Molecular characterization of a deep intronic TBX5 variant in a familial case of Holt-Oram syndrome
This study identifies and functionally validates a novel deep-intronic *TBX5* variant as the first regulatory splicing mutation causing Holt-Oram syndrome, demonstrating the critical necessity of RNA analysis for characterizing non-coding pathogenic variants.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Holt-Oram syndrome is a rare condition that affects how a person's heart and arms develop before birth. It runs in families and is caused by changes, or variants, in a specific gene called TBX5. This gene acts like a set of instructions for building the heart and the upper limbs during early pregnancy. When the instructions are clear and correct, the body builds these parts properly. When the instructions are scrambled, the result can be missing or malformed fingers, thumbs, or heart defects. For decades, doctors and scientists have known that most cases of this syndrome come from errors in the coding part of the gene, the section that directly spells out the protein. However, genes also contain long stretches of non-coding DNA between the instructions, which were once thought to be silent. Recent science has shown that these silent areas can actually control how the instructions are read, and errors hidden deep within them can cause disease just as surely as errors in the main text.
In a recent study, researchers at the Research Centre for Medical Genetics in Russia investigated a family where a father and his young son both suffered from Holt-Oram syndrome, yet standard genetic tests had failed to find the cause. The father had a history of heart surgery and significant hand deformities, including missing thumbs and fused fingers. His son, born with similar but milder arm issues and severe heart defects, had inherited the condition. When the family underwent whole-genome sequencing, which reads the entire genetic code, the scientists found a tiny change in the DNA. This change was located deep inside a non-coding gap between the main instructions of the TBX5 gene. It was a small deletion and insertion of genetic letters that had never been seen before. Because this change was in a region that does not code for protein, it was initially labeled as a variant of uncertain significance, meaning scientists did not know if it was the culprit or just a harmless difference.
To solve this mystery, the team turned to the father's cells. They grew skin cells from a small sample in a laboratory to produce RNA, which is the molecule that carries the genetic message from the DNA to the cell's protein-making machinery. By reading this RNA, the researchers could see exactly how the cell was interpreting the genetic instructions. They discovered that the tiny change in the non-coding gap had tricked the cell into reading a piece of DNA that should have been ignored. This extra piece, which the scientists called a pseudoexon, was being inserted into the final message. This insertion shifted the reading frame of the instructions, causing the cell to stop building the protein too early. The result was a broken, shortened version of the TBX5 protein that could not do its job.
The researchers confirmed this finding using a powerful technique called long-read sequencing, which allowed them to see the entire length of the genetic message in one go. They found that in the father's cells, about 22 percent of the TBX5 messages contained this unwanted extra piece. In healthy cells from a control donor, this error happened less than 0.2 percent of the time, suggesting that the genetic change in the family was indeed driving the mistake. The team also tested whether the cell tried to clean up these broken messages. They treated the cells with a substance that stops the cell's natural trash-disposal system, known as nonsense-mediated decay. When this system was paused, the amount of broken messages in the father's cells rose to 37 percent. This proved that the cell was actively trying to destroy the faulty messages, but some were escaping destruction and being made into the defective protein.
This study marks the first time a deep-intronic variant, a change hidden far from the main coding instructions, has been proven to cause Holt-Oram syndrome. The findings show that the genetic code is more complex than just the parts that build proteins; the spaces between them are critical for ensuring the instructions are read correctly. For the family in this study, the research provided a definitive answer after years of uncertainty, confirming that their condition was caused by this specific regulatory error. It also highlights that when standard genetic tests come back empty for families with clear symptoms, looking deeper into the non-coding regions and testing how genes actually function in cells can reveal the true cause of the disease.
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