contribute to its diagnosis. In this issue we found 577 relevant IMDs leading to ophthalmologic manifestations. This article is the seventh of a series attempting to create and maintain a comprehensive list of clinical and metabolic differential diagnoses according to system involvement.

Causes of poor eye contact in infants: a population-based study

BACKGROUND

Establishing eye contact between infants and parents is important for early parent-child bonding and lack of eye contact may be a sign of severe underlying disease. The aim of the study was to evaluate the causes of poor or lacking eye contact in infants.

METHODS

Cross-sectional study reviewing all referrals of infants ≤1 year of age from January 1rst, 2016 to December 31rst, 2018. Medical information was retrieved from patient files covering pregnancy, birth, diagnostic work-up and ocular parameters such as refraction, visual acuity and structural findings.

RESULTS

We identified 99 infants with poor or lacking eye contact. The relative frequency of causes was neurologic disease 36.4% (36/99), delayed visual maturation 24.2% (24/99), ocular disease 21.2% (21/99) and idiopathic infantile nystagmus 4.0% (4/99). Fourteen infants had a visual function within age-related norms at first examination despite poor eye contact at the time of referral. Of the infants with available data, 18/27 (33.3%) with neurologic cause, 15/23 (65.2%) with delayed visual maturation and 9/21 (42.9%) with ocular cause had visual acuity within the age-related norm at latest follow-up (0-41 months). In 23 infants, a genetic cause was found.

CONCLUSION

Poor eye contact in infants may be a sign of severe underlying disease, such as neurological or ocular disease. Close collaboration between pediatric ophthalmologists and neuro-pediatricians are warranted in the management of these infants.

Ocular manifestations of liver disease in children: Clinical aspects and implications

Liver and eyes are interlinked to each other in various medical conditions. There are certain ocular findings which directly indicate specific liver disorders. Thus, it becomes critical to identify disorders of liver and eyes early in the course of illness, so that prompt management may be initiated before the commencement of complications. It is highly advantageous in metabolic liver disorders as it offers prognostic value and spares the patient of unnecessary invasive and detailed work up. However, due to its silent and heterogeneous presentation, it is often unrecognized and ignored. Eye abnormalities could be due to, either direct toxic effects of abnormal metabolites, excess of normal metabolites, or by deficient energy metabolism. A number of inherited liver conditions have associated ocular lesions such as Kayser-Fleischer rings in Wilson's disease, posterior embryotoxon or optic drusen in Alagille's syndrome, and cherry-red spot in Niemann-Pick's type A. A thorough eye examination is important in distinguishing between several different forms of familial intrahepatic cholestasis which are associated with anomalies of the heart, bones, or kidneys. Early diagnosis is important, as in most cases, dietary restriction and early therapy prevents the onset of disability. The aim of this review is to sensitize and make pediatricians, hepatologists and ophthalmologists aware of specific ocular findings, suggestive of certain hepatobiliary disorders, thus helping in early referral. The pediatric and adult literature was thoroughly reviewed to organize the present review.

Optical coherence tomography in patients with Wilson's disease

OBJECTIVES

Wilson disease (WD) is an autosomal recessive disorder that leads to copper accumulation and deposition in different organs, frequently affecting visual pathways. Recent studies have detected morphological changes of the retina in patients with WD using optical coherence tomography (OCT). Measuring the thickness of the retinal nerve fibre layer (RNFL) with OCT provides an objective assessment of integrity and morphological abnormalities of the retina. The aim of this study was to evaluate the relationship between OCT parameters and form of the disease, therapy and symptoms duration, as well as severity of neurological impairment.

METHODS

The study comprised of 52 patients with WD and 52 healthy controls (HC). All the patients were on a regular and stable chelation therapy and/or zinc salts. Patients were divided into two groups, with neurological (NWD) or hepatic form of the disease (HWD). OCT was performed to assess the RNFL thickness.

RESULTS

The WD patients had significantly lower intraocular pressure in both eyes and lower RNFL thickness than the HC. There were no differences between NWD and HWD in any of the ophthalmologically tested parameters. No significant correlations were found between clinical features and retinal thickness parameters. Stratification of the cohort according to the disease duration showed that disease duration did not influence the RNFL thickness.

