Why Do Standard Genetic Tests Miss Spinocerebellar Ataxia?

It can be incredibly frustrating to have clear clinical signs of spinocerebellar ataxia (SCA), only for a comprehensive genetic test—like Whole Exome Sequencing (WES) or standard Whole Genome Sequencing (WGS)—to come back perfectly clear. The reason for this diagnostic gap lies in how these standard tests read your DNA. Broad genetic tests are excellent at finding simple “spelling mistakes” (single-letter changes) in your genes, but they are notoriously bad at detecting the “stuttering” DNA sequences that cause most SCAs ^[1]. Because of technical glitches like tricky DNA structures and short testing lengths, standard broad tests often skip right over the exact mutations your doctor is looking for ^[2][3].

The “Stuttering” Mutation

To understand why standard tests fail, it helps to know what they are looking for. Most SCAs are not caused by a missing or incorrect piece of DNA, but rather by a nucleotide repeat expansion ^[1].

Think of your DNA as an instruction manual. In a repeat expansion, a specific sequence of “letters” (nucleotides) is repeated over and over, like a stutter—for example, “CAG CAG CAG”. If the number of repeats crosses a certain threshold, it causes the gene to malfunction and leads to ataxia ^[4].

Why Broad Tests Miss the Mark

Standard broad genetic testing, generally called Next-Generation Sequencing (NGS), is an umbrella term that usually refers to “short-read” technologies. These standard tests struggle with repeating sequences for three main reasons:

1. Short Read Lengths

Standard testing works by taking your entire DNA manual, shredding it into millions of tiny pieces called “short reads,” reading those small fragments, and using a computer to stitch them back together ^[2]. However, if your repeating sequence (the stutter) is longer than the short fragment being read, the computer cannot figure out where the piece belongs or measure how long the full repeat actually is ^[5][6].

2. DNA “Knots” (Secondary Structures)

Many SCA mutations happen in regions of DNA that are rich in the letters G and C ^[7]. These highly repetitive regions have a tendency to fold in on themselves, forming tight molecular knots known as secondary structures ^[8]. During the chemical process of preparing your DNA for standard testing, the enzymes doing the reading physically stall or get stuck on these knots ^[9][10]. As a result, the final test data ends up with gaps or blank spots right where the critical SCA genes are located ^[11].

3. Looking in the Wrong Place

Whole Exome Sequencing (WES) is a highly popular test because it sequences the “exome”—the roughly 1-2% of your DNA that provides instructions for building proteins. However, several forms of SCA—including SCA10, SCA36, and the recently discovered SCA27B—are caused by repeat expansions hidden in the “introns,” which are the non-coding spaces between your genes ^[12][13]. Because standard WES only looks at coding regions, it completely ignores the areas where these specific mutations hide ^[14][15]. Even short-read Whole Genome Sequencing (WGS), which does look at non-coding areas, still struggles because of the short read lengths and knots mentioned above. Standard WES only provides a clear diagnosis in less than half of ataxia cases, in part because it simply cannot see these hidden repeat expansions ^[16].

The Right Tools for the Job

Because standard broad tests miss these expansions, doctors rely on specialized testing methods to confirm an SCA diagnosis.

• Targeted Repeat Expansion Panels: These are highly focused tests (such as Repeat-Primed PCR) designed specifically to push through DNA knots. They use special enzymes and chemicals that “unwind” the problematic knots to accurately count the exact number of repeats in known SCA genes ^[17].
• Long-Read Sequencing: This is a newer, advanced technology that can read long, continuous strands of DNA without shredding them into tiny pieces ^[18]. Long-read sequencing can easily scan across massive repeat expansions and non-coding regions without getting confused ^[19][20].

If your doctor strongly suspects SCA despite a negative exome test, you may simply need a different type of test designed to find these hidden mutations. If standard clinics cannot perform long-read sequencing, your doctor can often refer you to a neurogenetics clinic, an ataxia specialty center, or a university research hospital where these specialized tools are available.

It is also important to remember that genetics is a rapidly evolving field; even with the most advanced testing, the exact genetic cause sometimes remains undiscovered (idiopathic). Knowing this can help you and your care team emotionally prepare for all outcomes and maintain focus on symptom management and future research trials.