The Genetics of HDL1 and Family Risks

Genetic testing for Huntington disease-like 1 (HDL1) provides a definitive answer, but it also brings complex information about family risks. Because HDL1 is an inherited prion disease, understanding the specific “instruction error” in the DNA can help you and your family make informed decisions about the future ^[1][2].

The Genetic ‘Instruction Error’

HDL1 is caused by a mutation in the PRNP gene. This gene provides instructions for making the prion protein. In HDL1, a specific section of the gene called an octapeptide repeat is accidentally copied and inserted several extra times ^[1].

These are known as octapeptide-repeat insertions (OPRI). The most common versions in HDL1 involve 8 extra repeats (8-OPRI) or 5 extra repeats (5-OPRI) ^[1][3]. While insertions of 4 or more repeats are clearly known to be pathogenic (cause disease), smaller insertions (like 2-OPRI) are much less certain and may not always lead to symptoms. Interpretation alongside clinical symptoms by a specialist is always required ^[4].

How HDL1 is Inherited

HDL1 follows an autosomal dominant inheritance pattern ^[2]. This means:

• One Copy is Enough: A person only needs one pathogenic copy of the mutated gene (from either parent) to be at risk for the disease ^[2].
• The 50/50 Chance: Each child of a parent with one pathogenic copy of the mutation has a 50% (1 in 2) chance of inheriting that mutation ^[2].
• Apparent “Skipping”: Biologically, the mutation does not skip generations. If a child does not inherit the mutation, they cannot pass it on to their own children. However, because onset age varies widely, misdiagnoses are common, and some relatives may pass away from other causes before symptoms begin, the disease can sometimes appear to skip a generation in family history ^[2].

Factors Influencing the Disease

Researchers have noted factors that may influence how the disease looks and when it starts. Crucially, these are group-level trends based on very small case series, not reliable predictions for an individual. Even a specialist cannot predict your exact timeline. ^[5].

• Repeat Length: Generally, having a higher number of extra repeats (like 8 vs 5) has been associated in small studies with an earlier age of onset and a longer total disease duration ^[5].
• Codon 129: This is a naturally occurring variation in the PRNP gene that everyone has. You have two alleles for this spot and can be MM, MV, or VV ^[5][6].
• The Influence of ‘V’: Research suggests that having a ‘V’ at codon 129 on the exact same chromosome as the mutation (called being in cis) may be linked to a later age of onset and a faster disease course compared to having an ‘M’ ^[5].

The Importance of Genetic Counseling

Deciding whether to test at-risk family members who do not yet have symptoms (presymptomatic testing) is a major life decision. Because there is currently no cure, the decision is deeply personal and can be emotionally heavy ^[7][8].

Medical guidelines strongly recommend genetic counseling before and after any testing, adapting best-practice components to the person and local protocol ^[9][10]. A genetic counselor can help you:

1. Understand the Risks: Clarify the 50% inheritance risk and what the results would mean for employment, insurance (life, disability, and long-term care), and confidentiality, which vary heavily by jurisdiction ^[9].
2. Explore the ‘Right Not to Know’: Many people choose not to be tested, preferring to live without the burden of a definitive result. This choice is valid and respected by medical professionals ^[7]. Predictive testing is also generally deferred for asymptomatic minors until they are adults capable of informed consent, given it is an adult-onset condition.
3. Plan for the Future: Discuss reproductive options, such as preimplantation genetic testing (PGT-M). PGT-M involves IVF and allows families to select embryos without the mutation, though it is expensive, may not be available everywhere, and does not guarantee a successful pregnancy or a perfectly healthy child ^[9].