CONCLUSION

We found that involvement of the retina represented a subclinical finding in neurologically intact patients in the HWD group. Nevertheless, the value of OCT as a biomarker for the assessment of the clinical course and progression of WD still remains uncertain.

Evaluation of photoreceptor function in inherited retinal diseases using rod- and cone-enhanced flicker stimuli

PURPOSE

Clinical assessment of rod and cone photoreceptor sensitivity often involves the use of extended dark adaptation times to minimise cone involvement or the use of bright adapting backgrounds to saturate rods. In this study we examine a new rod/cone sensitivity test, which requires minimal dark adaptation. The aim was to establish whether rod/cone sensitivity losses could be measured reliably in patients with retinal diseases that selectively affect rods or cones when compared to age-matched subjects with normal vision.

METHODS

Flicker modulation thresholds (FMTs) were measured psychophysically, using cone- and rod-enhanced stimuli located centrally, and in four quadrants, at 5° retinal eccentricity in 20 patients (age range: 10-41 years) with cone-dominated (Stargardt's disease or macular dystrophy; n = 13) and rod-dominated (retinitis pigmentosa; n = 7) disease. These data were compared against age-matched normals tested with identical stimuli.

RESULTS

Across all retinal locations, cone FMTs in cone-dominated diseases (Median ± IQR: 32.32 ± 28.15% for central location) were greater than a majority (83%; 49/59) of corresponding rod FMTs (18.7 ± 3.29%; p = 0.05) and cone FMTs of controls (4.24 ± 2.00%). Similarly, rod FMTs in rod-dominant disease (14.99 ± 22.58%) were greater than a majority (88%; 29/39) of the corresponding cone FMTs (9.09 ± 10.33%) (p = 0.13) and rod FMT of controls (6.80 ± 2.60 %).

CONCLUSIONS

Cone-specific deficits were larger than rod-specific deficits in cone-dominated diseases, and vice versa in rod-dominated disease. These results suggest that the new method of assessing photoreceptor sensitivity has potential application in detecting specific rod/cone losses without the need for dark adaptation.

Metabolic Analysis of Vitreous/Lens and Retina in Wild Type and Retinal Degeneration Mice

Photoreceptors are the light-sensing cells of the retina and the major cell type affected in most inherited retinal degenerations. Different metabolic pathways sustain their high energetic demand in physiological conditions, particularly aerobic glycolysis. The principal metabolome of the mature retina has been studied, but only limited information is available on metabolic adaptations in response to key developmental events, such as eye opening. Moreover, dynamic metabolic changes due to retinal degeneration are not well understood. Here, we aimed to explore and map the ocular metabolic dynamics induced by eye opening in healthy (wild type) or Pde6b-mutant (retinal degeneration 1, Rd1) mice, in which photoreceptors degenerate shortly after eye opening. To unravel metabolic differences emerging before and after eye opening under physiological and pathophysiological conditions, we performed nuclear magnetic resonance (NMR) spectroscopy-based metabolome analysis of wild type and Rd1 retina and vitreous/lens. We show that eye opening is accompanied by changes in the concentration of selected metabolites in the retina and by alterations in the vitreous/lens composition only in the retinal degeneration context. As such, we identify NAcetylaspartate as a potential novel vitreous/lens marker reflecting progressive retinal degeneration. Thus, our data can help elucidating mechanisms underlying key events in retinal physiology and reveal changes occurring in pathology, while highlighting the importance of the vitreous/lens in the characterization of retinal diseases.

Neonatal Presentations of Metabolic Disorders

Metabolic disorders in a neonate can present with involvement of any organ system and can be challenging to diagnose. A newborn can present with an acute metabolic crisis such as hyperammonemia or seizures needing immediate management, with a more chronic clinical picture such as cholestatic liver disease, or with structural abnormalities such as skeletal manifestations. Early detection of treatable metabolic conditions is important to improve outcomes. Newborn screening has facilitated early detection and initiation of therapy for many metabolic disorders. However, normal testing does not rule out a metabolic disorder and a high index of suspicion should remain when caring for any critically ill neonate without a diagnosis. Whole exome sequencing (WES) or whole genome sequencing (WGS) can be powerful tools in rapid diagnosis of a potentially treatable metabolic condition in a critically ill neonate. This review presents classic clinical presentations of neonatal metabolic disorders and also highlights some uncommon neonatal manifestations of metabolic disorders to improve the recognition and diagnosis of these conditions.

