Recent insights into peroxisome biogenesis and associated diseases.

Peroxisomes are single-membrane organelles present in eukaryotes. The functional importance of peroxisomes in humans is represented by peroxisome-deficient peroxisome biogenesis disorders (PBDs), including Zellweger syndrome. Defects in the genes that encode the 14 peroxins that are required for peroxisomal membrane assembly, matrix protein import and division have been identified in PBDs. A number of recent findings have advanced our understanding of the biology, physiology and consequences of functional defects in peroxisomes. In this Review, we discuss a cooperative cell defense mechanisms against oxidative stress that involves the localization of BAK (also known as BAK1) to peroxisomes, which alters peroxisomal membrane permeability, resulting in the export of catalase, a peroxisomal enzyme. Another important recent finding is the discovery of a nucleoside diphosphate kinase-like protein that has been shown to be essential for how the energy GTP is generated and provided for the fission of peroxisomes. With regard to PBDs, we newly identified a mild mutation, Pex26-F51L that causes only hearing loss. We will also discuss findings from a new PBD model mouse defective in Pex14, which manifested dysregulation of the BDNF-TrkB pathway, an essential signaling pathway in cerebellar morphogenesis. Here, we thus aim to provide a current view of peroxisome biogenesis and the molecular pathogenesis of PBDs.

Novel REEP6 gene mutation associated with autosomal recessive retinitis pigmentosa.

PURPOSE: This study reports the ophthalmic and genetic findings of a Cameroonian patient with autosomal recessive retinitis pigmentosa (arRP) caused by a novel Receptor Expression Enhancing Protein 6 (REEP6) homozygous mutation. PATIENT AND METHODS: A 33-year-old man underwent comprehensive ophthalmic examinations, including visual acuity measurements, dilated fundus imaging, electroretinography (ERG), and spectral-domain optical coherence tomography (SD-OCT). Short-wavelength fundus autofluorescence (SW-AF) and near-infrared fundus autofluorescence (NIR-AF) were also evaluated. Whole exome sequencing (WES) was used to identify potential pathogenic variants. RESULTS: Fundus examination revealed typical RP findings with additional temporal ten micron yellow dots. SD-OCT imaging revealed cystoid macular edema and perifoveal outer retinal atrophy with centrally preserved inner segment ellipsoid zone (EZ) bands. Hyperreflective spots were seen in the inner retinal layers. On SW-AF images, a hypoautofluorescent area in the perifoveal area was observed. NIR-AF imaging revealed an irregularly shaped hyperautofluorescent ring. His visual acuity was mildly affected. ERG showed undetectable rod responses and intact cone responses. Genetic testing via WES revealed a novel homozygous mutation (c.295G>A, p.Glu99Lys) in the gene encoding REEP6, which is predicted to alter the charge in the transmembrane helix. CONCLUSIONS: This report is not only the first description of a Cameroonian patient with arRP associated with a REEP6 mutation, but also this particular genetic alteration. Substitution of p.Glu99Lys in REEP6 likely disrupts the interactions between REEP6 and the ER membrane. NIR-AF imaging may be particularly useful for assessing functional photoreceptor cells and show an "avocado" pattern of hyperautofluorescence in patients with the REEP6 mutation.

Mutations in SPATA5 Are Associated with Microcephaly, Intellectual Disability, Seizures, and Hearing Loss.

Using whole-exome sequencing, we have identified in ten families 14 individuals with microcephaly, developmental delay, intellectual disability, hypotonia, spasticity, seizures, sensorineural hearing loss, cortical visual impairment, and rare autosomal-recessive predicted pathogenic variants in spermatogenesis-associated protein 5 (SPATA5). SPATA5 encodes a ubiquitously expressed member of the ATPase associated with diverse activities (AAA) protein family and is involved in mitochondrial morphogenesis during early spermatogenesis. It might also play a role in post-translational modification during cell differentiation in neuronal development. Mutations in SPATA5 might affect brain development and function, resulting in microcephaly, developmental delay, and intellectual disability.

Human embryos commonly form abnormal nuclei during development: a mechanism of DNA damage, embryonic aneuploidy, and developmental arrest.

