Una nueva mutación en el gen CNGA3 causante de acromatopsia incompleta

Case reportA 56-year old male was diagnosed with incomplete achromatopsia. His molecular genetic analysis showed two heterozygous mutations in the CNGA3 gene associated with autosomal recessive achromatopsia.One of them, c.1495C>T, has not been previously reported in achromatopsia.DiscussionAchromatopsia is a congenital autosomal recessive retinal disorder. Mutations in the CNGA3 gene, located at chromosome positions 2q11, accounts for 5-25% of patients affected with this disorder. The vast majority of mutations are missense. This discovery confirms the clinical diagnosis and it allows us to provide genetic counselling.     

Novel causative variants in patients with achromatopsia

ABSTRACT

Purpose: To report five novel genetic variants in seven unrelated consanguineous families with achromatopsia (ACHM).

Methods: Patients were examined with multimodal retinal imaging and full-field electroretinography (ffERG). Genetic testing was conducted using next-generation sequencing (NGS).

Results: Three novel homozygous variants were detected in CNGA3: a missense c.967G > C (p.Ala323Pro) variant was detected in exon 8 (one patient), a splice site variant c.101 + 1G > A in intron 2 (three patients), and a splice site variant c.395 + 1G > T in intron 4(one patient). Another two novel variants were found in PDE6C: a homozygous missense variant c.1899C > A (p.His633Gln) in exon 15 (one patient) and a homozygous splice site variant c.1072-1G > C in intron 7 (one patient). Mutation segregation assessment was possible in 3 of the 7 families. All patients had nonrecordable ffERG 30-Hz flicker responses, reduced single-flash cone responses but preserved rod responses. Patients presented with variable degrees of foveal outer retinal layer loss and variable patterns of foveal hyperautofluorescence.

Conclusions: These novel variants expand the genotypes associated with ACHM and may help in future therapy development for ACHM.     

Incomplete penetrance of CRX gene for autosomal dominant form of cone-rod dystrophy

Purpose: Cone-rod dystrophy (CRD) is an inherited retinal dystrophy that is transmitted via different modes of inheritance. Mutations in more than 30 genes have been identified to cause the disease. We aimed to investigate the genetic agents of two unrelated cone-rod dystrophy affected Iranian families with autosomal recessive inheritance patterns.

Methods: Whole-exome sequencing (WES) was performed for identification of the disease-causing mutations in the probands of both families. The candidate mutations were further confirmed by Sanger sequencing. Samples from five available members of each family were then sequenced for the mutations present in the probands. Comprehensive ocular examinations for all members of the families carrying the mutations were completed by ophthalmologists.

Results: We identified a novel premature stop codon c.310C>T in CRX gene in heterozygote form in two symptomatic and two non-symptomatic members of one family (family-A), and a known CRX mutation c.122G>A in homozygote form in another (family B). c.122G>A has been reported to cause late-onset autosomal dominant form of the disease in previous studies. However, the middle-aged heterozygous carriers of the mutation in this family showed normal phenotype.

Conclusion: The CRX gene has been previously linked to the autosomal dominant form of cone-rod dystrophy. We report incomplete penetrance of CRX gene for autosomal dominant form of the disease. Incomplete penetrance of the mutations may be partly caused by the influence of other genes in the complex genetic network underlying retinal regulation.     

Compound heterozygous CNGA3 mutations (R436W, L633P) in a Japanese patient with congenital achromatopsia

