Ocular Pathology of Oculocerebrorenal Syndrome of Lowe: Novel Mutations and Genotype-Phenotype Analysis

Ocular manifestations of the genetic renal tubulopathies

BACKGROUND

The genetic tubulopathies are rare and heterogenous disorders that are often difficult to identify. This study examined the tubulopathy-causing genes for ocular associations that suggested their genetic basis and, in some cases, the affected gene.

METHODS

Sixty-seven genes from the Genomics England renal tubulopathy panel were reviewed for ocular features, and for retinal expression in the Human Protein Atlas and an ocular phenotype in mouse models in the Mouse Genome Informatics database. The genes resulted in disease affecting the proximal tubules (n = 24); the thick ascending limb of the loop of Henle (n = 10); the distal convoluted tubule (n = 15); or the collecting duct (n = 18).

RESULTS

Twenty-five of the tubulopathy-associated genes (37%) had ocular features reported in human disease, 49 (73%) were expressed in the retina, although often at low levels, and 16 (24%) of the corresponding mouse models had an ocular phenotype. Ocular abnormalities were more common in genes affected in the proximal tubulopathies (17/24, 71%) than elsewhere (7/43, 16%). They included structural features (coloboma, microphthalmia); refractive errors (myopia, astigmatism); crystal deposition (in oxalosis, cystinosis) and sclerochoroidal calcification (in Bartter, Gitelman syndromes). Retinal atrophy was common in the mitochondrial-associated tubulopathies. Structural abnormalities and crystal deposition were present from childhood, but sclerochoroidal calcification typically occurred after middle age.

CONCLUSIONS

Ocular abnormalities are uncommon in the genetic tubulopathies but may be helpful in recognizing the underlying genetic disease. The retinal expression and mouse phenotype data suggest that further ocular associations may become apparent with additional reports. Early identification may be necessary to monitor and treat visual complications.

Atlas of phosphoinositide signatures in the retina identifies heterogeneity between cell types

Phosphoinositides (PIPs) are a family of minor acidic phospholipids in the cell membrane. Phosphoinositide (PI) kinases and phosphatases can rapidly convert one PIP product into another resulting in the generation of seven distinct PIPs. The retina is a heterogeneous tissue composed of several cell types. In the mammalian genome, around 50 genes encode PI kinases and PI phosphatases; however, there are no studies describing the distribution of these enzymes in the various retinal cell types. Using translating ribosome affinity purification, we have identified the in vivo distribution of PI-converting enzymes from the rod, cone, retinal pigment epithelium (RPE), Müller glia, and retinal ganglion cells, generating a physiological atlas for PI-converting enzyme expression in the retina. The retinal neurons, rods, cones, and RGCs, are characterized by the enrichment of PI-converting enzymes, whereas the Müller glia and RPE are characterized by the depletion of these enzymes. We also found distinct differences between the expression of PI kinases and PI phosphatases in each retinal cell type. Since mutations in PI-converting enzymes are linked to human diseases including retinal diseases, the results of this study will provide a guide for what cell types are likely to be affected by retinal degenerative diseases brought on by changes in PI metabolism.

Dent disease: classification, heterogeneity and diagnosis

BACKGROUND

Dent disease is a rare tubulopathy characterized by manifestations of proximal tubular dysfunction, which occurs almost exclusively in males. It mainly presents symptoms in early childhood and may progress to end-stage renal failure between the 3rd and 5th decades of human life. According to its various genetic basis and to clinical signs and symptoms, researchers define two forms of Dent disease (Dent diseases 1 and 2) and suggest that these forms are produced by mutations in the CLCN5 and OCRL genes, respectively. Dent diseases 1 and 2 account for 60% and 15% of all Dent disease cases, and their genetic cause is generally understood. However, the genetic cause of the remaining 25% of Dent disease cases remains unidentified.

DATA SOURCES

All relevant peer-reviewed original articles published thus far have been screened out from PubMed and have been referenced.

RESULTS

Genetic testing has been used greatly to identify mutation types of CLCN5 and OCRL gene, and next-generation sequencing also has been used to identify an increasing number of unknown genotypes. Gene therapy may bring new hope to the treatment of Dent disease. The abuse of hormones and immunosuppressive agents for the treatment of Dent disease should be avoided to prevent unnecessary harm to children.

CONCLUSIONS

The current research progress in classification, genetic heterogeneity, diagnosis, and treatment of Dent disease reviewed in this paper enables doctors and researchers to better understand Dent disease and provides a basis for improved prevention and treatment.

Oculocerebrorenal syndrome of Lowe: Survey of ophthalmic presentations and management

BACKGROUND

Lowe syndrome is a rare X-linked disease that is characterized by renal dysfunction, developmental delays, congenital cataracts and glaucoma. Mutations in the oculocerebral renal syndrome of Lowe (OCRL) gene are found in Lowe syndrome patients. Although loss of vision is a major concern for families and physicians who take care of Lowe syndrome children, definitive cause of visual loss is still unclear. Children usually present with bilateral dense cataracts at birth and glaucoma, which occurs in more than half of cases, either concurrently or following cataract surgery.

MATERIALS AND METHODS

A retrospective review was conducted on the prevalence and characteristics of ocular findings among families of patients with Lowe syndrome with 137 uniquely affected individuals.

RESULTS

Of 137 patients, all had bilateral congenital cataracts. Nystagmus was reported in 69.3% of cases, glaucoma in 54.7%, strabismus in 35.0%, and corneal scar in 18.2% of patients. Glaucoma was reported as the most common cause of blindness (46%) followed by corneal scars (41%). Glaucoma occurred in 54.7% of patients and affected both eyes in the majority of cases. Of these patients, 55% underwent surgery for glaucoma, while the remaining patients used medications to control their eye pressure. Timolol and latanoprost were the most commonly used medications. Although trabeculectomy and goniotomy are commonly used for pressure management, aqueous tube shunts had the best outcomes.

