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Halsey’s article and SMN copies
So Halsey’s article today made me wonder if I was misunderstanding SMA and SMN1 and 2 with this passage: “but there’s another piece of information in my chart from my original diagnostic blood test in 1999 that further explains why they find my case to be particularly intriguing: I have zero copies of the survivor motor neuron 1 (SMN1) gene.
This gene carries the instructions for building SMN protein, which creates muscle. The fewer copies of it that someone has, the more severe their SMA should be.
Additionally, I have only two copies of the SMN2 gene, which supplements SMN1.”
After doing some checking, I think this section is off. Almost all cases of SMA will show zero copies of SMN1, that’s how it’s confirmed. A carrier will have 1, and normal will have 2 (with some occasional rare differences). It’s the number of copies of SMN2 that usually determine severity, but we know that’s not a hard rule.
https://clevelandcliniclabs.com/2017/10/18/tb-spinal-muscular-atrophy-copy-number-analysis/#sma-tb-interpretation: “Among patients with clinical presentations suggestive of SMA, detection of zero SMN1 copies confirms the diagnosis.”
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