Congenital achromatopsia is a stationary retinal disorder with autosomal recessive inheritance that is characterized by loss of color discrimination, low visual acuity, photophobia, and nystagmus. This disorder has been shown to be associated with CNGA3, CNGB3, and GNAT2 mutations, and the frequency of mutations in the CNGA3 gene (encoding alpha subunit of the cone-specific cGMP-gated cation channel) was 23-33% in European populations. The aim of this study was to test the hypothesis that CNGA3 mutations are also responsible for congenital achromatopsia in Japanese patients. DNA from venous blood samples from a total of 14 patients from 13 Japanese pedigrees was prepared. Mutation screening of the CNGA3 gene was performed using direct sequencing and PCR-single-strand conformation polymorphism analysis. Compound heterozygous missense mutations (p.R436W and p.L633P, the latter of which was novel) were identified in one patient only, a 22-year-old female. Neither of these two mutations was found in 150 Japanese control individuals. The patient's parents and sister carried one of these mutations each but were not affected. No mutations in the CNGB3 or GNAT2 genes were identified in the patient. Clinically, best-corrected visual acuity was 0.1 in both eyes. No specific findings were obtained in funduscopy. Optical coherence topography revealed a normal foveal thickness but a 20% decrease in parafoveal thickness. Ganzfeld full-field electroretinograms (ERGs) showed normal responses in rod and mixed rod-plus-cone ERGs but no response in cone or 30-Hz flicker ERGs. Spectral sensitivity on a white background revealed a curve with only one peak at around 500 nm, which fits the absorption spectrum of human rhodopsin. L633, conserved among vertebrate orthologs of human CNGA3, is a hydrophobic residue forming part of the carboxy-terminal leucine zipper (CLZ) domain, which is functionally important in the mediation of intracellular interactions. To our knowledge, this is the first report of a Japanese complete achromat with CNGA3 mutations, and of any patient with a missense mutation within the CLZ domain. The outcome suggests low frequency (7%, 1/14) of CNGA3 mutations in Japanese patients.     

Targeted next-generation sequencing reveals that a compound heterozygous mutation in phosphodiesterase 6a gene leads to retinitis pigmentosa in a Chinese family

Purpose: Retinitis pigmentosa (RP) is a genetically heterogeneous disease with over 70 causative genes identified to date. However, approximately 40% of RP cases remain genetically unsolved, suggesting that many novel disease-causing mutations are yet to be identified. The purpose of this study is to identify the causative mutations of a Chinese RP family.

Methods: Targeted next-generation sequencing (NGS) for a total of 163 genes which involved in inherited retinal disorders were used to screen the possible causative mutations. Sanger sequencing was used to verify the mutations.

Results: As results, we identified two heterozygous mutations: a splicing site mutation c.1407 + 1G>C and a nonsense mutation c. 1957C>T (p.R653X) in phosphodiesterase 6A (PDE6A) gene in the RP patient. These two mutations are inherited from his father and mother, respectively. Furthermore, these mutations are unique in our in-house database and are rare in human genome databases, implicating that these two mutations are pathological.

Conclusion: By using targeted NGS method, we identified a compound heterozygous mutation in PDE6A gene that is associated with RP in a Chinese family.     

CNGB3 mutations cause severe rod dysfunction

Purpose: Congenital achromatopsia or rod monochromatism is a rare autosomal recessive condition defined by a severe loss of cone photoreceptor function in which rods purportedly retain normal or near-to-normal function. This report describes the results of electroretinography in two siblings with CNGB3-associated achromatopsia.

Methods: Full field light- and dark-adapted electroretinograms (ERGs) were recorded using standard protocols detailed by the International Society for Clinical Electrophysiology of Vision (ISCEV). We also examined rod-mediated ERGs using series of stimuli that varied over a 6 log unit range of retinal illuminances (?1.9–3.5 log scotopic trolands).

Results: Dark-adapted ERGs in achromatopsia patients exhibited severely reduced b-wave amplitudes with abnormal b:a ratios (1.3 and 0.6). In comparison, the reduction in a-wave amplitude was less marked. The rod-mediated ERG took on an electronegative appearance at high-stimulus illuminances.

Conclusion: Although the defect that causes achromatopsia is primarily in the cone photoreceptors, our results reveal an accompanying disruption of rod function that is more severe than has previously been reported. The differential effects on the b-wave relative to the a-wave points to an inner-retinal locus for the disruption of rod function in these patients.     