STUDY QUESTION: What is the prevalence and developmental significance of morphologic nuclear abnormalities in human preimplantation embryos? SUMMARY ANSWER: Nuclear abnormalities are commonly found in human IVF embryos and are associated with DNA damage, aneuploidy, and decreased developmental potential. WHAT IS KNOWN ALREADY: Early human embryonic development is complicated by genomic errors that occur after fertilization. The appearance of extra-nuclear DNA, which has been observed in IVF, may be a result of such errors. However, the mechanism by which abnormal nuclei form and the impact on DNA integrity and embryonic development is not understood. STUDY DESIGN, SIZE, DURATION: Cryopreserved human cleavage-stage embryos (n = 150) and cryopreserved blastocysts (n = 105) from clinical IVF cycles performed between 1997 and 2008 were donated for research. Fresh embryos (n = 60) of poor quality that were slated for discard were also used. Immunohistochemical, microscopic and cytogenetic analyses at different developmental stages and morphologic grades were performed. PARTICIPANTS/MATERIALS, SETTING, METHODS: Embryos were fixed and stained for DNA, centromeres, mitotic activity and DNA damage and imaged using confocal microscopy. Rates of abnormal nuclear formation were compared between morphologically normal cleavage-stage embryos, morphologically normal blastocysts, and poor quality embryos. To control for clinical and IVF history of oocytes donors, and quality of frozen embryos within our sample, cleavage-stage embryos (n = 52) were thawed and fixed at different stages of development and then analyzed microscopically. Cleavage-stage embryos (n = 9) were thawed and all blastomeres (n = 62) were disaggregated, imaged and analyzed for karyotype. Correlations were made between microscopic and cytogenetic findings of individual blastomeres and whole embryos. MAIN RESULTS AND THE ROLE OF CHANCE: The frequency of microscopic nuclear abnormalities was lower in blastocysts (5%; 177/3737 cells) than in cleavage-stage embryos (16%, 103/640 blastomeres, P < 0.05) and highest in arrested embryos (65%; 44/68 blastomeres, P < 0.05). DNA damage was significantly higher in cells with microscopic nuclear abnormalities (γH2AX (phosphorylated (Ser139) histone H2A.X): 87.1%, 74/85; replication protein A: 72.9%, 62/85) relative to cells with normal nuclear morphology (γH2AX: 9.3%, 60/642; RPA: 5.6%, 36/642) (P < 0.05). Blastomeres containing nuclear abnormalities were strongly associated with aneuploidy (Fisher exact test, two-tailed, P < 0.01). LIMITATIONS, REASONS FOR CAUTION: The embryos used were de-identified, and the clinical and IVF history was unknown. WIDER IMPLICATIONS OF THE FINDINGS: This study explores a mechanism of abnormal embryonic development post-fertilization. While most of the current data have explored abnormal meiotic chromosome segregation in oocytes as a primary mechanism of reproductive failure, abnormal nuclear formation during early mitotic cell division in IVF embryos also plays a significant role. The detection of abnormal nuclear formation may have clinical application in noninvasive embryo selection during IVF. STUDY FUNDING/COMPETING INTERESTS: The study was supported by Columbia University and the New York Stem Cell Foundation. Authors declare no competing interest.

Organ size is limited by the number of embryonic progenitor cells in the pancreas but not the liver.

The determinants of vertebrate organ size are poorly understood, but the process is thought to depend heavily on growth factors and other environmental cues. In the blood and central nervous system, for example, organ mass is determined primarily by growth-factor-regulated cell proliferation and apoptosis to achieve a final target size. Here, we report that the size of the mouse pancreas is constrained by an intrinsic programme established early in development, one that is essentially not subject to growth compensation. Specifically, final pancreas size is limited by the size of the progenitor cell pool that is set aside in the developing pancreatic bud. By contrast, the size of the liver is not constrained by reductions in the progenitor cell pool. These findings show that progenitor cell number, independently of regulation by growth factors, can be a key determinant of organ size.

Rare variants and HLA haplotypes associated in patients with neuromyelitis optica spectrum disorders.

