Complement System Disorder Panel

ICD codes
Commonly used ICD-10 code(s) when ordering the Complement System Disorder Panel

ICD-10	Disease
D84.1	Defects in the complement system
D58.8	Atypical hemolytic uremic syndrome (aHUS)

Sample requirements:

• EDTA blood, min. 1 ml
• Purified DNA, min. 3μg
• Saliva (Oragene DNA OG-500 kit)

Label the sample tube with your patient’s name, date of birth and the date of sample collection. Note that we do not accept DNA samples isolated from formalin-fixed paraffin-embedded (FFPE) tissue.

The complement system disorders are a group of primary immunodeficiencies resulting in absent or suboptimal function of complement system proteins. In general, deficiencies of the classical and alternative complement pathways are rather rare, while deficiencies of the proteins in the mannose-binding lectin (MBL) pathway are more common. C2-deficiency is the most common classical pathway complement deficiency in many populations and its prevalence is estimated to be 1:10,000. There are two main categories of complement system disorders, originating from mutations in genes encoding proteins either inhibiting or activating the complement system and thus resulting in overactive or underactive responses, respectively. Complement system disorders predispose patients for example to (Neisserial) infections, atypical hemolytic uremic syndrome, age-related macular degeneration, systemic lupus erythematosus SLE and preeclampsia. This panel covers genes associated with autosomal recessive, autosomal dominant as well as X-linked forms of complement deficiencies. In addition to complement system disorders, this panel has the ability to diagnose other conditions, such as primary ciliary dyskinesia, that is characterized by recurrent respiratory infections. OTHER INFORMATION ON CFH AND CFHR1-4 GENES CFH gene have multiple exons that are pseudogenic (exons 8-9, 11, 21-23). Moreover, the function of CFHR1, CFHR2, CFHR3 and CFHR4 has not been established and they are highly homologues (see below chapter ‘CFHR1-4 genes‘). Genetics of atypic hemolytic uremic syndrome (aHUS) Mutations in CFH account for approximately 30% of the cases, CD46 (also known as MCP) 12%, CFI 5%-10%, C3 5%, THBD 3%-5%. In early onset aHUS, disease manifesting before age 1 year, mutations in DGKE explain 27% of the cases. Inheritance mode is difficult to determine for most of the genes related to aHUS due to low penetrance but the predisposition to disease is commonly autosomal dominant. In the ClinVar mutation database, vast majority of the novel disease associated variants in major aHUS genes such as CFH, CD46, CFI and C3 are classified as risk factors but not pathogenic or likely pathogenic. In most of the families where probands has novel variant in aHUS genes, some of the unaffected parents or other family members carry the same variant. However, one study showed fully penetrant recessive aHUS relating to homozygous CFH mutations in a large Bedouin pedigree with 10 aHUS cases (PubMed: 9811382). In addition, deletions in CFHR1 to CFHR5 genes have shown to increase slightly a risk for aHUS. The Newcastle cohort of 66 aHUS patients showed deletions in CFHR1 were more frequent in aHUS patients compared to controls (zero copies 10% vs. 2%; one copy 35%vs 9% and two copies 55% vs.89%) indicating odds ratios (OR) 6.3 for homozygous deletion and 3.8 for heterozygous. Absence of CFHR1 and/or CFHR3 was shown to contribute to the defective regulation of complement activation on cell and tissue surfaces (PubMed: 17367211). Hofer et al evaluated 116 aHUS patients and 118 control. Homozygous deletion in CFHR1 was detected in 32% of the patients with aHUS tested and in 2.5% of controls. CFH antibodies were present in 25% of the patients and none of the controls. CFH antibodies were detected in 82% of patients with homozygous CFHR1 deletion and in 6% of patients without. CFH antibody-positive patients with aHUS showed a significantly lower platelet nadir at disease onset and significantly less frequent involvement of the central nervous system than did antibody-negative patients. Antibody-positive patients also received plasma therapy more often (PubMed: 23243267). It is noteworthy that disease activity appears to correlate better with immune complex titers than FHAA titers (PubMed: 22922817). In 2016, Challis et al described novel CFH/CFHR3 hybrid gene in a patient with aHUS secondary to a de novo 6.3-kb deletion that arose through microhomology-mediated end joining rather than nonallelic homologous recombination. Secreted protein product lacked the recognition domain of factor H and exhibits impaired cell surface complement regulation. The fact that the formation of this hybrid gene arose as a de novo event suggests that this cluster is a dynamic area of the genome in which additional genomic disorders may arise (PubMed: 26490391). CFHR1-4 genes In August 25 2017, Blueprint Genetics excluded CFHR1, CFHR2, CFHR3, CFHR4 genes from three diagnostic NGS panels including Primary Immunodeficiency Panel, Complement System Disorder Panel and Hemolytic Uremic Syndrome Panel. This was done due to extensive homology between these genes making it difficult or even impossible to determine copy number reliably from these genes with short read length NGS methods. Moreover, homozygous or heterozygous deletions involving these gene are common in population even though enriched in patients with aHUS. By relying on three estimates: 1) higher end of aHUS prevalence (9 per 1,000,000), 2) frequency of homozygous CFHR1 deletion (2%) and 3) assuming that all aHUS cases would be caused by this defect (over estimating the effect), we are left with the fact that 99.95% of the individuals with homozygous CFHR1 deletion will never get aHUS. Thus, we consider releasing copy number from CFHR1-4 genes may be misleading, and is not considered helpful in clinical practice. We believe that fusion genes between CFH and CFHR1-4 may be the mechanism that explain the association between CFHR1-4 gene deletions and aHUS. However, this kind of alterations are not reliably detected by targeted sequencing approaches.

