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Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2018.

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Educational Materials — Genetic Testing: Current Approaches

, MD and , PhD.

Author Information

Initial Posting: ; Last Update: February 12, 2018.

Note: This information, provided by the editors of GeneReviews, is intended both for individuals who have limited experience with comprehensive genetic testing (see Introductory Information) and for clinicians who routinely order comprehensive genetic testing (see Detailed Information). – The Editors

Table of Contents
Introductory informationMultigene panels
Clinical exome sequencing
Clinical genome sequencing
Chromosomal microarray (CMA)
Detailed information for clinicians ordering genetic tests
Multigene panels: FAQsWhat variables affect the diagnostic sensitivity of multigene panels?
What kinds of multigene panels are available?
How do off-the-shelf multigene panels compare with custom multigene panels?
What are the disadvantages of custom multigene panels compared to off-the-shelf multigene panels?
Comprehensive
genomic
testing
Clinical
exome
sequencing:
FAQs
When does exome sequencing provide the best test value?
What types of disorders can be reliably diagnosed by exome sequencing?
What types of genetic alterations cannot be reliably identified by exome sequencing?
What types of disorders are not reliably identified by exome sequencing?
What variables affect the sensitivity of exome sequencing?
Clinical
genome
sequencing:
FAQs
When does genome sequencing provide the best test value?
Benefits and limitations of genome sequencing
What types of disorders can be reliably diagnosed by genome sequencing?
What types of genetic alterations are not reliably identified by genome sequencing?
What types of disorders are not reliably identified by genome sequencing?
Comparison:
multigene
panels &
comprehensive
genomic
testing
Advantages of multigene panels over exome sequencing and genome sequencing
Advantages of exome sequencing or genome sequencing over multigene panels
Resources
for genetics
professionals
DatabasesPopulation databases
Curated variant databases
Tables (pdf)Table 1. Genetic Disorders Caused by Imprinting Errors and Uniparental Disomy
Table 2. Genes and Related Disorders Caused by Nucleotide Repeat Expansions and Contractions
Table 3. Genes and Related Disorders with Highly Homologous Gene Family Members and Pseudogene(s)
ReferencesLiterature Cited
Suggested Reading

Introductory Information

This discussion addresses clinical tests available through CLIA-certified laboratories in the United States. Research testing is not discussed.

Multigene Panels

Many inherited disorders and phenotypes are genetically heterogeneous – that is, pathogenic variants in more than one gene can cause one phenotype (e.g., dilated cardiomyopathy, ataxia, hereditary hearing loss and deafness) or one genetic disorder (e.g., Noonan syndrome). Prior to the development of massively parallel sequencing (also known as next-generation sequencing), the only cost-effective way to test more than one gene was serial single-gene testing (i.e., complete testing of one gene that might account for the phenotype before proceeding to testing of the next gene) ‒ an expensive and time-consuming approach with a potentially low yield. In the past ten years, improvements in massively parallel sequencing techniques have led to the development and widespread clinical use of multigene panels, which allow simultaneous testing of two to more than 150 genes. The methods used in multigene panels may include sequence analysis, deletion/duplication analysis, and/or other non-sequencing-based tests.

There are two types of multigene panels:

  • Off the shelf. These are designed by a laboratory to include genes commonly associated with a broad phenotype (e.g., cardiomyopathy, ataxia, intellectual disability) or a recognizable syndrome with genetic heterogeneity (e.g., Noonan syndrome).
  • Custom designed. These include genes selected by a clinician for analysis by clinical sequencing. Results for each gene on the custom multigene panel are reported to the ordering clinician, whereas the results from the remaining genes sequenced (but not requested by the clinician) are not analyzed or included in the final laboratory report. Custom multigene panels offered by some reference laboratories are marketed under names such as XomeDxSlice® and ExomeNext-Select®.

Comprehensive Genomic Testing

Clinical Exome Sequencing

The human exome includes all coding nuclear DNA sequences, approximately 180,000 exons that are transcribed into mature RNA. (Note that mitochondrial DNA is not included in the exome.) Comprising only 1%-2% of the human genome, the exome nonetheless contains the majority of currently recognized disease-causing variants.

Exome sequencing is a laboratory test designed to identify and analyze the sequence of all protein-coding nuclear genes in the genome. Approximately 95% of the exome can be sequenced with currently available techniques. The diagnostic utility of exome sequencing has consistently been 20%-30% (i.e., a diagnosis is identified in 20%-30% of individuals who were previously undiagnosed but had features suggestive of a genetic condition) [Gahl et al 2012, Lazaridis et al 2016].

