A technique called whole-exome sequencing (WES) that screens the protein-coding region of the DNA enabled the identification of mutations in the COL4A5 gene in two girls with Alport syndrome and no family history of the disease, a case study reports.
X-linked Alport syndrome (XLAS) represents about 80% of all disease cases and is caused by mutations in the type IV collagen alpha5 chain gene (COL4A5) in the X chromosome. Women have a milder clinical presentation that is associated with different alterations in the glomerular basement membrane (GBM), which plays a key role in the kidney’s filtration barrier.
A COL4A5 mutation test, available in the U.S. and Europe, has a reported 80% detection rate in males with XLAS. However, insufficient data on COL4A3 and COL4A4 mutations complicate the distinction between autosomal recessive and autosomal dominant Alport syndrome.
WES has previously been used in Alport patients. The team used this technique to identify mutations in two Korean girls with suspicious GBM changes associated with Alport syndrome, but without relevant family history.
The first case describes a previously healthy 11-year-old girl who first went to the hospital due to proteinuria, or excess amounts of protein in urine. The patient had episodic hematuria, large corneal astigmatism — irregularly shaped cornea causing blurred vision — and mild lumbar scoliosis, or curvature of the spine. She had no family history of kidney disorders and had normal hearing, as assessed by pure tone audiometry and speech audiometry, and renal ultrasound results were unremarkable. The patient then underwent a kidney biopsy.
The second case was a 12-year-old girl who had had recurrent hematuria since she was 24 months old. She had been diagnosed with Alport when she was around 7. However, direct DNA sequencing did not reveal a COL4A5 mutation. She also did not have extra-renal symptoms, including hearing loss or ocular lesions, nor a family history of kidney diseases. Her kidney function was normal, except for persistent hematuria and proteinuria.
For WES, the team extracted DNA from the patients’ and the parents’ peripheral blood leukocytes, or white blood cells. The investigators focused on gene variants that could alter protein function. A total of 31 genes, including COL4A3, COL4A4, and COL4A5, were then analyzed.
The first patient revealed a likely pathogenic (disease-causing) variant in COL4A5, known as c.4688G>A, causing the substitution of an arginine for a glutamine amino acid, not observed in her parents. As such, this is a de novo mutation, which refers to DNA alterations that occur during embryonic development, as opposed to inherited mutations. This variant has previously been reported in a family with Alport. This patient also had one variant of uncertain significance in COL4A4 (c.4817G>A).
The second patient revealed a different likely pathogenic variant (c.2714G>A), causing the substitution of a glycine for an aspartic acid amino acid, also not found in either parent. This de novo mutation has been reported in an Alport patient.
“We therefore suggest that WES is an effective approach to obtain genetic information in [Alport syndrome], particularly in female patients without a relevant family history, to detect unexpected DNA variations,” the scientists wrote.