The Genetics and Subtypes of Huntington-Like Syndromes

When a patient shows the classic signs of Huntington’s Disease—involuntary movements, cognitive changes, and mood shifts—but tests negative for the HTT gene mutation, they are said to have a phenocopy ^[1].

A phenocopy is a medical “look-alike.” It is a condition that mimics the outward appearance of another disease but is caused by an entirely different genetic “instruction” ^[2][1]. In the case of Huntington Disease-Like (HDL) syndromes, your ancestry and family history are the most important clues doctors use to find the specific genetic cause. Discovering the precise genetic cause is crucial, not just for your treatment plan, but because it has immediate implications for the risks to your biological children and siblings.

How Your Ancestry Points to the Cause

Genetics are often tied to geography. Because certain mutations first appeared in specific populations, your ethnic background can make some HDL subtypes much more likely than others.

African Ancestry: HDL2

Huntington Disease-Like 2 (HDL2) is the most common mimic found in people of African descent ^[2]. It is caused by a repeat expansion in a gene called JPH3 ^[3]. Clinically, it is almost impossible to tell apart from classic HD without a genetic test ^[3].

European Ancestry: C9orf72

In people of European descent, the most frequent cause of an HD phenocopy is an expansion in the C9orf72 gene ^[4]. While this mutation is more famous for causing ALS (Lou Gehrig’s disease) or Frontotemporal Dementia, it can also present with the involuntary movements (chorea) typical of Huntington’s ^[5][6].

Asian Ancestry: SCA17 (HDL4)

Spinocerebellar Ataxia Type 17 (SCA17), also known as HDL4, is caused by a mutation in the TBP gene ^[7]. While it occurs globally, it has a notable presence in East Asian populations, including Chinese, Korean, and Japanese families ^[8]. It often includes ataxia—a specific type of clumsiness or “drunken” walk ^[9].

Rare and Global Subtypes

Some syndromes are “ultra-rare” and do not seem to cluster in any specific ethnic group.

• HDL1: Caused by a mutation in the PRNP gene (the prion protein gene) ^[10]. It is extremely rare and can progress much faster than classic HD ^[10].
• Neuroacanthocytosis (ChAc and McLeod): These syndromes involve misshapen red blood cells (“acanthocytes”) and are caused by mutations in the VPS13A or XK genes ^[11]. They are found globally and often involve unique symptoms like seizures or involuntary tongue biting ^[11][12].

Comparing the Genetic “Blueprints”

Understanding the biology helps explain why these diseases look so similar. Most involve “repeat expansions”—places where a tiny piece of the genetic code stutters and repeats itself too many times, eventually creating a protein that is toxic to brain cells ^[13].

It is incredibly important to understand the inheritance pattern of your specific syndrome.

• Dominant inheritance means you only need one copy of the mutated gene to develop the disease, giving your children a 50% chance of inheriting it.
• Recessive inheritance means you must inherit two copies (one from each parent) to get the disease. If you have a recessive syndrome, your children will only inherit the disease if your partner is also a carrier (which gives a 25% chance of a child having the disease).

Note: HDL3 was a term once used for a single family in 2001, but a specific gene was never found, and it is no longer a standard diagnosis ^[10].