Blood samples used to detect kEDS-causing mutation: Case study
Unusual mutation caused 7-year-old boy's condition, researchers report
An unusual mutation in the PLOD1 gene caused kyphoscoliotic Ehlers-Danlos syndrome (kEDS) type 1 in a 7-year-old boy, a case study reported. Instead of a skin biopsy to investigate the clinical effect of this mutation, researchers used whole blood samples because PLOD1, unlike the genes that cause other types of EDS, is physiologically active in the bloodstream.
“Analysis on peripheral blood was a rapid and reliable tool to clinically characterize a [mutation] in PLOD1-kEDS,” the researchers wrote.
Case details were reported in the study, “Whole Blood Multi-OMIC Analysis Is Effective in Clinical Interpretation of Splicing Aberrations in PLOD1-Related Kyphoscoliotic Ehlers-Danlos Syndrome,” published in the American Journal of Medical Genetics Part A.
kEDS type 1 is caused by mutations in both copies of the PLOD1 gene, one inherited from each biological parent. Such mutations lead to a deficiency in lysyl hydroxylase 1 (LH1), an enzyme that modifies and strengthens collagen, the main protein in connective tissue.
A lack of this enzyme weakens connective tissue, giving rise to kEDS symptoms, including hypermobile joints, stretchy skin, eye fragility, low muscle tone, and kyphoscoliosis, a sideways curvature of the spine combined with a hunched back.
Splice variant
The most common kEDS-causing PLOD1 mutation results from a large portion of the gene being copied, resulting in a nonfunctional version of LH1. Other mutations introduce premature stop signals in the gene that prevent any enzyme production.
A research team in Italy described the case of a 7-year-old boy with kEDS who carried an unusual mutation called a splice variant in both copies of his PLOD1 gene.
He was the second child of unaffected parents who were first cousins. Examination at birth revealed low muscle tone, multiple joint contractures, and left hip dysplasia, an abnormality of the hip joint where the socket does not fully cover the ball at the top of the leg bone.
Physical therapy and cast application resolved the contractures on his hands and feet and the hip dysplasia. While the boy improved, low muscle tone persisted over the years, delaying his ability to walk until he was 4. He also developed kyphoscoliosis, for which he received physical therapy and an orthopedic brace.
An examination when the boy was 5 showed several abnormalities in head shape, including a head that was relatively long compared to its width, a bulge on the back of the head, a vertical bony ridge on the center of the face, and skin folds on the upper lids of both eyes.
He had marked kyphosis, or a hunched back, with his upper arms rotated toward the front side of his body. He also had hypermobile hand and foot joints, severe flat feet, soft and stretchy skin, and a raised scar on the left leg.
Genetic analysis revealed two identical mutations, called c.1756-13C > A, one in each copy of his PLOD1 gene. Based on the clinical and genetic findings, he was diagnosed with kEDS type 1.
His parents and unaffected sister each carried one mutated copy of the gene. The mutation was found in a section of the gene called an intron.
When cells make a protein, the information in a gene is first copied into precursor messenger RNA (mRNA). This single-stranded molecule contains exons, or segments that encode the protein, and introns, which are removed in a process called splicing. The remaining exons are then stitched together to create a mature mRNA that is read by the cellular machinery to produce a protein.
Although the mutation had been reported in a published case study, no information was given on its effect on LH1 production. So the team conducted an mRNA analysis using whole blood samples because PLOD1 is active in the bloodstream, unlike the genes that cause other EDS types.
The results showed that the mutation was located in a position in the gene’s intron that altered the splicing process such that a short intron segment between two exons was not removed. This led to the production of a shortened non-functional version of LH1, causing the boy’s kEDS.
“Our work documents for the first time the reliable use of whole blood RNA-seq to demonstrate the deleteriousness of … splicing variants in PLOD1-related kEDS without proceeding with a skin biopsy, a procedure that is questioned in specific real-world circumstances,” the scientists wrote.
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