Genes in the Complement System Disorder Panel and their clinical significance

Non-coding variants covered by the panel

The strengths of this test include:

• CAP and ISO-15189 accreditations covering all operations at Blueprint Genetics including all Whole Exome Sequencing, NGS panels and confirmatory testing
• CLIA-certified personnel performing clinical testing in a CLIA-certified laboratory
• Powerful sequencing technologies, advanced target enrichment methods and precision bioinformatics pipelines ensure superior analytical performance
• Careful construction of clinically effective and scientifically justified gene panels
• Our Nucleus online portal providing transparent and easy access to quality and performance data at the patient level
• Our publically available analytic validation demonstrating complete details of test performance
• ~1,500 non-coding disease causing variants in Blueprint WES assay (please see below ‘Non-coding disease causing variants covered by this panel’)
• Our rigorous variant classification based on modified ACMG variant classification scheme
• Our systematic clinical interpretation workflow using proprietary software enabling accurate and traceable processing of NGS data
• Our comprehensive clinical statements

Test limitations The following exons are not included in the panel as they are not sufficiently covered with high quality sequence reads: *PPA2* (11, 12). Genes with partial, or whole gene, segmental duplications in the human genome are marked with an asterisk if they overlap with the UCSC pseudogene regions. The technology may have limited sensitivity to detect variants in genes marked with these symbols (please see the Panel content table above).

This test does not detect the following:

• Complex inversions
• Gene conversions
• Balanced translocations
• Mitochondrial DNA variants
• Repeat expansion disorders unless specifically mentioned
• Non-coding variants deeper than ±20 base pairs from exon-intron boundary unless otherwise indicated (please see above Panel Content / non-coding variants covered by the panel).

This test may not reliably detect the following:

• Low level mosaicism
• Stretches of mononucleotide repeats
• Indels larger than 50bp
• Single exon deletions or duplications
• Variants within pseudogene regions/duplicated segments

The sensitivity of this test may be reduced if DNA is extracted by a laboratory other than GHC Genetics.

For additional information, please refer to the Test performance section and see our Analytic Validation.

The GHC Genetics panel covers classical genes associated with Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia (CPVT), cardiac arrest underlying cardiac condition, cardiac arrest cause unspecified, syncope and collapse, abnormal ECG, Long QT syndrome, arrhythmogenic right ventricular cardiomyopathy (ARVC) and Short QT syndrome. The genes on the panel have been carefully selected based on scientific literature, mutation databases and our experience.

Our panels are sliced from our high-quality whole exome sequencing data. Please see our sequencing and detection performance table for different types of alterations at the whole exome level (Table).

Assays have been validated for different starting materials including EDTA-blood, isolated DNA (no FFPE), saliva and dry blood spots (filter card) and all provide high-quality results. The diagnostic yield varies substantially depending on the assay used, referring healthcare professional, hospital and country. Blueprint Genetics’ Plus Analysis (Seq+Del/Dup) maximizes the chance to find a molecular genetic diagnosis for your patient although Sequence Analysis or Del/Dup Analysis may be a cost-effective first line test if your patient’s phenotype is suggestive of a specific mutation type.

Performance of GHC Genetics Whole Exome Sequencing (WES) assay.
All individual panels are sliced from WES data.

Our mission is to improve the quality of the sequencing process and each modification is followed by our standardized validation process. Detection of Del/Dup of several genes is by MLPA analysis (MS Holland). All genes are performed by CNV analysis through the genome depending on exon size, sequencing coverage and sequence content. We have validated the assays for different starting materials including isolated DNA from EDTA blood that provide high-quality results.

