- Design
- Methods comparison study
- Population
- 20 patients with suspected 21-hydroxylase deficiency and known CYP21A2 structural variants
- Primary outcome
- Detection and characterisation of structural variants vs MLPA, CNVplex and SNaPshot
- Effect
- Long-read sequencing detected masked '2+0' carriers, resolved CH-4/CH-9 chimeras and TNXA/TNXB deletions missed or misclassified by conventional assays
Molecular diagnosis of 21-hydroxylase deficiency, the commonest cause of congenital adrenal hyperplasia, is hampered by a neighbouring pseudogene (CYP21A1P) that is nearly identical. This Chinese study took 20 patients whose structural variants had already been found by MLPA screening and re-analysed them with CNVplex, SNaPshot and then long-read sequencing.
Long-read sequencing identified and mapped a novel large deletion, and detected '2+0' carriers — people with two gene copies on one chromosome and none on the other — whom the conventional methods called normal because a duplication masked the deletion. It also separated chimera subtypes the other assays could not, and correctly identified TNXA/TNXB chimeric deletions that are often misclassified; those matter because they cause the CAH-X syndrome with connective tissue features.
The practical point for molecular laboratories is that a normal MLPA copy number does not exclude carrier status in a family at risk.
- Do not report a normal MLPA copy number as excluding CYP21A2 carrier status in an at-risk family.
- Consider long-read sequencing, or referral to a laboratory offering it, for unresolved or discordant 21-hydroxylase results.
- Report TNXB involvement explicitly, because CAH-X carries connective tissue implications.
- Include the limits of the method used in every CYP21A2 report.
Why it matters
Missed '2+0' carriers mean wrong reassurance in genetic counselling for a common inherited disorder.
Don't overread it
Twenty pre-selected patients; the study shows what long-read sequencing can detect, not how often standard methods fail in routine testing.
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