In the past five years, exome sequencing has increasingly become clinically available because:

  • Continuous improvements in massively parallel sequencing and bioinformatics tools for data analysis have lowered the cost and decreased the turn-around time;
  • Reports of clinically actionable results have led to improved coverage by medical insurance [Lazaridis et al 2016].

Clinical Genome Sequencing

The human genome includes all coding and noncoding nuclear and mitochondrial DNA sequences. Nuclear DNA encodes most of the more than 20,000 genes in humans; mitochondrial DNA encodes 37 genes. Most of the more than 3.2 million base pairs that comprise the human genome are repetitive DNA or noncoding sequences – including noncoding RNAs, variants in which have been attributed to specific inherited disorders.

Genome sequencing is a laboratory test designed to identify and analyze the sequence of all coding and noncoding nuclear DNA. Mitochondrial DNA is part of the genome; however, mitochondrial sequencing is often ordered as a separate laboratory test.

Genome sequencing continues to be significantly more costly than exome sequencing because of the high cost of data analysis. However, the diagnostic utility (20%-30%) is roughly the same for the two test methods: although genome sequencing can identify variants outside of the coding regions, determination of pathogenicity of these variants is often not possible. Therefore, most confirmed pathogenic variants identified by genome sequencing are within exons [Taylor et al 2015].

Chromosomal Microarray

A chromosomal microarray (CMA) is a molecular genetic test used to detect copy number variants (CNVs); CNVs are deletions (loss) or duplications (gain) of chromosome material that range in size from approximately one kilobase (kb) to multiple megabases (Mb), with the largest CNVs resulting in a loss or gain of an entire chromosome. Depending on the size and genomic location of a CNV, the deletion or duplication may contain zero, one, or many genes. CNVs may be benign, pathogenic, or of uncertain clinical significance.

The most common types of CMA are oligonucleotide array comparative genomic hybridization (oligo aCGH), single-nucleotide polymorphism genotyping array (SNP array), and oligo aCGH / SNP combination array. CMA can be designed to identify deletions and duplications across the genome or in a targeted region(s) of the genome.

CMA is more sensitive at detecting CNVs than karyotype analysis, which has largely been supplanted by CMA. High-resolution karyotype analysis can detect deletions as small as 3-5 Mb and duplications larger than ~5 Mb, whereas most CMA can detect CNVs as small as 100 kb. Oligo aCGH arrays, specifically, can be designed to detect CNVs as small as a single exon.

CMA has been available as a clinical diagnostic test since 2004 and is recommended as a first-line test for individuals with developmental delay, intellectual disability, multiple congenital anomalies, and/or autism spectrum disorder. For these disorders, CMA has a diagnostic yield of 15%-20%, compared to the 3% yield of a traditional karyotype [Manning et al 2010, Miller et al 2010].

Detailed Information for Clinicians Ordering Genetic Tests

Multigene Panels: FAQs

What Variables Affect the Diagnostic Sensitivity of Multigene Panels?

  • The genes included in multigene panels vary by laboratory.
  • Methods used in a multigene panel may include sequence analysis, deletion/duplication analysis, and/or other non-sequencing-based tests.
  • Sequence enrichment methods vary.
  • Laboratories frequently update multigene panels to include analysis of:
    • Noncoding regions (e.g., promoters); and
    • Additional genes as they are discovered.

What Kinds of Multigene Panels Are Available?

  • Off-the-shelf. These are designed by a laboratory to include genes commonly associated with a broad phenotype (e.g., ataxia, intellectual disability, cardiomyopathy) or a recognizable syndrome with genetic heterogeneity (e.g., Noonan syndrome). Off-the-shelf multigene panels may include additional test methods (e.g., deletion/duplication analysis or other non-sequencing-based tests).
  • Custom designed. These include genes selected by a clinician for analysis by sequencing. Sequencing results for each gene on the custom multigene panel are reported to the ordering clinician, whereas the sequencing data from the remaining genes sequenced (but not requested by the clinician) are not analyzed or included in the final laboratory report. Custom multigene panels offered by some reference laboratories are marketed under names such as XomeDxSlice® and ExomeNext-Select®.

How Do Off-the-Shelf Multigene Panels Compare with Custom Multigene Panels?

  • Custom multigene panels allow clinicians to design a single molecular genetic test for individuals with multisystem involvement, for which one off-the-shelf multigene panel is not clinically available.
  • If a pathogenic variant(s) is not identified in one of the genes analyzed on the custom multigene panel, reflex analysis of other targeted genes may be faster and less expensive, and it typically does not require an additional sample from the individual being tested or from the biological parents of the individual being tested (if samples from the parents were sent when ordering the custom multigene panel).

