Cone Rod Dystrophy, Comprehensive Genetic Testing

The Comprehensive Genetic Test for Cone Rod Dystrophy utilizes next-generation sequencing (NGS) to examine 44 genes associated with inherited retinal dystrophies and cone rod degeneration.It is a targeted gene panel specifically designed to support accurate diagnosis, risk assessment, and prevention.

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The Comprehensive Genetic Test for Cone Rod Dystrophy is a specialized genetic test designed to evaluate genes associated with cone-rod dystrophies (CRDs), including the assessment of selected non-coding variants. It is intended for use in individuals with a clinical suspicion or established diagnosis of cone-rod dystrophies, a group of inherited retinal disorders. The comprehensive genetic test for cone-rod dystrophy serves as part of a broader retinal dystrophy testing approach and enables the identification of genetic alterations contributing to progressive visual impairment. By targeting genes known to be implicated in retinal degeneration, the comprehensive genetic test for cone-rod dystrophy supports a more precise characterization of the underlying molecular basis of the disease.

The comprehensive genetic test for cone-rod dystrophy includes genes involved in photoreceptor structure, function, and survival, such as ABCA4, GUCY2D, CRX, and RPGR. These genes play critical roles in phototransduction, retinoid cycling, and the maintenance of cone and rod photoreceptors. For example, ABCA4 is essential for retinoid transport in photoreceptor outer segments, while GUCY2D is involved in cyclic GMP synthesis, a key component of visual signal recovery. Disruption of these pathways leads to progressive retinal dysfunction and degeneration. The comprehensive genetic test for cone-rod dystrophy is indicated in individuals presenting with clinical features suggestive of cone-rod dystrophy or related inherited retinal disorders.

Cone rod dystrophies are characterized by primary dysfunction of cone photoreceptors, followed by secondary degeneration of rod cells. Early symptoms typically include reduced visual acuity, impaired color vision, photophobia, and decreased sensitivity in the central visual field. As the disease progresses, peripheral vision loss and nyctalopia may develop. Fundoscopic findings often reveal macular atrophy and pigmentary changes. The clinical presentation is heterogeneous, with variability in age of onset, severity, and rate of progression. Both nonsyndromic and syndromic forms exist, with associations reported in conditions such as Bardet-Biedl syndrome. Inheritance patterns may be autosomal dominant, autosomal recessive, or X-linked.

The purpose of the comprehensive genetic test for cone-rod dystrophy is to identify genetic variants associated with cone-rod dystrophy, enabling a more accurate molecular diagnosis. It contributes to improved disease classification, supports risk assessment, and enhances understanding of genotype–phenotype correlations. The identification of causative variants provides valuable insights into disease mechanisms and may inform patient stratification for clinical research and emerging therapeutic approaches. Additionally, it facilitates differentiation from other retinal dystrophies with overlapping clinical features.

A higher genetic risk is confirmed when pathogenic mutations are found in genes associated with cone-rod dystrophy. A lower risk may be inferred when no mutations are detected, though comprehensive clinical follow-up is still essential. The integration of genetic data with clinical findings and ophthalmologic evaluation is critical for precise diagnosis, prognosis, and long-term patient care.

The test is performed in a clinical laboratory accredited to ISO 15189 and certified by CLIA and CAP, ensuring the validity, accuracy and international recognition of the results.

Genes and Associated Phenotypes (Comprehensive Genetic Testing for Cone Rod Dystrophy)

Genetic testing for this gene is performed by taking a blood sample or by taking genetic material from inside the cheeks (with a swab), and can be done:

• By taking the sample at Diagnostiki Athinon. Book your appointment in real-time and purchase the test online
• By taking the sample at home. We send you all the necessary collection materials to your home via courier. The courier also returns the sample to the laboratory. Purchase the test online

Test Strengths

The test is characterized by the following strengths:

