Mutations in the COL4A5 gene that lead to the production of shorter versions of the encoded protein are linked to a more severe form of Alport syndrome than other mutations, researchers report in a study.
This finding suggests that analysis of the COL4A5-encoded protein can be used to predict renal outcomes in Alport syndrome patients. Additionally, strategies to prevent the underlying mechanism involved in the production of these abnormally short proteins may hold potential to improve the outcomes of these patients.
Findings were published in the study, “Detection of Splicing Abnormalities and Genotype-Phenotype Correlation in X-linked Alport Syndrome,” in the Journal of the American Society of Nephrology.
About 85 percent of Alport syndrome cases are caused by a genetic mutation on the COL4A5 gene, which is located on the X chromosome and encodes for one of the chains of type IV collagen. Encoding means the gene provides the instructions for the production of a specific molecule.
X-linked Alport syndrome (XLAS) is known to be more severe in men than in women. This is because women have two copies of the X chromosome in each cell, although only one copy is active — but that X chromosome could be normal in some cells, and compensate for those cells in which is the gene is mutated. Men, conversely, have only one X chromosome and it will carry the genetic error in those who are XLAS patients.
But studies have also suggested that the severity of the disease can also be dependent on the mutation affecting the gene, whether it leads to a slightly altered protein or if it has more drastic effects on the protein sequence.
When genes are processed into their coded proteins, they go through a process called splicing, which consists of gluing together all the protein-coding sequences of the gene, known as exons, while removing small parts that are not required, called introns.
However, this process can be severely altered if mutations occur and disturb the way the machinery involved recognizes where to cut and paste the different parts. This can give rise to abnormally longer or shorter versions of the encoded protein, which will ultimately regulate its function.
Although such splicing abnormalities have been reported in patients with XLAS, it has never been demonstrated that they could be linked to disease presentation.
In this study, Japanese researchers evaluated 14 patients who were followed between January 2006 and July 2017 at Kobe University, and who were suspected of having COL4A5 splicing abnormalities. Researchers also recruited 46 males from 29 families who were known to have such genetic abnormalities to further evaluate their association with disease severity.
Analysis of the disease-causing mutations and the sequence of the produced protein in each case revealed several splicing abnormalities already reported in XLAS patients. But researchers were also able to identify 14 new atypical COL4A5 splicing patterns, which led to the exclusion of exons in some cases, as well as the inclusion of introns in others.
Among the identified genetic abnormalities, the researchers found that, in 21 cases, the produced protein was smaller than normal, and in 25 cases, the alteration did not significantly change the size of the protein. Interestingly, when they compared the two groups, the researchers found that those with smaller proteins reached end-stage renal disease a median of nine years earlier than those with normal-size proteins.
“Renal prognosis differs significantly for patients with truncating [shortened] versus non-truncating splicing abnormalities,” they wrote.
Supported by these results, the team believes that protein analysis can help to “accurately estimate renal prognosis and provide better information during genetic counseling.” The “results suggest that splicing modulation should be explored as a therapy for XLAS with truncating mutations,” they added.