Testing for Single Gene Disorders

Single gene (monogenic) disorders result from heritable changes in the DNA sequence (pathogenic variants) that perturb protein structure and function.
Mutations causing these disorders include single base substitutions (missense, nonsense, or silent mutations), deletions (in-frame or frameshift), insertions, and triplet repeat expansions.
They exhibit classical Mendelian inheritance patterns, including autosomal dominant, autosomal recessive, and X-linked inheritance.
Testing for single-gene disorders primarily utilizes three approaches: direct pathogenic variant analysis (DNA sequencing), chromosome microarray (CMA), and linkage analysis.

Direct DNA-based testing targets specific gene sequence changes and provides high specificity if a previously described deleterious variant is found.
Sanger Sequencing: Considered the gold standard for mutation screening, this method is based on the selective incorporation of chain-terminating dideoxynucleotides during in vitro DNA replication. It evaluates only part of a gene at a time and is ideal for confirming known mutations or evaluating conditions with minimal locus heterogeneity.
Next-Generation Sequencing (NGS): A high-throughput technique that runs thousands or millions of sequences in parallel at high speed and lower cost.
Targeted Gene Panels: Utilizes NGS to test a specific subset of genes associated with a particular phenotype (e.g., neuromuscular disorders or deafness).
Whole Exome Sequencing (WES): Analyzes the exome, which comprises the protein-coding regions representing approximately 1% of the genome. It is highly useful for unexplained intellectual disability, extreme heterogeneity, or atypical clinical presentations.
Whole Genome Sequencing (WGS): Evaluates the entire genome, including both coding and noncoding (intronic or regulatory) regions. It provides improved detection of structural variations, copy number variations, and repeat expansions compared to WES.

Specific polymerase chain reaction (PCR) methods amplify target sequences to detect known mutations rapidly.
Size analysis of PCR products is utilized for testing cystic fibrosis.
Restriction fragment length polymorphism (RFLP) is applied in testing for spinal muscular atrophy.
Amplification-refractory mutation system (ARMS) PCR is used for detecting thalassemia.
Real-time PCR uses special kits to analyze rapid results within one hour.

CMA resolves copy number variations (CNVs) such as deletions or duplications of several kilobases within one or more genes.
It detects small intragenic deletions and duplications that would typically be missed by both traditional chromosome analysis and direct DNA sequencing.
A limitation of CMA is that it can miss small sequence-level deletions or insertions depending on the resolution of the array utilized.

Linkage analysis tracks a genetic trait through an extended family using closely linked polymorphic markers as a surrogate for the disease trait.
It is deployed when a specific disease-causing genetic variant cannot be identified or is impractical to find due to the large size or number of variants in a gene.
This approach requires multiple family members with a documented Mendelian inheritance pattern.
Linkage analysis is vulnerable to pitfalls such as genetic recombination between the marker and the gene, locus heterogeneity, and incorrect clinical diagnosis in the proband.

Testing Modality	Clinical Indications	Examples
Single Gene Sequencing	Minimal locus heterogeneity; distinctive clinical findings pointing to a specific gene.	CFTR for cystic fibrosis; PAH for phenylketonuria.
Targeted Gene Panel	Locus heterogeneity; overlapping phenotypes; disorders sharing a common pathway.	Muscular dystrophy panel; cardiomyopathy panel; epilepsy panel.
Exome Sequencing (WES)	Extreme heterogeneity; de novo mutations; indistinct phenotypes; nondiagnostic initial tests.	Autism, intellectual disability, Kabuki syndrome.
Genome Sequencing (WGS)	Suspected noncoding variation; suspected structural variation; critical illness requiring rapid data; nondiagnostic WES.	DiGeorge syndrome; severe infantile intensive care presentations.

Genetic test interpretation relies on analytic validity (accuracy of the test), clinical validity (prediction of disease presence or absence), and clinical utility (guidance for medical management).
Variants are classified into five categories based on their functional impact: pathogenic, likely pathogenic, variant of unknown significance (VUS), likely benign, and benign.
A negative DNA sequence test does not entirely rule out a single gene disorder, as pathogenic variants may reside in unsequenced noncoding regions, or the disorder may result from undetected structural variants and large deletions.