CONCLUSION

Ocular manifestations in individuals with Lowe syndrome and carriers with OCRL mutation are reported which may help familiarize clinicians with the ocular manifestations and management of a rare and complex syndrome.

Insights into the Effect of Lowe Syndrome-Causing Mutation p.Asn591Lys of OCRL-1 through Protein-Protein Interaction Networks and Molecular Dynamics Simulations

Inositol polyphosphate 5-phosphatase (OCRL-1) participates in the regulation of multiple cellular processes, through the conversion of phosphatidylinositol 4,5-phosphate to phosphatidylinositol 4-phosphate. Mutations in this protein are related to Lowe syndrome (LS) and Dent-2 disease. In this study, the impact of Lowe syndrome mutations on the interactions of OCRL-1 with other proteins was evaluated through bioinformatic and computational approaches. In the functional analysis of the interaction network of the proteins, we found that the terms of gene ontology (GO) of greater significance were related to the intracellular transport of proteins, the signal transduction mediated by small G proteins and vesicles associated with the Golgi apparatus. From the proteins present in the GO terms of greater significance Rab8a was selected because its interaction facilitates the intracellular distribution of OCRL-1. The mutation p.Asn591Lys, present in the interaction domain of OCRL-1 and Rab8a, was studied using molecular dynamics. The molecular dynamics analysis showed that the presence of this mutation causes changes in the positional fluctuations of the amino acids and affects the flexibility of the protein making the interaction with Rab8a weaker. Rab proteins establish some specific interactions, which are important for the intracellular localization of OCRL-1; therefore, our findings suggest that the phenotype observed in patients with LS, in this case, is due to the destabilizing effect of p.Asn591Lys affecting the localization of OCRL-1 and indirectly its 5-phosphatase activity in the Golgi apparatus, endosomes, and cilia.

Hypotonia and delayed motor development as an early presentation of Lowe syndrome: case report and literature review

We describe a boy who presented with neonatal hypotonia, followed by delayed motor development and growth impairment. Further evaluation revealed rickets caused by proximal renal tubular dysfunction. At age 3, the boy exhibited dysmorphic features and bilateral cataract. Genetic analysis of the OCRL gene showed a novel variant in exon 13: c.1250T>A, p.Val417Asp; in silico and segregation analysis confirmed the variant to be pathogenic, compatible with the diagnosis of the oculocerebrorenal syndrome of Lowe. Lowe syndrome is a rare multisystemic disorder; the diagnostic triad requires involvement of the eye, central nervous system and the proximal renal tubule. Typical clinical features are congenital cataract, glaucoma, hypotonia, mental and behavioral problems, benign skin lesions, platelet dysfunction and dental abnormalities. Phenotypic features early in life may be nonspecific, which is illustrated by this case with a late manifestation of cataract. Because an early diagnosis can lead to better counseling and treatment, we suggest urinary testing for proteinuria as a part of the evaluation of children with unexplained hypotonia.

Alport syndrome with bilateral simultaneous anterior and posterior lenticonus with severe temporal macular thinning

Alport syndrome (AS) is a hereditary disease with various modes of inheritance, X-linked being the the most common. Anterior lenticonus is the characteristic abnormality along with perimacular and peripheral fleck retinopathy. Our two cases of AS had simultaneous anterior and posterior lenticonus with severe temporal macular thinning on optical coherence tomography with no specific renal symptomatology and were diagnosed as AS without any invasive renal biopsy. First patient was a 19-year-old man who presented with compound myopia due to bilateral anterior and posterior lenticonus with perimacular fleck retinopathy and lozenge sign and bilateral moderate sensorineural hearing loss (SNHL). Second patient was a 24-year-old man who presented with difficulty in vision due to bilateral anterior and posterior lenticonus with bilateral severe SNHL. Our cases emphasise the crucial role of an ophthalmologist in diagnosing AS before the onset of renal symptoms and prompting further nephrological work-up in the patient or the carrier.

Loss of OCRL increases ciliary PI(4,5)P2 in Lowe oculocerebrorenal syndrome

Lowe syndrome is a rare X-linked disorder characterized by bilateral congenital cataracts and glaucoma, mental retardation, and proximal renal tubular dysfunction. Mutations in OCRL, an inositol polyphosphate 5-phosphatase that dephosphorylates PI(4,5)P₂, cause Lowe syndrome. Previously we showed that OCRL localizes to the primary cilium, which has a distinct membrane phospholipid composition, but disruption of phosphoinositides in the ciliary membrane is poorly understood. Here, we demonstrate that cilia from Lowe syndrome patient fibroblasts exhibit increased levels of PI(4,5)P₂ and decreased levels of PI4P. In particular, subcellular distribution of PI(4,5)P₂ build-up was observed at the transition zone. Accumulation of ciliary PI(4,5)P₂ was pronounced in mouse embryonic fibroblasts (MEFs) derived from Lowe syndrome mouse model as well as in Ocrl-null MEFs, which was reversed by reintroduction of OCRL. Similarly, expression of wild-type OCRL reversed the elevated PI(4,5)P₂ in Lowe patient cells. Accumulation of sonic hedgehog protein in response to hedgehog agonist was decreased in MEFs derived from a Lowe syndrome mouse model. Together, our findings show for the first time an abnormality in ciliary phosphoinositides of both human and mouse cell models of Lowe syndrome.