The sequencing data generated in our laboratory is analysed by our bioinformatic pipeline, integrating state-of-the art algorithms and industry-standard software solutions. We use also JSI medical systems software for sequencing data analysis. JSI medical systems is a certified system offering sophisticated bioinformatic software solutions covering a wide field of sequencing techniques.

Incorporation of rigorous quality control steps throughout the workflow of the pipeline ensures the consistency, validity and accuracy of results.

Every pathogenic or probably pathogenic variant is confirmed by the Sanger sequencing method. Sanger sequencing is also used occasionally with other variants reported in the statement. In the case of variant of uncertain significance (VUS) we do not recommend risk stratification based on the genetic finding. The analysis of detected variants was performed on the basis of the reference database of polymorphisms and international mutation databases: UMD, LOVD and ClinVar.

The consequence of variants in coding and splice regions are estimated using Alamut software. The Alamut database contains more than 28000 coding genes, non-protein coding genes and pseudogenes. This database (shared with the high throughput annotation engine for NGS data, Alamut Batch) is frequently updated. Information comes from different public databases such as NCBI, EBI, and UCSC, as well as other sources including gnomAD, ESP, Cosmic, ClinVar, or HGMD and CentoMD (for those a separate subscription from Qiagen/Biobase and Centogene respectively is required). Alamut Visual finds information about nucleotide conservation data through many vertebrates’ species, with the phastCons and phyloP scores, amino acid conservation data through orthologue alignments and information on protein domains.

Moreover, we integrate several missense variant pathogenicity prediction tools and algorithms such as SIFT, PolyPhen, AlignGVGD or MutationTaster. It also offers a window dedicated to the in silico study of variants’ effect on RNA splicing, allowing the assessment of their potential impact on splice junctions and visualization of cryptic or de novo splice sites. Impact on splicing regulation is also assessed.

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Clinical interpretation

At GHC Genetics our geneticists and clinicians, who together evaluate the results from the sequence analysis pipeline in the context of phenotype information provided in the requisition form, prepare the clinical report. We recommend an interpretation of the findings of this molecular genetic analysis, including subsequent oncological consultation for the patient in the context of genetic counselling for the patient.

We strive to continuously monitor current genetic literature identifying new relevant information and findings and adapting them to our diagnostics. This enables relevant novel discoveries to be rapidly translated and adopted into our ongoing diagnostics development without delay. The undertaking of such comprehensive due diligence ensures that our diagnostic panels and clinical statements are the most up-to-date on the market.

Variant classification is the corner stone of clinical interpretation and resulting patient management decisions. Minor modifications were made to increase reproducibility of the variant classification and improve the clinical validity of the report. Our experience with tens of thousands of clinical cases analysed at our laboratories enables us to further develop the industry standard.

The final step in the analysis of sequence variants is confirmation of variants classified as pathogenic or likely pathogenic using bi-directional Sanger sequencing. Variant(s) fulfilling all of the following criteria are not Sanger confirmed: 1) the variant quality score is above the internal threshold for a true positive call, 2) an unambiguous IGV in-line with the variant call and 3) previous Sanger confirmation of the same variant three times at GHC Genetics. Reported variants of uncertain significance (VUS) are confirmed with bi-directional Sanger sequencing only if the quality score is below our internally defined quality score for true positive call. Reported copy number variations with a size >10 exons are confirmed by orthogonal methods such as qPCR if the specific CNV has been seen less than three times at GHC Genetics.

Our clinical statement includes tables for sequencing and copy number variants that include basic variant information (genomic coordinates, HGVS nomenclature, zygosity, allele frequencies, in silico predictions, OMIM phenotypes and classification of the variant). In addition, the statement includes detailed descriptions of the variant, gene and phenotype(s) including the role of the specific gene in human disease, the mutation profile, information about the gene’s variation in population cohorts and detailed information about related phenotypes. We also provide links to the references used, and mutation databases to help our customers further evaluate the reported findings if desired. The conclusion summarizes all of the existing information and provides our rationale for the classification of the variant.

Identification of pathogenic or likely pathogenic variants in dominant disorders or their combinations in different alleles in recessive disorders are considered molecular confirmation of the clinical diagnosis. In these cases, family member testing can be used for risk stratification within the family. In the case of variants of uncertain significance (VUS), we do not recommend family member risk stratification based on the VUS result. Furthermore, in the case of VUS, we do not recommend the use of genetic information in patient management or genetic counselling.

Our Clinical interpretation team analyses millions of variants from thousands of individuals with rare diseases. Thus, our database, and our understanding of variants and related phenotypes, is growing by leaps and bounds. Our laboratories are therefore well positioned to re-classify previously reported variants as new information becomes available. If a variant previously reported by GHC Genetics is re-classified, our laboratories will issue a follow-up statement to the original ordering health care provider at no additional cost.