• The majority of X-linked retinitis pigmentosa (X-linked RP) cases are caused by mutations in the RPGR gene. A significant proportion of these variants occur within a specific region of the gene, the ORF15 exon (567 amino acids), which represents a mutational hotspot and accounts for approximately two-thirds of all pathogenic variants. The ORF15 exon contains a highly repetitive, purine-rich sequence, which is typically challenging for analysis using standard NGS-based methods. The present test utilizes a specialized assay that achieves complete coverage (>20x) with high mapping quality (mapping quality >20) across 100.0% of the targeted regions of the RPGR gene. Validation data demonstrated a high mean coverage of 139x for this gene. Therefore, this NGS panel is not expected to have significant limitations in detecting variants in the RPGR gene, including those located within the ORF15 exon.
• Analysis is performed in a CAP-accredited laboratory with the participation of CLIA-certified personnel.
• Advanced sequencing technologies, high-performance target enrichment methods, and precise bioinformatics pipelines ensure superior analytical performance.
• Gene panels are carefully designed to be clinically effective and scientifically validated.
• In some panels, the entire mitochondrial genome is included (see Panel Content section).
• Approximately 2,000 non-coding disease-causing variants are covered in the clinical-grade NGS assay (see Non-coding disease-causing variants covered by this panel in the Panel Content section).
• A rigorous variant classification system is applied.
• A systematic interpretation workflow, supported by proprietary software, enables accurate and traceable processing of NGS data.
• Comprehensive and clinically meaningful reports are provided.

Test Limitations

Specific technical and biological limitations apply:

• Genes with suboptimal coverage are marked with a number sign (#), while those with partial or complete segmental duplications overlapping UCSC pseudogene regions are marked with an asterisk (*).
• A gene is considered to have suboptimal coverage when more than 90% of its target nucleotides are not covered at a depth of greater than 20x with mapping quality score (MQ > 20) reads. In such cases, detection of variants may be limited.

This test does not detect:

• Complex inversions
• Gene conversions
• Balanced translocations
• Repeat expansion disorders, unless otherwise specified
• Non-coding variants beyond ±20 base pairs from the exon–intron boundary, unless otherwise indicated
• The complete mitochondrial genome in all panels (see Panel Content section

This test may not reliably detect:

• Low-level mosaicism in nuclear genes (variants with a minor allele fraction of ~14.6% are detected with 90% probability)
• Stretches of mononucleotide repeats
• Low-level heteroplasmy in mtDNA (>90% detected at 5% level)
• Insertions or deletions larger than 50 bp
• Single-exon deletions or duplications
• Variants within pseudogene regions or duplicated genomic segments
• Disease-causing mtDNA variants not present in blood; in such cases, post-mitotic tissue (e.g., skeletal muscle) may be required to establish a molecular diagnosis

For further details, please refer to the Bioinformatics & Performance section.

Bioinformatics

The target region for each gene encompasses the coding exons as well as ±20 base pairs from the exon–intron boundaries. In addition, the panel may include non-coding and regulatory variants, as listed in the “Non-coding variants covered by the panel” section. Specific regions of certain gene(s) may be excluded from analysis if noted in the “Test limitations” section. When the test includes the mitochondrial genome, the target region also comprises the complete set of mitochondrial genes.

Sequencing data is processed through a proprietary analysis and annotation pipeline that integrates state-of-the-art algorithms and industry-standard software solutions. Multiple quality control steps are embedded throughout the workflow to ensure the accuracy, validity, and consistency of the results. The pipeline is optimized to maximize sensitivity without compromising specificity.

For variant interpretation, the system incorporates data from multiple reference populations and mutation databases, including (but not limited to) the 1000 Genomes Project, gnomAD, ClinVar, and HGMD. In silico tools such as SIFT, PolyPhen, and MutationTaster are also applied to support the classification of missense variants.

All findings are summarized in a clear and detailed clinical report. This report includes sequencing quality metrics, gene-level coverage information, and highlights any regions where coverage is limited (<20x for nuclear genes and <1000x for mtDNA, when applicable). This ensures transparency and supports accurate clinical decision-making.