A diagnostic approach to syndromic retinal dystrophies with intellectual disability

Inherited retinal dystrophies are a group of monogenic disorders that, as a whole, contribute significantly to the burden of ocular disease in both pediatric and adult patients. In their syndromic forms, retinal dystrophies can be observed in association with intellectual disability, frequently alongside other systemic manifestations. There are now over 80 genes implicated in syndromic retinal dystrophies with intellectual disability. Identifying and accurately characterizing these disorders allows the clinician to narrow the differential diagnosis, evaluate for relevant associated features, arrive at a timely and accurate diagnosis, and address both sight-threatening ocular manifestations and morbidity-causing systemic manifestations. The co-occurrence of retinal dystrophy and intellectual disability in an individual can be challenging to investigate, diagnose, and counsel given the considerable phenotypic and genotypic heterogeneity that exists within this broad group of disorders. We performed a review of the current literature and propose an algorithm to facilitate the evaluation, and clinical and mechanistic classification, of these individuals.

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

Inherited retinal degeneration (RD) leads to the impairment or loss of vision in millions of individuals worldwide, most frequently due to the loss of photoreceptor (PR) cells. Animal models, particularly the laboratory mouse, have been used to understand the pathogenic mechanisms that underlie PR cell loss and to explore therapies that may prevent, delay, or reverse RD. Here, we reviewed entries in the Mouse Genome Informatics and PubMed databases to compile a comprehensive list of monogenic mouse models in which PR cell loss is demonstrated. The progression of PR cell loss with postnatal age was documented in mutant alleles of genes grouped by biological function. As anticipated, a wide range in the onset and rate of cell loss was observed among the reported models. The analysis underscored relationships between RD genes and ciliary function, transcription-coupled DNA damage repair, and cellular chloride homeostasis. Comparing the mouse gene list to human RD genes identified in the RetNet database revealed that mouse models are available for 40% of the known human diseases, suggesting opportunities for future research. This work may provide insight into the molecular players and pathways through which PR degenerative disease occurs and may be useful for planning translational studies.

Pediatric liver diseases and ocular changes: What hepatologists and ophthalmologists should know and share with each other

Several rare pediatric liver disorders are accompanied by ophthalmic signs whose awareness and early identification may be of value in confirming/accelerating their diagnosis. Many of these signs are asymptomatic and can only be detected with an ophthalmological examination. Corneal signs are described in patients with Wilson's disease, Alagille's syndrome and some liver storage diseases. Cataract plays an important role to diagnose galactosemia. Retinal involvement is seen in some peroxisomal disorders (e.g. Zellweger's syndrome), in mucopolysaccharidoses (pigmentary retinopathy), and in Niemann-Pick disease (macular cherry red spot). In mucopolysaccharidoses optic nerve can be involved as optic atrophy secondary to pigmentary retinopathy or to chronic papilledema. Children with neonatal cholestasis due to hypopituitarism may present septo-optic dysplasia. Several infectious agents have an ophthalmological/hepatic involvement in the fetal life and/or thereafter. Some mitochondrial liver diseases, such as Pearson's syndrome, present pigmentary retinopathy and a chronic progressive external ophthalmoplegia. Finally, some drugs while protecting the liver may damage the ocular system as seen with long-term glucocorticoids and Nitisinone administration. This review provides a synopsis of those conditions that hepatologists and ophthalmologists should share among themselves to better take care of patients. Synoptic tables are presented to facilitate the mutual understanding of the issues.

Sodium Iodate Disrupted the Mitochondrial-Lysosomal Axis in Cultured Retinal Pigment Epithelial Cells

PURPOSE

Low doses of sodium iodate (NaIO₃) impair visual function in experimental animals with selective damage to retinal pigment epithelium (RPE) and serve as a useful model to study diseases caused by RPE degeneration. Mitochondrial dysfunction and defective autophagy have been suggested to play important roles in normal aging as well as many neurodegenerative diseases. In this study, we examined whether NaIO₃ treatment disrupted the mitochondrial-lysosomal axis in cultured RPE.

