Mutations in NYX, encoding the leucine-rich proteoglycan nyctalopin, cause X-linked complete congenital stationary night blindness

During development, visual photoreceptors, bipolar cells and other neurons establish connections within the retina enabling the eye to process visual images over approximately 7 log units of illumination. Within the retina, cells that respond to light increment and light decrement are separated into ON- and OFF-pathways. Hereditary diseases are known to disturb these retinal pathways, causing either progressive degeneration or stationary deficits. Congenital stationary night blindness (CSNB) is a group of stable retinal disorders that are characterized by abnormal night vision. Genetic subtypes of CSNB have been defined and different disease actions have been postulated. The molecular bases have been elucidated in several subtypes, providing a better understanding of the disease mechanisms and developmental retinal neurobiology. Here we have studied 22 families with 'complete' X-linked CSNB (CSNB1; MIM 310500; ref. 4) in which affected males have night blindness, some photopic vision loss and a defect of the ON-pathway. We have found 14 different mutations, including 1 founder mutation in 7 families from the United States, in a novel candidate gene, NYX. NYX, which encodes a glycosylphosphatidyl (GPI)-anchored protein called nyctalopin, is a new and unique member of the small leucine-rich proteoglycan (SLRP) family. The role of other SLRP proteins suggests that mutant nyctalopin disrupts developing retinal interconnections involving the ON-bipolar cells, leading to the visual losses seen in patients with complete CSNB.     

Mutations in WDR62, encoding a centrosome-associated protein, cause microcephaly with simplified gyri and abnormal cortical architecture

Genes associated with human microcephaly, a condition characterized by a small brain, include critical regulators of proliferation, cell fate and DNA repair. We describe a syndrome of congenital microcephaly and diverse defects in cerebral cortical architecture. Genome-wide linkage analysis in two families identified a 7.5-Mb locus on chromosome 19q13.12 containing 148 genes. Targeted high throughput sequence analysis of linked genes in each family yielded > 4,000 DNA variants and implicated a single gene, WDR62, as harboring potentially deleterious changes. We subsequently identified additional WDR62 mutations in four other families. Magnetic resonance imaging and postmortem brain analysis supports important roles for WDR62 in the proliferation and migration of neuronal precursors. WDR62 is a WD40 repeat-containing protein expressed in neuronal precursors as well as in postmitotic neurons in the developing brain and localizes to the spindle poles of dividing cells. The diverse phenotypes of WDR62 suggest it has central roles in many aspects of cerebral cortical development.     

Nuclear gene OPA1, encoding a mitochondrial dynamin-related protein, is mutated in dominant optic atrophy

Optic atrophy type 1 (OPA1, MIM 165500) is a dominantly inherited optic neuropathy occurring in 1 in 50,000 individuals that features progressive loss in visual acuity leading, in many cases, to legal blindness. Phenotypic variations and loss of retinal ganglion cells, as found in Leber hereditary optic neuropathy (LHON), have suggested possible mitochondrial impairment. The OPA1 gene has been localized to 3q28-q29 (refs 13-19). We describe here a nuclear gene, OPA1, that maps within the candidate region and encodes a dynamin-related protein localized to mitochondria. We found four different OPA1 mutations, including frameshift and missense mutations, to segregate with the disease, demonstrating a role for mitochondria in retinal ganglion cell pathophysiology.     

Mutations in the gene encoding pejvakin, a newly identified protein of the afferent auditory pathway, cause DFNB59 auditory neuropathy

Auditory neuropathy is a particular type of hearing impairment in which neural transmission of the auditory signal is impaired, while cochlear outer hair cells remain functional. Here we report on DFNB59, a newly identified gene on chromosome 2q31.1-q31.3 mutated in four families segregating autosomal recessive auditory neuropathy. DFNB59 encodes pejvakin, a 352-residue protein. Pejvakin is a paralog of DFNA5, a protein of unknown function also involved in deafness. By immunohistofluorescence, pejvakin is detected in the cell bodies of neurons of the afferent auditory pathway. Furthermore, Dfnb59 knock-in mice, homozygous for the R183W variant identified in one DFNB59 family, show abnormal auditory brainstem responses indicative of neuronal dysfunction along the auditory pathway. Unlike previously described sensorineural deafness genes, all of which underlie cochlear cell pathologies, DFNB59 is the first human gene implicated in nonsyndromic deafness due to a neuronal defect.     

Wnt-beta-catenin signaling initiates taste papilla development

Fungiform taste papillae form a regular array on the dorsal tongue. Taste buds arise from papilla epithelium and, unusually for epithelial derivatives, synapse with neurons, release neurotransmitters and generate receptor and action potentials. Despite the importance of taste as one of our five senses, genetic analyses of taste papilla and bud development are lacking. We demonstrate that Wnt-beta-catenin signaling is activated in developing fungiform placodes and taste bud cells. A dominant stabilizing mutation of epithelial beta-catenin causes massive overproduction of enlarged fungiform papillae and taste buds. Likewise, genetic deletion of epithelial beta-catenin or inhibition of Wnt-beta-catenin signaling by ectopic dickkopf1 (Dkk1) blocks initiation of fungiform papilla morphogenesis. Ectopic papillae are innervated in the stabilizing beta-catenin mutant, whereas ectopic Dkk1 causes absence of lingual epithelial innervation. Thus, Wnt-beta-catenin signaling is critical for fungiform papilla and taste bud development. Altered regulation of this pathway may underlie evolutionary changes in taste papilla patterning.     