Test Performace

The genes included in this panel have been carefully selected based on evidence from scientific literature, mutation databases, and accumulated clinical expertise.

All panels are derived from a high-quality, clinical-grade NGS assay. Details regarding the detection of different types of genetic alterations are provided in the sequencing and detection performance table (see Table).

The assay has been validated for several sample types, including EDTA blood, isolated DNA (excluding DNA from formalin-fixed paraffin-embedded tissue), saliva, and dried blood spots (filter cards). These sample types were chosen to maximize the probability of obtaining high-quality DNA. The diagnostic yield may vary depending on the assay performed, the referring healthcare professional, the hospital, and the country in which the test is conducted. The Plus Analysis further enhances the chance of reaching a genetic diagnosis, as large deletions and duplications cannot be detected by sequencing alone. This approach integrates both sequencing and deletion/duplication (copy number variant, CNV) analysis.

The performance metrics shown below are based on initial validation studies at an accredited clinical laboratory, with equivalent results confirmed across additional laboratory sites.

Performance of a high-quality, clinical-grade NGS sequencing assay for panels

Coverage metrics

Performance of Mitochondrial Sequencing Assay

Mitochondrial assay coverage metrics

Comprehensive clinical reports are delivered, designed to provide healthcare professionals with meaningful and actionable insights. Clinical interpretation requires a solid understanding of both clinical genetics and genetic principles. Reports are prepared by teams of molecular geneticists, medical experts, and other highly experienced professionals, who evaluate identified variants in the context of the patient’s phenotypic information provided in the requisition form.

The goal is to ensure that reports are clinically valuable and understandable to all healthcare providers, regardless of their level of training in genetics and genetics-related fields. Variant classification forms the foundation of clinical interpretation and guides patient management decisions. Classifications follow the ACMG 2015 guidelines. Sequence and copy number variants classified as pathogenic, likely pathogenic, or of uncertain significance (VUS) are confirmed using bidirectional Sanger sequencing or orthogonal methods such as qPCR/ddPCR whenever NGS quality metrics do not meet strict thresholds for a true positive call.

Each report includes tables for sequence and copy number variants, presenting essential information such as genomic coordinates, HGVS nomenclature, zygosity, allele frequencies, in silico predictions, phenotypic correlations, and variant classification. In addition, detailed descriptions are provided for the variant, the gene, and the related phenotypes, covering the gene’s role in human disease, mutation spectrum, variation in population cohorts, and links to associated conditions. References, abstracts, and variant databases are also cited, enabling healthcare providers to further evaluate reported findings if desired.

Reports are structured into two sections: primary findings and additional findings.

The primary findings consist of variants known to cause disease or rare variants that may explain the patient’s phenotype. The conclusion summarizes all available evidence and presents the rationale for classification.

Additional findings include variants that are not currently sufficient to explain the patient’s phenotype, based on existing knowledge. Examples include heterozygous variants in autosomal recessive genes, VUS in autosomal dominant genes (when a pathogenic or likely pathogenic variant already explains the phenotype), or risk alleles in genes included in the panel.

The identification of pathogenic or likely pathogenic variants in dominant disorders, or biallelic variants in recessive disorders, is considered molecular confirmation of the clinical diagnosis. In such cases, family testing may assist in risk assessment. Variants of uncertain significance are not recommended for patient management or family risk stratification. Genetic counseling is strongly advised.

Interpretation teams routinely analyze millions of variants across thousands of cases of rare diseases. With each case, internal knowledge of variant–phenotype relationships is enriched, enabling continuous refinement of classifications. As new evidence becomes available, previously reported variants may be reclassified. If a variant initially reported as a primary or secondary finding is later reclassified (for example, from VUS to pathogenic/likely pathogenic, or from pathogenic/likely pathogenic to VUS, likely benign, or benign), an updated statement is issued to the original ordering healthcare provider at no additional cost.