What Are the Disadvantages of Custom Multigene Panels Compared to Off-the-Shelf Multigene Panels?

  • Clinical sensitivity for custom multigene panels is not known and may be lower than for larger panels.
  • Custom multigene panels cannot detect larger deletions or duplications within the genes of interest.
  • Custom multigene panels may not include ancillary assays necessary to cover regions with (e.g.) highly homologous pseudogenes, deep intronic pathogenic variants, and expanded nucleotide repeats.

Comprehensive Genomic Testing

Clinical Exome Sequencing: FAQs

When Does Exome Sequencing Provide the Best Test Value?

  • When the clinical features in a patient are not highly suggestive of a known genetic condition
  • When the clinical features in a patient are suggestive of several genetic conditions that are not included in one multigene panel
  • When the clinical features in a patient and the family history are suggestive of a highly penetrant Mendelian condition that cannot be identified by phenotype alone

What Types of Disorders Can Be Reliably Diagnosed by Exome Sequencing?

  • Mendelian disorders caused by missense or nonsense variants that:
    • Are rare in the population; and
    • Have been previously reported as pathogenic in the literature or HGMD® (Human Gene Mutation Database)
  • Mendelian disorders caused by small insertions or deletions (<50 bp) within non-repetitive, coding DNA

What Types of Genetic Alterations Cannot Be Reliably Identified by Exome Sequencing?

Exome sequencing requires sequence enrichment to target exons and sequence each exon. Genetic alterations that cannot reliably be detected by exome sequencing include alterations that:

  • Disrupt probe binding during the enrichment process; thus, the exon is not sequenced and not included in the analysis;
  • Influence gene expression without changing DNA sequence (e.g., imprinting errors, uniparental heterodisomy);
  • Are present in a highly repetitive region of DNA that is difficult to sequence (e.g., nucleotide repeat expansions and contractions);
  • Involve a gene that is highly homologous to other gene family members or a pseudogene;
  • Are present in a deep intronic (noncoding) region;
  • Are present in mitochondrial DNA, not nuclear DNA;
  • Are somatic mosaic changes (i.e., the genetic change is present in a small percentage of cells and not present in the germline, i.e., all cells); thus, either base calls do not pass quality thresholds or the genetic change was not present in the cells from which the DNA was extracted;
  • Result from large copy number variants (i.e., insertions or deletions >50 bp). Although copy number variants may be identified on exome sequencing by comparing actual read depth to expected read depth through intra- and inter-sample comparisons, variations in read depth (e.g., due to guanine-cytosine [GC] content of a region) can lead to false positive results.
  • Result from structural chromosome rearrangements (e.g., inversions, translocations). Note that chromosome rearrangements may be identified by using paired-end and mate-pair mapping to identify malalignment of sequences to a reference genome.

What Types of Disorders Are Not Reliably Identified by Exome Sequencing?

Disorders resulting from genetic alterations that are not reliably identified by exome sequencing because of technical limitations:

Genetic disorders that may not be recognized during analysis of exome sequencing due to unexpected inheritance pattern:

  • Disorders with incomplete penetrance or variable age of onset (e.g., variant present in an unaffected parent)
  • Disorders with previously unreported inheritance patterns (e.g., autosomal dominant inheritance in a disorder previously reported as autosomal recessive)

Disorders for which associated genes and/or pathogenic variants have not been reported:

  • Mendelian disorders associated with multiple unidentified genes
  • Disorders caused by pathogenic variants that are not described as disease-associated in the literature or HGMD®

Disorders that are not known to be genetic:

  • Conditions with a non-genetic etiology (e.g., fetal alcohol syndrome)
  • Conditions for which there is no known genetic etiology

What Variables Affect the Sensitivity of Exome Sequencing?

Laboratory-dependent variables. Read depth (sometimes called exome coverage) and accuracy of base calling

  • "Read" refers to the nucleotide sequence generated from the laboratory process of sequencing a segment of DNA or RNA. Read depth refers to the number of times each nucleotide is sequenced. Read depth of an exome can vary significantly because some exons are easier to capture with probes and sequence than others. Read depth can refer to a single nucleotide, but is typically reported as the percentage of nucleotides that are sequenced either an average or minimum number of times (e.g., 30x average read depth for 95% of the exome).
    Exome coverage refers to the number of times each nucleotide is sequenced or the percentage of the exome sequenced an average or minimum number of times (e.g., 95% of exome at ≥20x coverage).
  • Accuracy of base calling, the reported nucleotide sequence compared to the actual nucleotide sequence, is measured by the Phred quality score. Phred scores are logarithmically related to nucleotide identification error probability. A Phred score of 10 indicates a one-in-ten chance of an inaccurate nucleotide determination. A Phred score of 20 indicates a one-in-100 chance of an inaccurate nucleotide determination, or a 99% likelihood of correct nucleotide assignment.