Slowly progressive retinitis pigmentosa caused by two novel mutations in the MAK gene

Background: The growing number of clinical trials currently underway for inherited retinal diseases has highlighted the importance of achieving a molecular diagnosis for all new cases presenting to hospital eye services. The male germ cell-associated kinase (MAK) gene encodes a cilium-associated protein selectively expressed in the retina and testis, and has recently been implicated in autosomal recessive retinitis pigmentosa (RP). Whole exome sequencing has previously identified a homozygous Alu insertion in probands with recessive RP and nonsense and missense mutations have also been reported.

Materials and methods: Here we describe two novel mutations in different alleles of the MAK gene in a 75-year-old British female, who had a clinical diagnosis of RP () with onset in the fourth decade and no relevant family history. The mutations were established through next generation sequencing of a panel of 111 genes associated with RP and RP-like phenotypes.

Results: Two novel null mutations were identified within the MAK gene. The first c.1195_1196delAC p.(Thr399fs), was a two base-pair deletion creating a frame-shift in exon 9 predicted to result in nonsense-mediated decay. The second, c.279-2A>G, involved the splice acceptor consensus site upstream of exon 4, predicted to lead to aberrant splicing.

Conclusions: The natural history of this individual’s RP is consistent with previously described MAK mutations, being significantly milder than that associated with other photoreceptor ciliopathies. We suggest inclusion of MAK as part of wider genetic testing in all individuals presenting with RP.     

Novel compound heterozygous mutation in the POC1B gene underlie peripheral cone dystrophy in a Chinese family

Purpose: To describe the clinical characteristics of a Chinese family with peripheral cone dystrophy (PCD) and identify the gene mutations causing PCD.

Methods: The Chinese PCD pedigree underwent comprehensive ophthalmic examinations, including visual acuity, slit lamp examination, fundoscopy, visual field examination, autofluorescence, fluorescence fundus angiography and indocyanine green angiography, full-field electroretinograms, and spectral-domain optical coherence tomography. The targeted next-generation sequencing of COD or cone-rod dystrophy (CORD) genes was used to identify the causative mutation.

Result: The fundus characteristics of the Chinese patient were consistent with PCD. The novel compound heterozygous mutation, c.1354C>T and c.710A>G, in POC1B was identified in the patient, the mutations were segregated with the PCD phenotype in the family and were absent from ethnically matched control chromosomes. Prediction analysis demonstrated the novel missense mutation, POC1B c.710A>G, might be damaging.

Conclusions: PCD was a type of COD or CORD and the novel compound heterozygous mutation in POC1B was responsible for PCD phenotype in the family.     

Case of cone dystrophy with normal fundus appearance associated with biallelic POC1B variants

Background: Biallelic variants of POC1B were recently reported to cause autosomal recessive non-syndromic cone dystrophy. However, the number of studies supporting this is limited, and the clinical phenotypes of cone dystrophy have not been definitively determined. The purpose of this study was to report the phenotype of a case of POC1B-associated cone dystrophy.

Materials and methods: The medical chart of one case diagnosed with cone dystrophy was reviewed.

Results: The patient was a 20-year-old Japanese man whose chief complaint was a progressive decrease in his central vision. His decimal best-corrected visual acuity was 0.2 for the right and 0.3 for the left. Fundus examinations showed no abnormalities. The photopic electroretinograms were nonrecordable, but the scotopic electroretinograms were within normal limits. Optical coherence tomography detected a blurry line in the region of the external limiting membrane and ellipsoid zone. Adaptive optics images showed sparsely distributed cone cells around the fovea. The patient was initially diagnosed with incomplete achromatopsia. Whole-exome sequence with targeted analysis identified new compound heterozygous mutations of c.G1355A (p R452Q) and c.C987A (pY329X) in the POC1B gene. The patient was then diagnosed with cone dystrophy.

Conclusions: The cone dystrophy associated with POC1B variants has features similar to achromatopsia, and genetic analyses is useful in discriminating these two diseases.