METHODS

The human RPE cell line, ARPE-19, was treated with low concentrations (≤500 μM) of NaIO₃. The expression of proteins involved in the autophagic pathway and mitochondrial biogenesis was examined with Western blot. Intracellular acidic compartments and lipofuscinogenesis were evaluated by acridine orange staining and autofluorescence, respectively. Mitochondrial mass, mitochondrial membrane potential (MMP), and mitochondrial function were quantified by MitoTracker Green staining, tetramethylrhodamine methyl ester staining, and the MTT assay, respectively. Phagocytosis and the degradation of photoreceptor outer segments (POS) were assessed by fluorescence-based approaches and Western blot against rhodopsin.

RESULTS

Treatment with low concentrations of NaIO₃ decreased cellular acidity, blocked autophagic flux, and resulted in increased lipofuscinogenesis in ARPE-19 cells. Despite increases in protein levels of Sirtuin 1 and PGC-1α, mitochondrial function was compromised, and this decrease was attributed to disrupted MMP. POS phagocytic activities decreased by 60% in NaIO₃-treated cells, and the degradation of ingested POS was also impaired. Pretreatment and cotreatment with rapamycin partially rescued NaIO₃-induced RPE dysfunction.

CONCLUSIONS

Low concentrations of NaIO₃ disrupted the mitochondrial-lysosomal axis in RPE and led to impaired phagocytic activities and degradation capacities.

Screening, genetics, risk factors, and treatment of neonatal cataracts

Neonatal cataracts remain the most common cause of visual loss in children worldwide and have diverse, often unknown, etiologies. This review summarizes current knowledge about the detection, treatment, genetics, risk factors, and molecular mechanisms of congenital cataracts. We emphasize significant progress and topics requiring further study in both clinical cataract therapy and basic lens research. Advances in genetic screening and surgical technologies have improved the diagnosis, management, and visual outcomes of affected children. For example, mutations in lens crystallins and membrane/cytoskeletal components that commonly underlie genetically inherited cataracts are now known. However, many questions still remain regarding the causes, progression, and pathology of neonatal cataracts. Further investigations are also required to improve diagnostic criteria for determining the timing of appropriate interventions, such as the implantation of intraocular lenses and postoperative management strategies, to ensure safety and predictable visual outcomes for children. Birth Defects Research 109:734-743, 2017. © 2017 Wiley Periodicals, Inc.

Proteomic analysis of the Rett syndrome experimental model mecp2Q63X mutant zebrafish

Rett syndrome (RTT) is a severe genetic disorder resulting from mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene. Recently, a zebrafish carrying a mecp2-null mutation has been developed with the resulting phenotypes exhibiting defective sensory and thigmotactic responses, and abnormal motor behavior reminiscent of the human disease. Here, we performed a proteomic analysis to examine protein expression changes in mecp2-null vs. wild-type larvae and adult zebrafish. We found a total of 20 proteins differentially expressed between wild-type and mutant zebrafish, suggesting skeletal and cardiac muscle functional defects, a stunted glycolysis and depleted energy availability. This molecular evidence is directly linked to the mecp2-null zebrafish observed phenotype. In addition, we identified changes in expression of proteins critical for a proper redox balance, suggesting an enhanced oxidative stress, a phenomenon also documented in human patients and RTT murine models. The molecular alterations observed in the mecp2-null zebrafish expand our knowledge on the molecular cascade of events that lead to the RTT phenotype.

BIOLOGICAL SIGNIFICANCE

We performed a proteomic study of a non-mammalian vertebrate model (zebrafish, Danio rerio) for Rett syndrome (RTT) at larval and adult stages of development. Our results reveal major protein expression changes pointing out to defects in energy metabolism, redox status imbalance, and muscle function, both skeletal and cardiac. Our molecular analysis grants the mecp2-null zebrafish as a valuable RTT model, triggering new research approaches for a better understanding of the RTT pathogenesis and phenotype expression. This non-mammalian vertebrate model of RTT strongly suggests a broad impact of Mecp2 dysfunction.