The Science of HDL2: Genetics and Biology

While Huntington disease-like 2 (HDL2) looks almost exactly like Huntington’s Disease (HD) on the outside, the “blueprint” of the disease—the genetics—is found on a completely different chromosome. Understanding the biology of HDL2 helps explain why it is so rare and why it often requires a specific search to find.

The Genetic Blueprint: JPH3 vs. HTT

Most people with “Huntington-like” symptoms are first tested for a mutation in the HTT gene on chromosome 4 ^[1]. If that test is negative, doctors look at the JPH3 (junctophilin-3) gene on chromosome 16 ^[2].

In HDL2, a specific section of the JPH3 gene contains too many repeats of a three-letter genetic code. You may see this written as CAG or CTG on lab reports; these are not two different mutations, but rather two ways of describing the complementary strands of the same DNA segment ^[2][3]. This is known as a trinucleotide repeat expansion. While everyone has some repeats in this gene, people with HDL2 have an expanded number that disrupts how the cell functions ^[2].

How the Mutation Affects the Brain

The JPH3 gene provides the instructions for making a protein called junctophilin-3 ^[4]. This protein acts like a bridge or a “staple,” holding different parts of a brain cell’s internal machinery together so they can communicate properly ^[4][3]. Research suggests that when the gene is mutated, the brain may be damaged through a combination of mechanisms that are still being studied:

• RNA Toxicity (Gain-of-Function): The “messenger” molecules carrying the genetic instructions may trap other important proteins, preventing them from doing their jobs ^[5][3].
• Protein Aggregates: The mutation might produce abnormal proteins that clump together into toxic “knots” (aggregates) inside the cell’s nucleus, contributing to cell injury ^[6][7].
• Loss of Function: Because the gene is damaged, the cell may fail to produce enough healthy junctophilin-3 protein, breaking the communication bridges the brain needs to control movement and mood ^[3][8].

The Path to Diagnosis

The most common “pitfall” in diagnosing HDL2 is stopping the search after a negative Huntington’s test ^[1]. Because the two diseases are phenocopies—meaning they look identical in a clinical exam—specific genetic testing for JPH3 is required ^[9][1].

Current diagnostic evaluations usually involve:

1. HTT Testing First: Rule out standard Huntington’s Disease ^[9].
2. JPH3 Testing: If HTT is negative or nondiagnostic, JPH3 testing should be considered based on symptoms, specialist assessment, and family history. While it is strongly associated with African ancestry, testing should not be denied to patients who do not identify with that ancestry if their symptoms strongly suggest the disease ^[1][10].
3. Broad Screen: If JPH3 is also negative, doctors may look for other rare mimics like SCA17, C9orf72, or DRPLA using broader genetic panels ^[11][9].

Inheritance and Genetic Counseling

HDL2 follows an autosomal dominant inheritance pattern ^[2]. This means that if a person has the expansion, there is a 50% chance of passing it to each child in every pregnancy ^[12].

A feature of repeat expansion diseases is anticipation. As the gene is passed from parent to child, the number of repeats can sometimes grow larger ^[13]. Generally, a higher number of repeats has been associated with an earlier age of onset ^[14][2]. However, in a rare condition like HDL2, repeat sizes and anticipation patterns are based on limited data. A repeat count cannot be used to precisely forecast an individual’s age of onset, severity, or exact course ^[13].

Adult predictive testing for relatives should always be voluntary and accompanied by professional genetic counseling to explore family implications and privacy concerns.

Understanding Your Genetic Report

A genetic test for HDL2 is complex. When you receive your report, ensure it includes these key pieces of data to discuss with your doctor or genetic counselor:

Because research into HDL2 is still ongoing, intermediate or borderline repeat results can be difficult to interpret and require expert medical-genetics guidance ^[12][16].