Cone photoreceptor disorders form a clinical spectrum of diseases that include progressive cone dystrophy (CD) and complete and incomplete achromatopsia (ACHM). The underlying disease mechanisms of autosomal recessive (ar)CD are largely unknown. Our aim was to identify causative genes for these disorders by genome-wide homozygosity mapping. We investigated 75 ACHM, 97 arCD, and 20 early-onset arCD probands and excluded the involvement of known genes for ACHM and arCD. Subsequently, we performed high-resolution SNP analysis and identified large homozygous regions spanning the PDE6C gene in one sibling pair with early-onset arCD and one sibling pair with incomplete ACHM. The PDE6C gene encodes the cone α subunit of cyclic guanosine monophosphate (cGMP) phosphodiesterase, which converts cGMP to 5′-GMP, and thereby plays an essential role in cone phototransduction. Sequence analysis of the coding region of PDE6C revealed homozygous missense mutations (p.R29W, p.Y323N) in both sibling pairs. Sequence analysis of 104 probands with arCD and 10 probands with ACHM revealed compound heterozygous PDE6C mutations in three complete ACHM patients from two families. One patient had a frameshift mutation and a splice defect; the other two had a splice defect and a missense variant (p.M455V). Cross-sectional retinal imaging via optical coherence tomography revealed a more pronounced absence of cone photoreceptors in patients with ACHM compared to patients with early-onset arCD. Our findings identify PDE6C as a gene for cone photoreceptor disorders and show that arCD and ACHM constitute genetically and clinically overlapping phenotypes.

Rapid intracellular transport and secretion of cytotoxic granules through the immunological synapse requires a balanced interaction of several proteins. Disturbance of this highly regulated process underlies familial hemophagocytic lymphohistiocytosis (FHL), a genetically heterogeneous autosomal-recessive disorder characterized by a severe hyperinflammatory phenotype. Here, we have assigned FHL-5 to a 1 Mb region on chromosome 19p by using high-resolution SNP genotyping in eight unrelated FHL patients from consanguineous families. Subsequently, we found nine different mutations, either truncating or missense, in STXBP2 in twelve patients from Turkey, Saudi Arabia, and Central Europe. STXBP2 encodes syntaxin binding protein 2 (Munc18-2), involved in the regulation of vesicle transport to the plasma membrane. We have identified syntaxin 11, a SNARE protein mutated in FHL-4, as an interaction partner of STXBP2. This interaction is eliminated by the missense mutations found in our FHL-5 patients, which leads to a decreased stability of both proteins, as shown in patient lymphocytes. Activity of natural killer and cytotoxic T cells was markedly reduced or absent, as determined by CD107 degranulation. Our findings thus identify a key role for STXBP2 in lytic granule exocytosis.

Inherited and isolated nail malformations are rare and heterogeneous conditions. We identified two consanguineous pedigrees in which some family members were affected by isolated nail dysplasia that suggested an autosomal-recessive inheritance pattern and was characterized by claw-shaped nails, onychauxis, and onycholysis. Genome-wide SNP array analysis of affected individuals from both families showed an overlapping and homozygous region of 800 kb on the long arm of chromosome 8. The candidate region spans eight genes, and DNA sequence analysis revealed homozygous nonsense and missense mutations in FZD6, the gene encoding Frizzled 6. FZD6 belongs to a family of highly conserved membrane-bound WNT receptors involved in developmental processes and differentiation through several signaling pathways. We expressed the FZD6 missense mutation and observed a quantitative shift in subcellular distribution from the plasma membrane to the lysosomes, where the receptor is inaccessible for signaling and presumably degraded. Analysis of human fibroblasts homozygous for the nonsense mutation showed an aberrant response to both WNT-3A and WNT-5A stimulation; this response was consistent with an effect on both canonical and noncanonical WNT-FZD signaling. A detailed analysis of the Fzd6−/− mice...