Control of neurulation by the nucleosome assembly protein-1-like 2

Neurulation is a complex process of histogenesis involving the precise temporal and spatial organization of gene expression. Genes influencing neurulation include proneural genes determining primary cell fate, neurogenic genes involved in lateral inhibition pathways and genes controlling the frequency of mitotic events. This is reflected in the aetiology and genetics of human and mouse neural tube defects, which are of both multifactorial and multigenic origin. The X-linked gene Nap1l2, specifically expressed in neurons, encodes a protein that is highly similar to the nucleosome assembly (NAP) and SET proteins. We inactivated Nap1l2 in mice by gene targeting, leading to embryonic lethality from mid-gestation onwards. Surviving mutant chimaeric embryos showed extensive surface ectoderm defects as well as the presence of open neural tubes and exposed brains similar to those observed in human spina bifida and anencephaly. These defects correlated with an overproduction of neuronal precursor cells. Protein expression studies showed that the Nap1l2 protein binds to condensing chromatin during S phase and in apoptotic cells, but remained cytoplasmic during G1 phase. Nap1l2 therefore likely represents a class of tissue-specific factors interacting with chromatin to regulate neuronal cell proliferation.     

Mutations in the gene encoding lecithin retinol acyltransferase are associated with early-onset severe retinal dystrophy

The chromophore of the visual pigments, 11-cis retinal, is derived from vitamin A (all-trans retinol) through a series of reactions that take place in retinal pigment epithelium (RPE); (ref. 1). The first of these reactions is catalyzed by lecithin retinol acyltransferase (LRAT); (ref. 2). We screened 267 retinal dystrophy patients for mutations in LRAT and identified disease-associated mutations (S175R and 396delAA) in three individuals with severe, early-onset disease. We showed that the S175R mutant has no acyltransferase activity in transfected COS-7 cells. Our findings highlight the importance of genetic defects in vitamin A metabolism as causes of retinal dystrophies and extend prospects for retinoid replacement therapy in this group of diseases.     

Mutations in RDH12 encoding a photoreceptor cell retinol dehydrogenase cause childhood-onset severe retinal dystrophy

We identified three consanguineous Austrian kindreds with 15 members affected by autosomal recessive childhood-onset severe retinal dystrophy, a genetically heterogeneous group of disorders characterized by degeneration of the photoreceptor cells. A whole-genome scan by microarray analysis of single-nucleotide polymorphisms (ref. 2) identified a founder haplotype and defined a critical interval of 1.53 cM on chromosome 14q23.3-q24.1 that contains the gene associated with this form of retinal dystrophy. RDH12 maps in this region and encodes a retinol dehydrogenase proposed to function in the visual cycle. A homozygous 677A-->G transition (resulting in Y226C) in RDH12 was present in all affected family members studied, as well as in two Austrian individuals with sporadic retinal dystrophy. We identified additional mutations in RDH12 in 3 of 89 non-Austrian individuals with retinal dystrophy: a 5-nucleotide deletion (806delCCCTG) and the transition 565C-->T (resulting in Q189X), each in the homozygous state, and 146C-->T (resulting in T49M) and 184C-->T (resulting in R62X) in compound heterozygosity. When expressed in COS-7 cells, Cys226 and Met49 variants had diminished and aberrant activity, respectively, in interconverting isomers of retinol and retinal. The severe visual impairment of individuals with mutations in RDH12 is in marked contrast to the mild visual deficiency in individuals with fundus albipunctatus caused by mutations in RDH5, encoding another retinal dehydrogenase. Our studies show that RDH12 is associated with retinal dystrophy and encodes an enzyme with a unique, nonredundant role in the photoreceptor cells.     

Loss of BBS proteins causes anosmia in humans and defects in olfactory cilia structure and function in the mouse

Defects in cilia are associated with several human disorders, including Kartagener syndrome, polycystic kidney disease, nephronophthisis and hydrocephalus. We proposed that the pleiotropic phenotype of Bardet-Biedl syndrome (BBS), which encompasses retinal degeneration, truncal obesity, renal and limb malformations and developmental delay, is due to dysfunction of basal bodies and cilia. Here we show that individuals with BBS have partial or complete anosmia. To test whether this phenotype is caused by ciliary defects of olfactory sensory neurons, we examined mice with deletions of Bbs1 or Bbs4. Loss of function of either BBS protein affected the olfactory, but not the respiratory, epithelium, causing severe reduction of the ciliated border, disorganization of the dendritic microtubule network and trapping of olfactory ciliary proteins in dendrites and cell bodies. Our data indicate that BBS proteins have a role in the microtubule organization of mammalian ciliated cells and that anosmia might be a useful determinant of other pleiotropic disorders with a suspected ciliary involvement.     

