Genomic coordinate (human 12:111,452,214 ATXN2).
Cytoband (human 12q24.12 ATXN2).
OMIM’ genes @ 12q24.12 = 32 genes.
Hyperbolic Umbilic Chromosome-12 is 1,200 genes.
Here I present: “Spinocerebellar Ataxia”, Victor McKusick, Mendelian Inheritance in Man’, 1966. (ATXN2)
INTRODUCTION.
In genetics, the ATXN2 gene contains a repeating sequence of three DNA building blocks: cytosine, adenine, and guanine (CAG repeats). The exact number of these repeats determines whether a person remains in the stable “Normal” region of the cusp landscape or shifts into a neurodegenerative disease state.
The clinical thresholds for the ATXN2 gene are defined below:
🟩 Normal Range (13 to 31 Repeats)
- Threshold: ≤ 31 CAG repeats.
- Context: Over 90% of the global population carries exactly 22 repeats. Lengths up to 31 do not cause spinocerebellar ataxia.
🟨 Intermediate Range & ALS Risk (27 to 33 Repeats)
- Threshold: 27 to 32/33 CAG repeats.
- Context: While these intermediate lengths do not cause standard ataxia, they fall into the early slopes of the “cusp.” Having an intermediate expansion of ≥27 to 33 repeats significantly increases the risk of developing Amyotrophic Lateral Sclerosis (ALS) or parkinsonian syndromes.
🟧 Reduced Penetrance Range (32 to 34 Repeats)
- Threshold: 32, 33, or 34 CAG repeats.
- Context: This is the literal “bifurcation cusp” or boundary zone. Individuals with 32 to 34 repeats may or may not develop signs of SCA2. If they do experience symptoms, it typically manifests as a very late-onset variant of the disease.
🟥 Pathogenic Range / Positive SCA2 (35+ Repeats)
- Threshold: ≥ 35 CAG repeats (Fully penetrant at 37 to 39+ repeats).
- Context: This represents the upper unstable folded sheet where the “sudden collapse” happens. At this length, the gene produces a toxic polyglutamine tract that leads to progressive brain cell death, directly causing Spinocerebellar Ataxia Type-2. Highly expanded alleles (over 45) can trigger juvenile or early teen onset.
