Understanding Your HDL1 Diagnosis

Receiving a diagnosis of Huntington disease-like 1 (HDL1) often marks the end of a long and confusing journey. Many families spend years searching for answers, frequently being told they have Huntington’s disease (HD) because the symptoms—involuntary movements, personality changes, and memory problems—look so similar ^[1][2]. Finding out your diagnosis is actually HDL1 is a pivotal moment that provides a clear genetic explanation for what you have been experiencing.

What is HDL1?

HDL1 is a very rare, inherited prion disease ^[1]. While it shares a name with Huntington’s disease, it is caused by a completely different genetic instruction. In Huntington’s disease, the problem lies in a gene called HTT ^[3]. In HDL1, the issue is a mutation in the PRNP gene ^[1][4].

The specific mutation in HDL1 is usually an octapeptide-repeat insertion (OPRI), most commonly involving eight extra “repeats” of a specific genetic sequence ^[1], though others like 5-OPRI can sometimes be related. Think of it like a recipe where one instruction has been accidentally copied and pasted several extra times, causing the resulting protein to be misshapen.

Understanding Prion Diseases

The term prion disease refers to a group of conditions caused by the misfolding of proteins.

• Normal Proteins: We all have “prion proteins” in our brains that fold into a specific, healthy shape.
• Misfolded Proteins: In HDL1, the genetic variant—which is present from birth, even though symptoms start later—causes these proteins to fold into an abnormal shape ^[5].
• The Chain Reaction: These misfolded proteins can cause other healthy proteins nearby to also misfold. Over time, these abnormal proteins clump together, which interferes with how brain cells work ^[5][6].

It is important to understand that while some prion diseases (like “Mad Cow Disease”) are acquired through infection, HDL1 is strictly genetic ^[1]. You did not “catch” this from anything you ate or any person you touched. It is a condition that is passed down through families in an autosomal dominant pattern, meaning there is a 50% chance for a parent with one pathogenic copy to pass it to a child ^[1][7].

Why the Huntington’s Confusion?

HDL1 is often called a phenocopy of Huntington’s disease, which means it “copies” or mimics the appearance of the more common condition ^[1][8]. Because the symptoms are so similar, many patients only receive an HDL1 diagnosis after a standard Huntington’s genetic test comes back negative ^[4].

HDL1 is extraordinarily rare. While Huntington’s disease affects thousands of people, HDL1 has only been documented in a handful of families worldwide (based on limited literature, approximately five to six pedigrees have been formally reported) ^[1][9]. Because it is so rare, many doctors may never have seen a case before your family’s.

A Variable Journey

The way HDL1 progresses can vary significantly, even between members of the same family.

• Age of Onset: Symptoms have been reported in small case series to start as early as age 18 or as late as age 54 ^[1].
• Speed of Progression: For some, the disease may progress rapidly over just a few months. For others, it may be a much slower process lasting 20 years or more ^[1][9].
• Symptoms: While it often looks like Huntington’s—with chorea (fidgety, dance-like movements) and cognitive decline—some people may also experience seizures or more rapid changes in personality ^[1][9][10].

Living with the Diagnosis

One of the most common fears after hearing the words “prion disease” is the concern about being “contagious.” HDL1 is not contagious through casual contact. You cannot spread it to your family, friends, or caregivers through normal daily contact, such as hugging, sharing meals, or direct care, and home isolation is never needed ^[11][7].

Standard precautions are routinely used in healthcare settings. However, additional prion-specific precautions may be required for certain invasive medical procedures, such as brain or eye surgeries, where specialized cleaning protocols are used for surgical instruments and tissue handling ^[12][13]. You should always inform your healthcare teams, including dentists and surgeons, about your diagnosis before any invasive procedure. For day-to-day life, you can continue to interact with your loved ones exactly as you always have. Focus on working with your care team to manage symptoms and planning for the road ahead with the clarity that a definitive diagnosis provides.