Additional laboratory-dependent variables. Sequence enrichment method used to target exons, sequencing technique, and length of sequence generated

  • Sequence enrichment method used to target exons: fixed array-based probes versus solution-based probes. Fixed array-based probes were the first method used to capture exons; while newer solution-based probes require less sample DNA, they may not capture regions with low GC content as well as array-based probes.
    Laboratories select probes that will target well-annotated genes associated with genetic conditions, thereby increasing the read depth of these genes. Laboratories frequently update the number of probes used in an assay to include noncoding regions of the genome (e.g., promoters, highly conserved regulatory sequences). Solution-based methods are more adaptable for updates than array-based methods.
  • Sequencing technique: paired-end reads (both ends of each DNA fragment are sequenced) versus single-end reads (only one end of a DNA fragment is sequenced). Paired-end reads are better than single-end reads at unambiguously determining alignment of a sequence to the reference exome, particularly in repetitive regions. However, sequencing of paired-end reads requires more laboratory time and is more expensive.
  • Length of sequence generated: longer reads reduce false positives that result from mapping ambiguity better than shorter reads.

Laboratory-dependent variables introduced by analysis of data. Application of filters and analysis of remaining unfiltered variants

  • Filters, applied during bioinformatics analysis, are used to select from the large number of variants identified by sequencing those variants that can be reasonably investigated for possible pathogenicity. Filters exclude variants that are unlikely to be disease related based on (a) the mode of inheritance inferred from the pedigree, (b) the frequency of a variant in the population, (c) a low Phred score, or (d) the prediction that a variant is non-pathogenic.
  • Variant classification guidelines updated by the American College of Medical Genetics and Genomics set forth objective variant classification methods [Richards et al 2015]. Nonetheless, to determine the pathogenicity of every variant identified, molecular and/or clinical geneticists in each laboratory develop their own approach to variant classification, often using the following:
    • Population databases (e.g., 1000 Genomes, ExAC, dbSNP)
    • Curated variant databases (HGVS, locus-specific databases, LOVD, ClinVar)
    • Published literature
    • Unpublished in-house laboratory data
    • In-silico predictive tools
    Because the expertise of the molecular geneticist(s) and the data available vary, variant classification, and therefore clinical sensitivity, are laboratory dependent to some degree.

Laboratory-independent variables. Source of the DNA; GC (guanine-cytosine) content of the region

  • The source of DNA (e.g., blood, skin, saliva) affects the quantity and quality of DNA. For example, saliva samples have a lower quantity of DNA and higher contamination rates (e.g., bacterial DNA) than blood and skin samples. Lower DNA quantity will decrease achievable read depth. Higher contamination rates will decrease the accuracy of base calling.
  • The GC content of the region (i.e., proportion of guanine [G] and cytosine [C] nucleotides compared to adenine [A] and thymine [T] nucleotides) varies throughout the exome. Regions with high GC content (≥60%) (e.g., first exons, promoters) and low GC content (≤25%) are more difficult to sequence, resulting in decreased coverage of these regions compared to regions with a balanced number of nucleotides [Rieber et al 2013].

Clinical Genome Sequencing: FAQs

When Does Genome Sequencing Provide the Best Test Value?

Benefits and Limitations of Genome Sequencing

Genome sequencing is typically performed by next-generation sequencing of sheared genomic DNA. Genome sequencing techniques have nonstandardized, highly variable coverage.

While genome sequencing is significantly more costly than exome sequencing, it has distinct advantages:

  • Simpler sample preparation (no need for sequence enrichment strategies to target coding DNA that results in more even coverage [read depth] across coding regions)
  • The ability to identify structural variants and chromosome breakpoints in noncoding regions

The coverage of the genome is less than 100% and varies by laboratory. Telenti et al [2016] sequenced more than 10,000 genomes at a mean read depth of 30-40x (i.e., each DNA fragment was sequenced an average of 30 to 40 times); the authors reported that 91.5% of exons and 95.2% of known pathogenic variant positions could be sequenced with high confidence. The clinical sensitivity of genome sequencing is unknown.

Although genome sequencing can identify variants outside of the coding regions, most of the confirmed pathogenic variants identified by genome sequencing are within the exome [Taylor et al 2015]. The diagnostic utility of exome sequencing and genome sequencing (~20%-30%) remains similar. As more noncoding pathogenic variants are identified, the clinical sensitivity and value of genome sequencing should increase.

