Astral Codex Ten

Chromosome 21

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Chromosome 21

Article

Chromosome 21 is a recurring concept in the Astral Codex Ten archive, appearing 4 times across 4 issues between July 12, 2024 and August 14, 2025. The archive places it in contexts such as “mutation on Chromosome 21”; “Chromosome 21 includes some anti-cancer genes”; “Mutations in the APP gene on chromosome 21”. It most often appears alongside Down syndrome, Alzheimer’s, aducanumab.

Metadata

  • Category: Concepts
  • Mention count: 4
  • Issue count: 4
  • First seen: July 12, 2024
  • Last seen: August 14, 2025

Appears In

Source Context

Recovered passages from the original issue text. When the raw archive preserved outbound links inside the source passage, they are listed directly under the quote.

Your Book Review: The Family That Couldn’t Sleep
July 12, 2024 · Original source
I think DTM’s explanation is erroneous. He identifies his disorder as resembling “a form of CMT caused by a mutation on Chromosome 21”, then goes on to describe the specific gene affected. That particular gene is on 8p21, so...understandable game of telephone. There is a reported form of CMT caused by a mutation on 21q22, but it seems to have been first described in 2019.
Links For July 2025
July 01, 2025 · Original source
42: Did you know: people with Down Syndrome have only half the expected rate of solid cancers (X). Chromosome 21 includes some anti-cancer genes, and having an extra copy gives extra protection. Kesin says that there’s “no free lunch” because Down Syndrome patients also have much higher rates of leukemia, but that’s only true on a chromosome-wide basis; the specific genes that protect against solid tumors don’t cause the leukemia. What’s the downside of increasing those genes? Seems less well-understood, but might harm wound healing, placental implantation, and neurodevelopment.
Inline links: people with Down Syndrome have only half the expected rate of solid cancers (X)
Your Review: Of Mice, Mechanisms, and Dementia
July 11, 2025 · Original source
Second, the hypothesis was backed by powerful genetic evidence. Mutations in the APP gene on chromosome 21 were associated with early-onset Alzheimer’s. The case grew stronger with the observation that more than 50% of individuals with Down syndrome, who carry an extra copy of chromosome 21 (and thus extra APP), develop Alzheimer’s-like pathology by age 40.
In Defense Of The Amyloid Hypothesis
August 14, 2025 · Original source
Anti-amyloid drugs (like Aduhelm) don't reverse the disease, and only slow progression a relatively small amount. Opponents call the amyloid hypothesis zombie science, propped up only by pharmaceutical companies hoping to sell off a few more anti-amyloid me-too drugs before it collapses. Meanwhile, mainstream scientists . . . continue to believe it without really offering any public defense. Scott was so surprised by the size of the gap between official and unofficial opinion that he asked if someone from the orthodox camp would speak out in its favor. I am David Schneider-Joseph, an engineer formerly with SpaceX and Google, now working in AI safety. Alzheimer’s isn’t my field, but I got very interested in it, spent six months studying the literature, and came away believing the amyloid hypothesis was basically completely solid. I thought I’d share that understanding with current skeptics. The ATN model The most plausible variant of the amyloid hypothesis is the A → T → N model: amyloid causes tau causes neurodegeneration. 1: Amyloid The common entrypoint, typically at least 15 years before clinically detectable symptoms [1], is accumulation of amyloid-β deposits (especially Aβ42, one of several variants). Amyloid-β is a peptide produced in healthy human beings and many other animals, probably for antimicrobial purposes [2, 3]. Factors which cause overproduction of amyloid also cause Alzheimer’s. Factors that cause decreased clearance of amyloid also cause Alzheimer’s. The clearest relationship is various genes which massively increase amyloid production (while doing nothing else); these genes are Alzheimer’s risk factors, with some of the rarer and more severe ones causing extreme versions of the disease that manifest at otherwise almost-never-seen ages. One of the clearest examples is Down syndrome, which is caused by three (rather than the usual two) copies of chromosome 21. People with Down syndrome are at much higher risk of Alzheimer’s than the general population: two-thirds will have the condition by age sixty, and 15% have it by age forty. APP, the gene for the amyloid precursor protein, is on chromosome 21. This means that people with Down syndrome will have an extra copy. This extra copy has been observed to lead to higher-than-normal amyloid levels. But there are many genes on chromosome 21; do we have additional evidence that it’s the amyloid one that’s involved? Yes. Dozens of other mutations on APP cause the same sort of extremely young and severe Alzheimer’s. So do mutations on PSEN1 and 2, the genes for the enzyme that processes amyloid precursor protein into amyloid. So do mutations on several other amyloid-related genes. [6, 91 - 96] Researchers call these autosomal-dominant Alzheimer’s, meaning Alzheimer’s cases that get inherited from a single parent in a simple fashion typical of single-gene disorders. They make up about 1% of all cases, and are our strongest evidence for the causal role of amyloid in the disorder. To my knowledge, there is no serious claim that these genes could be working through any pathway other than their shared role in the amyloid system. But these autosomal-dominant cases only make up about 1% of all Alzheimer’s patients. Might they be a different disease than the usual sporadic Alzheimer’s that strikes people without strong family histories at normal ages? Probably not: the presentation and trajectory of autosomal-dominant and sporadic Alzheimer’s cases are strikingly similar. Both show an initial appearance of amyloid pathology starting in intrinsic connectivity networks in both autosomal-dominant [14] and sporadic [15–18] types, cortical tau appearing first in the medial temporal lobe and with the exact same fold in both disease types [97] (despite human tauopathies having at least seven other possible characteristic folds [36]), that tau pathology worsening and spreading outside this region only once amyloid pathology reaches sufficient severity [65], neurodegeneration progressing closely in step with the tau pathology, and the same usual approximate trajectory of cognitive symptoms due to the sequence of affected regions. So it’s as if two bank robberies occurred hours apart, in the same town, and in a highly similar and idiosyncratic manner, and we can positively identify the culprit of one on security camera footage. It’s a good bet the culprit of the other is the same. Increased amyloid production → Alzheimer’s is an especially clear and simple pathway, but any other change in amyloid can also cause the disease. For example Overproduction or reduced clearance of amyloid due to impaired slow wave sleep. Aβ production is neuronal activity-dependent, and toxins (perhaps including Aβ) are cleared from the brain during sleep via the glymphatic system. Thus Aβ can accumulate if the brain is more active and/or has less opportunity for clearance. [7, 8, 9, 10, 11]
Inline links: Aduhelm, David Schneider-Joseph