Neuromyelitis optica spectrum disorders (NMOSD) are rare, debilitating autoimmune diseases of the central nervous system. Many NMOSD patients have antibodies to Aquaporin-4 (AQP4). Prior studies show associations of NMOSD with individual Human Leukocyte Antigen (HLA) alleles and with mutations in the complement pathway and potassium channels. HLA allele associations with NMOSD are inconsistent between populations, suggesting complex relationships between the identified alleles and risk of disease. We used a retrospective case-control approach to identify contributing genetic variants in patients who met the diagnostic criteria for NMOSD and their unaffected family members. Potentially deleterious variants identified in NMOSD patients were compared to members of their families who do not have the disease and to existing databases of human genetic variation. HLA sequences from patients from Belgrade, Serbia, were compared to the frequency of HLA haplotypes in the general population in Belgrade. We analyzed exome sequencing on 40 NMOSD patients and identified rare inherited variants in the complement pathway and potassium channel genes. Haplotype analysis further detected two haplotypes, HLA-A*01, B*08, DRB1*03 and HLA-A*01, B*08, C*07, DRB1*03, DQB1*02, which were more prevalent in NMOSD patients than in unaffected individuals. In silico modeling indicates that HLA molecules within these haplotypes are predicted to bind AQP4 at several sites, potentially contributing to the development of autoimmunity. Our results point to possible autoimmune and neurodegenerative mechanisms that cause NMOSD, and can be used to investigate potential NMOSD drug targets.

Phenotypic expansion of autosomal dominant retinitis pigmentosa associated with the D477G mutation in RPE65.

Mutations in the gene RPE65 (OMIM: 180069) are recessively inherited and known to cause Leber congenital amaurosis. Recently, the mutation D477G in RPE65 has been identified as a cause of autosomal dominant retinitis pigmentosa (RP). Variable expressivity of this disease has been reported, as carrier individuals can present with mild, nonpenetrant, or, most commonly, a severe chorioretinal phenotype that resembles choroideremia. We report the case of a 57-yr-old male who presented to our clinic with nyctalopia and decreasing visual acuity for 1 yr. Dilated fundus examination revealed retinal atrophy and peripheral mottling of the retinal pigment epithelium (RPE). SW-AF revealed patchy hypoautofluorescence throughout the posterior pole with separate lacunae-like areas in the macula of severe RPE atrophy along with foveal sparing. Full-field electroretinogram suggested a rod-cone dystrophy. Whole-exome sequencing revealed the heterozygous mutation c.1430A > G (p.D477G) in the RPE65 gene. This phenotype of peripheral RPE mottling and severe macular lacunae-like atrophy has not been previously reported with RPE65 autosomal dominant RP, supporting the variable expressivity of the disease and expanding the known phenotypic presentations.

Progressive RPE atrophy and photoreceptor death in KIZ-associated autosomal recessive retinitis pigmentosa.

Background: To evaluate the long-term progression of autosomal recessive retinitis pigmentosa (RP) due to mutations in KIZ using multimodal imaging and a quantitative analytical approach.Methods: Whole exome sequencing (WES) and targeted capture sequencing were used to identify mutation. Fundus photography, short-wavelength autofluorescence (SW-AF), spectral-domain optical coherence tomography (SD-OCT) imaging, and electroretinography (ERG) were analyzed. Serial measurements of peripheral retinal pigment epithelium (RPE) atrophy area with SW-AF, as well as the ellipsoid zone (EZ) width using SD-OCT were performed.Results: Two homozygous variants in KIZ-a c.226C>T mutation as well as a previously unreported c.119_122delAACT mutation-were identified in four unrelated patients. Fundus examination and ERG revealed classic rod-cone dysfunction, and SD-OCT demonstrated outer retinal atrophy with centrally preserved EZ line. SW-AF imaging revealed hyperautofluorescent rings with surrounding parafoveal, mid-peripheral and widespread loss of autofluorescence. The RPE atrophy area increased annually by 4.9%. Mean annual exponential rates of decline for KIZ patients were 8.5% for visual acuity and 15.9% for 30 Hz Flicker amplitude. The average annual reduction distance of the EZ distance was 66.5 µm per year.Conclusions: RPE atrophy progresses along with a loss of photoreceptors, and parafoveal RPE hypoautofluorescence is commonly seen in KIZ-associated RP patients. KIZ-associated RP is an early-onset severe rod-cone dystrophy.

Fundoscopy-directed genetic testing to re-evaluate negative whole exome sequencing results.