Rb regulates proliferation and rod photoreceptor development in the mouse retina

The retinoblastoma protein (Rb) regulates proliferation, cell fate specification and differentiation in the developing central nervous system (CNS), but the role of Rb in the developing mouse retina has not been studied, because Rb-deficient embryos die before the retinas are fully formed. We combined several genetic approaches to explore the role of Rb in the mouse retina. During postnatal development, Rb is expressed in proliferating retinal progenitor cells and differentiating rod photoreceptors. In the absence of Rb, progenitor cells continue to divide, and rods do not mature. To determine whether Rb functions in these processes in a cell-autonomous manner, we used a replication-incompetent retrovirus encoding Cre recombinase to inactivate the Rb1(lox) allele in individual retinal progenitor cells in vivo. Combined with data from studies of conditional inactivation of Rb1 using a combination of Cre transgenic mouse lines, these results show that Rb is required in a cell-autonomous manner for appropriate exit from the cell cycle of retinal progenitor cells and for rod development.     

Amino-terminal fragments of mutant huntingtin show selective accumulation in striatal neurons and synaptic toxicity

Huntington disease (HD) is caused by expansion of a glutamine repeat in the amino-terminal region of huntingtin. Despite its widespread expression, mutant huntingtin induces selective neuronal loss in striatal neurons. Here we report that, in mutant mice expressing HD repeats, the production and aggregation of N-terminal huntingtin fragments preferentially occur in HD-affected neurons and their processes and axonal terminals. N-terminal fragments of mutant huntingtin form aggregates and induce neuritic degeneration in cultured striatal neurons. N-terminal mutant huntingtin also binds to synaptic vesicles and inhibits their glutamate uptake in vitro. The specific processing and accumulation of toxic fragments of N-terminal huntingtin in HD-affected striatal neurons, especially in their neuronal processes and axonal terminals, may contribute to the selective neuropathology of HD.     

Cryptic boundaries in roof plate and choroid plexus identified by intersectional gene activation

The hindbrain roof plate and choroid plexus are essential organizing centers for inducing dorsal neuron fates and sustaining neuron function. To map the formation of these structures, we developed a broadly applicable, high resolution, recombinase-based method for mapping the fate of cells originating from coordinates defined by intersecting combinations of expressed genes. Using this method, we show that distinct regions of hindbrain roof plate originate from discrete subdomains of rhombencephalic neuroectoderm expressing Wnt1; that choroid plexus, a secretory epithelium important for patterning later-formed hindbrain structures and maintaining neuron function, derives from the same embryonic primordium as the hindbrain roof plate; and that, unlike the floor plate, these dorsal organizing centers develop in a patterned, segmental manner, built from lineage-restricted compartments. Our data suggest that the roof plate and choroid plexus may be formed of functional units that are capable of differentially organizing the generation of distinct neuronal cell types at different axial levels.     

Mutations in the gene encoding 11-cis retinol dehydrogenase cause delayed dark adaptation and fundus albipunctatus.

The metabolic pathways that produce 11-cis retinal are important for vision because this retinoid is the chromophore residing in rhodopsin and the cone opsins. The all-trans retinal that is generated after cone and rod photopigments absorb photons of light is recycled back to 11-cis retinal by the retinal pigment epithelium and Müller cells of the retina. Several of the enzymes involved have recently been purified and molecularly cloned; here we focus on 11-cis retinol dehydrogenase (encoded by the gene RDH5; chromosome 12q13-14; ref. 4), the first cloned enzyme in this pathway. This microsomal enzyme is abundant in the retinal pigment epithelium, where it has been proposed to catalyse the conversion of 11-cis retinol to 11-cis retinal. We evaluated patients with hereditary retinal diseases featuring subretinal spots (retinitis punctata albescens and fundus albipunctatus) and patients with typical dominant or recessive retinitis pigmentosa for mutations in RDH5. Mutations were found only in two unrelated patients, both with fundus albipunctatus; they segregated with disease in the respective families. Recombinant mutant 11-cis retinol dehydrogenases had reduced activity compared with recombinant enzyme with wild-type sequence. Our results suggest that mutant alleles in RDH5 are a cause of fundus albipunctatus, a rare form of stationary night blindness characterized by a delay in the regeneration of cone and rod photopigments.     

High levels of mitochondrial DNA deletions in substantia nigra neurons in aging and Parkinson disease

Here we show that in substantia nigra neurons from both aged controls and individuals with Parkinson disease, there is a high level of deleted mitochondrial DNA (mtDNA) (controls, 43.3% +/- 9.3%; individuals with Parkinson disease, 52.3% +/- 9.3%). These mtDNA mutations are somatic, with different clonally expanded deletions in individual cells, and high levels of these mutations are associated with respiratory chain deficiency. Our studies suggest that somatic mtDNA deletions are important in the selective neuronal loss observed in brain aging and in Parkinson disease.