Cone-rod dystrophy (CRD) and retinitis pigmentosa (RP) are clinically and genetically overlapping heterogeneous retinal dystrophies. By using homozygosity mapping in an individual with autosomal-recessive (ar) RP from a consanguineous family, we identified three sizeable homozygous regions, together encompassing 46 Mb. Next-generation sequencing of all exons, flanking intron sequences, microRNAs, and other highly conserved genomic elements in these three regions revealed a homozygous nonsense mutation (c.497T>A [p.Leu166∗]) in C8orf37, located on chromosome 8q22.1. This mutation was not present in 150 ethnically matched control individuals, single-nucleotide polymorphism databases, or the 1000 Genomes database. Immunohistochemical studies revealed C8orf37 localization at the base of the primary cilium of human retinal pigment epithelium cells and at the base of connecting cilia of mouse photoreceptors. C8orf37 sequence analysis of individuals who had retinal dystrophy and carried conspicuously large homozygous regions encompassing C8orf37 revealed a homozygous splice-site mutation (c.156−2A>G) in two siblings of a consanguineous family and homozygous missense mutations (c.529C>T [p.Arg177Trp]; c.545A>G [p.Gln182Arg]) in siblings of two other consanguineous families. The missense mutations affect highly conserved amino acids...

Olmsted syndrome (OS) is a rare congenital disorder characterized by palmoplantar and periorificial keratoderma, alopecia in most cases, and severe itching. The genetic basis for OS remained unidentified. Using whole-exome sequencing of case-parents trios, we have identified a de novo missense mutation in TRPV3 that produces p.Gly573Ser in an individual with OS. Nucleotide sequencing of five additional affected individuals also revealed missense mutations in TRPV3 (which produced p.Gly573Ser in three cases and p.Gly573Cys and p.Trp692Gly in one case each). Encoding a transient receptor potential vanilloid-3 cation channel, TRPV3 is primarily expressed in the skin, hair follicles, brain, and spinal cord. In transfected HEK293 cells expressing TRPV3 mutants, much larger inward currents were recorded, probably because of the constitutive opening of the mutants. These gain-of-function mutations might lead to elevated apoptosis of keratinocytes and consequent skin hyperkeratosis in the affected individuals. Our findings suggest that TRPV3 plays essential roles in skin keratinization, hair growth, and possibly itching sensation in humans and selectively targeting TRPV3 could provide therapeutic potential for keratinization or itching-related skin disorders.

We report that hypofunctional alleles of WNT1 cause autosomal-recessive osteogenesis imperfecta, a congenital disorder characterized by reduced bone mass and recurrent fractures. In consanguineous families, we identified five homozygous mutations in WNT1: one frameshift mutation, two missense mutations, one splice-site mutation, and one nonsense mutation. In addition, in a family affected by dominantly inherited early-onset osteoporosis, a heterozygous WNT1 missense mutation was identified in affected individuals. Initial functional analysis revealed that altered WNT1 proteins fail to activate canonical LRP5-mediated WNT-regulated β-catenin signaling. Furthermore, osteoblasts cultured in vitro showed enhanced Wnt1 expression with advancing differentiation, indicating a role of WNT1 in osteoblast function and bone development. Our finding that homozygous and heterozygous variants in WNT1 predispose to low-bone-mass phenotypes might advance the development of more effective therapeutic strategies for congenital forms of bone fragility, as well as for common forms of age-related osteoporosis.

Optic nerve atrophy and hypoplasia can be primary disorders or can result from trans-synaptic degeneration arising from cerebral visual impairment (CVI). Here we report six individuals with CVI and/or optic nerve abnormalities, born after an uneventful pregnancy and delivery, who have either de novo heterozygous missense mutations in NR2F1, also known as COUP-TFI, or deletions encompassing NR2F1. All affected individuals show mild to moderate intellectual impairment. NR2F1 encodes a nuclear receptor protein that regulates transcription. A reporter assay showed that missense mutations in the zinc-finger DNA-binding domain and the putative ligand-binding domain decrease NR2F1 transcriptional activity. These findings indicate that NR2F1 plays an important role in the neurodevelopment of the visual system and that its disruption can lead to optic atrophy with intellectual disability.