BACKGROUND: Whole exome sequencing (WES) allows for an unbiased search of the genetic cause of a disease. Employing it as a first-tier genetic testing can be favored due to the associated lower incremental cost per diagnosis compared to when using it later in the diagnostic pathway. However, there are technical limitations of WES that can lead to inaccurate negative variant callings. Our study presents these limitations through a re-evaluation of negative WES results using subsequent tests primarily driven by fundoscopic findings. These tests included targeted gene testing, inherited retinal gene panels, whole genome sequencing (WGS), and array comparative genomic hybridization. RESULTS: Subsequent genetic testing guided by fundoscopy findings identified the following variant types causing retinitis pigmentosa that were not detected by WES: frameshift deletion and nonsense variants in the RPGR gene, 353-bp Alu repeat insertions in the MAK gene, and large exonic deletion variants in the EYS and PRPF31 genes. Deep intronic variants in the ABCA4 gene causing Stargardt disease and the GUCY2D gene causing Leber congenital amaurosis were also identified. CONCLUSIONS: Negative WES analyses inconsistent with the phenotype should raise clinical suspicion. Subsequent genetic testing may detect genetic variants missed by WES and can make patients eligible for gene replacement therapy and upcoming clinical trials. When phenotypic findings support a genetic etiology, negative WES results should be followed by targeted gene sequencing, array based approach or whole genome sequencing.

Mononuclear muscle cells in Drosophila ovaries revealed by GFP protein traps.

Genetic analysis of muscle specification, formation and function in model systems has provided valuable insight into human muscle physiology and disease. Studies in Drosophila have been particularly useful for discovering key genes involved in muscle specification, myoblast fusion, and sarcomere organization. The muscles of the Drosophila female reproductive system have received little attention despite extensive work on oogenesis. We have used newly available GFP protein trap lines to characterize of ovarian muscle morphology and sarcomere organization. The muscle cells surrounding the oviducts are multinuclear with highly organized sarcomeres typical of somatic muscles. In contrast, the two muscle layers of the ovary, which are derived from gonadal mesoderm, have a mesh-like morphology similar to gut visceral muscle. Protein traps in the Fasciclin 3 gene produced Fas3::GFP that localized in dots around the periphery of epithelial sheath cells, the muscle surrounding ovarioles. Surprisingly, the epithelial sheath cells each contain a single nucleus, indicating these cells do not undergo myoblast fusion during development. Consistent with this observation, we were able to use the Flp/FRT system to efficiently generate genetic mosaics in the epithelial sheath, suggesting these cells provide a new opportunity for clonal analysis of adult striated muscle.

A newly identified mutation in the PEX26 gene is associated with a milder form of Zellweger spectrum disorder.

Using clinical exome sequencing (ES), we identified an autosomal recessive missense variant, c.153C>A (p.F51L), in the peroxisome biogenesis factor 26 gene (PEX26) in a 19-yr-old female of Ashkenazi Jewish descent who was referred for moderate to severe hearing loss. The proband and three affected siblings are all homozygous for the c.153C>A variant. Skin fibroblasts from this patient show normal morphology in immunostaining of matrix proteins, although the level of catalase was elevated. Import rate of matrix proteins was significantly decreased in the patient-derived fibroblasts. Binding of Pex26-F51L to the AAA ATPase peroxins, Pex1 and Pex6, is severely impaired and affects peroxisome assembly. Moreover, Pex26 in the patient's fibroblasts is reduced to ∼30% of the control, suggesting that Pex26-F51L is unstable in cells. In the patient's fibroblasts, peroxisome-targeting signal 1 (PTS1) proteins, PTS2 protein 3-ketoacyl-CoA thiolase, and catalase are present in a punctate staining pattern at 37°C and in a diffuse pattern at 42°C, suggesting that these matrix proteins are not imported to peroxisomes in a temperature-sensitive manner. Analysis of peroxisomal metabolism in the patient's fibroblasts showed that the level of docosahexaenoic acid (DHA) (C22:6n-3) in ether phospholipids is decreased, whereas other lipid metabolism, including peroxisomal fatty acid ß-oxidation, is normal. Collectively, the functional data support the mild phenotype of nonsyndromic hearing loss in patients harboring the F51L variant in PEX26.

Spectrum of Disease Severity and Phenotype in Choroideremia Carriers.