What Types of Disorders Can Be Reliably Diagnosed by Genome Sequencing?

Mendelian disorders caused by:

  • Missense or nonsense variants that:
    • Are rare in the population; and
    • Have been previously reported as pathogenic in the literature or HGMD® (Human Gene Mutation Database)
  • Small insertions or deletions (<50 bp) within non-repetitive DNA that:
    • Have been previously reported as pathogenic in the literature; or
    • Disrupt a gene reported in HGMD® (Human Gene Mutation Database)

What Types of Genetic Alterations Are Not Reliably Identified by Genome Sequencing?

Alterations that:

  • Influence gene expression without changing DNA sequence (e.g., imprinting errors, uniparental heterodisomy);
  • Are present in a highly repetitive region of DNA that is difficult to sequence (e.g., nucleotide repeat expansions and contractions);
  • Involve a gene that is highly homologous to other gene family members or a pseudogene;
  • Are present in mitochondrial DNA, not nuclear DNA;
  • Are somatic mosaic changes (i.e., the genetic change is present in a small percentage of cells and absent in a large percentage of cells); thus, either base calls do not pass quality thresholds or the genetic change was not present in the cells from which the DNA was extracted.

What Types of Disorders Are Not Reliably Identified by Genome Sequencing?

Disorders caused by genetic alterations that are not identified on genome sequencing because of technical limitations:

Disorders for which associated genes and/or pathogenic variants have not been reported:

  • Mendelian disorders associated with multiple unidentified genes
  • Disorders caused by pathogenic variants that are not described as disease-associated in the literature or HGMD®

Disorders that are not known to be genetic:

  • Conditions with a non-genetic etiology (e.g., fetal alcohol syndrome)
  • Conditions for which there is no known genetic etiology

Comparison of Multigene Panels with Comprehensive Genomic Testing

Advantages of Multigene Panels over Exome Sequencing and Genome Sequencing

Clinical sensitivity (the ability to identify pathogenic variants causative of known clinical disorders) can be superior.

Results can be easier to analyze. Because fewer genes are sequenced, fewer variants will be identified. Therefore, multigene panels often have the following advantages:

  • Faster turnaround time
  • Lower cost
  • No incidental findings (identification of pathogenic variants in genes that do not account for the patient phenotype that prompted the diagnostic testing)
  • No routine requirement of parental testing for interpretation of test results. Note: Parental samples are required to further evaluate variants of uncertain significance.

Advantages of Exome Sequencing or Genome Sequencing over Multigene Panels

Exome sequencing and genome sequencing do not require the clinician to determine which disorders (and, hence, which genes) are likely to be involved; thus, testing can be ordered earlier in a patient’s diagnostic evaluation because extensive clinical evaluations, laboratory testing, and radiographic evaluations are not needed to identify diagnostic clues that would lead the clinician to suspect a specific disorder(s).

Exome sequencing and genome sequencing can detect the presence of two or more genetically distinct disorders (the phenotypic presentation of which may have complicated diagnosis) in the same individual [Yang et al 2013, Adams et al 2014].

Using a multigene panel forces the clinician to select the best panel for the patient. Selection is often difficult because:

  • The less well defined the patient’s phenotype, the more difficult it is to identify the most appropriate multigene panel;
  • Genes for rare disorders or newly discovered genes may not be included in a multigene panel;
  • The testing method required to detect variants (e.g., exon or whole-gene deletions) commonly observed in some disorders may not be utilized in a given multigene panel;
  • The clinical sensitivity (which can vary widely among multigene panels) is not provided for some panels;
  • Laboratories with multigene panels comprising very similar lists of genes may manage variants of uncertain significance differently, potentially causing a substantial clinical burden in interpretation (e.g., testing additional family members to clarify the result), and genetic counseling (e.g., clarifying the difference between a pathogenic variant and a variant of uncertain significance). For example:
    • Laboratories may not reveal the probability of identifying a variant of uncertain significance, a factor to consider for large multigene panels for which the probability of identifying a variant of uncertain significance can exceed 75%;
    • Laboratories may not include variants of uncertain significance in the test result provided to the ordering clinician.

Resources

Population Databases

Database of Genomic Variants: dgv.tcag.ca/dgv/app/home

Curated Variant Databases

ClinGen Dosage Sensitivity Map: www.ncbi.nlm.nih.gov/projects/dbvar/clingen

DECIPHER: decipher.sanger.ac.uk

ECARUCA: ecaruca.radboudumc.nl/ecaruca/queryDatabase.jsp

References

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Suggested Reading

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  • Genome in a Bottle Consortium (www​.genomeinabottle.org)
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