PURPOSE: To characterize and bring awareness to the disease spectrum of female choroideremia patients, as severity can vary from mild to severe disease, comparable to that observed in male patients. DESIGN: Retrospective cohort study. METHODS: Twelve female carriers of disease-causing variants in the CHM gene confirmed by molecular genetic sequencing were characterized clinically and imaged with short-wave fundus autofluorescence (SW-FAF), spectral-domain optical coherence tomography (OCT), and color fundus imaging. RESULTS: Twelve unrelated female patients with a clinical and genetic diagnosis of choroideremia carriers were included in this study. Disease severity among these phenotypes ranged from mild to severe, resembling the typical presentation of choroideremia in male patients. Mild disease presented with retinal pigment epithelium mottling, a patchy pattern of hypoautofluorescent speckles on SW-FAF, and intact retinal layers on spectral-domain OCT. Severe disease presented with widespread chorioretinal atrophy as shown by SW-FAF and spectral-domain OCT. Each of the identified genetic variants in CHM was predicted to be disease-causing according to in silico prediction software. Disease progression analysis of 4 patients with follow-up showed a decline in visual acuity for 2 patients, with progression observed on spectral-domain OCT in 1 of the patients. No significant disease progression on SW-FAF was observed for any of the patients. CONCLUSIONS: Female carriers of choroideremia can present with a wide range of clinical phenotypes and disease severity, from mild to severe disease, similar to male subjects. Symptomatic female subjects should be considered for current and upcoming gene replacement therapy clinical trials.

Compound heterozygous novel frameshift variants in the PROM1 gene result in Leber congenital amaurosis.

The PROM1 (prominin 1) gene encodes an 865-amino acid glycoprotein that is expressed in retinoblastoma cell lines and in the adult retina. The protein is localized to photoreceptor outer segment disc membranes, where it plays a structural role, and in the retinal pigment epithelium (RPE), where it acts as a cytosolic protein that mediates autophagy. Mutations in PROM1 are typically associated with cone-rod dystrophy 12 (OMIM#3612657), autosomal dominant retinal macular dystrophy 2 (OMIM#608051), autosomal recessive retinitis pigmentosa 41 (OMIM#612095), and Stargardt disease 4 (OMIM#603786). Here we describe the first case of PROM1-associated Leber congenital amaurosis (LCA) in a 12-yr-old Asian male, caused by two not previously described deleterious frameshift variants in the compound heterozygous state. Clinical features include the presence of bull's eye maculopathy, pendular horizontal nystagmus, and photodysphoria consistent with the clinical diagnosis of LCA. The patient was evaluated using ophthalmic imaging, electroretinography, and whole-exome sequencing. Electroretinography revealed extinguished retinal activity.

De novo variants in EBF3 are associated with hypotonia, developmental delay, intellectual disability, and autism.

Using whole-exome sequencing, we identified seven unrelated individuals with global developmental delay, hypotonia, dysmorphic facial features, and an increased frequency of short stature, ataxia, and autism with de novo heterozygous frameshift, nonsense, splice, and missense variants in the Early B-cell Transcription Factor Family Member 3 (EBF3) gene. EBF3 is a member of the collier/olfactory-1/early B-cell factor (COE) family of proteins, which are required for central nervous system (CNS) development. COE proteins are highly evolutionarily conserved and regulate neuronal specification, migration, axon guidance, and dendritogenesis during development and are essential for maintaining neuronal identity in adult neurons. Haploinsufficiency of EBF3 may affect brain development and function, resulting in developmental delay, intellectual disability, and behavioral differences observed in individuals with a deleterious variant in EBF3.

De novo pathogenic variants in CHAMP1 are associated with global developmental delay, intellectual disability, and dysmorphic facial features.

We identified five unrelated individuals with significant global developmental delay and intellectual disability (ID), dysmorphic facial features and frequent microcephaly, and de novo predicted loss-of-function variants in chromosome alignment maintaining phosphoprotein 1 (CHAMP1). Our findings are consistent with recently reported de novo mutations in CHAMP1 in five other individuals with similar features. CHAMP1 is a zinc finger protein involved in kinetochore-microtubule attachment and is required for regulating the proper alignment of chromosomes during metaphase in mitosis. Mutations in CHAMP1 may affect cell division and hence brain development and function, resulting in developmental delay and ID.

De novo mutations in PURA are associated with hypotonia and developmental delay.

PURA is the leading candidate gene responsible for the developmental phenotype in the 5q31.3 microdeletion syndrome. De novo mutations in PURA were recently reported in 15 individuals with developmental features similar to the 5q31.3 microdeletion syndrome. Here we describe six unrelated children who were identified by clinical whole-exome sequencing (WES) to have novel de novo variants in PURA with a similar phenotype of hypotonia and developmental delay and frequently associated with seizures. The protein Purα (encoded by PURA) is involved in neuronal proliferation, dendrite maturation, and the transport of mRNA to translation sites during neuronal development. Mutations in PURA may alter normal brain development and impair neuronal function, leading to developmental delay and the seizures observed in patients with mutations in PURA.