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PubMed This is a summary of 7 peer-reviewed journal articles Updated
Genetics · Huntington's Disease

Does Huntington's Disease Get Worse in Each Generation?

At a Glance

Yes, Huntington's disease can appear at an earlier age and with more severe symptoms in each successive generation. This happens because the gene mutation that causes the disease can expand when passed from parent to child, especially when the gene is inherited from the father.

Huntington’s disease can indeed appear at an earlier age and cause more severe symptoms in each successive generation [1][2]. In genetics, this phenomenon is called genetic anticipation. It is one of the most common and understandably frightening concerns for families tracking Huntington’s disease across generations.

To understand why this happens, it is helpful to look at the gene that causes the disease.

The Role of CAG Repeats

Huntington’s disease is caused by an expansion in the HTT gene. Inside this gene, a specific sequence of DNA building blocks—known as a CAG repeat—is repeated multiple times. The number of times this sequence repeats is crucial in determining the risk of the disease [3]:

  • Fewer than 26 repeats (Normal): The person will not develop Huntington’s disease, and their children are not at risk.
  • 27 to 35 repeats (Intermediate): The person will not develop the disease, but the gene is somewhat unstable. This means the repeat count could expand when passed to their children, putting the next generation at risk.
  • 36 to 39 repeats (Reduced Penetrance): The person may or may not develop symptoms in their lifetime. If they do, symptoms typically begin at an older age.
  • 40 or more repeats (Full Penetrance): The person will almost certainly develop Huntington’s disease if they live a normal lifespan.

For people who will develop the disease, the exact number of repeats is a strong predictor of when they will begin to show symptoms. In general, a higher CAG repeat count leads to an earlier age of symptom onset [4][5].

How the Repeat Count Changes Between Generations

When a parent with the Huntington’s gene passes it on to a child, the number of CAG repeats doesn’t always stay exactly the same. The DNA sequence can be unstable, meaning the repeat count can shrink or, more commonly, expand when it is passed down to the next generation [1].

If the repeat count expands and the child inherits a higher number of CAG repeats than their parent had, the child will likely develop symptoms at a younger age than the parent did [4].

Paternal vs. Maternal Inheritance

The risk of the CAG repeat count expanding depends heavily on which parent passes down the gene:

  • Paternal Inheritance (from the father): When the gene is inherited from the father, the CAG repeat sequence is highly unstable during the production of sperm [6][1]. Because of this instability, the repeat count is significantly more likely to undergo a large expansion when passed from father to child [7]. This large expansion can cause the child to develop symptoms much earlier than the father. Most cases of Juvenile Huntington’s disease (where symptoms begin before age 20) are the result of the gene being inherited from the father.
  • Maternal Inheritance (from the mother): When the gene is inherited from the mother, the repeat count is much more stable. The number of repeats might stay the same, decrease slightly, or increase slightly. While large expansions are rare, they are still possible. Typically, children who inherit the gene from their mother develop symptoms around the same age as their mother did, though some variation occurs.

Genetic Counseling and Family Planning

It is important to remember that while genetic anticipation is a real risk—particularly with paternal inheritance—it does not happen in every single case. The only way to know the exact number of CAG repeats an individual has inherited is through a specialized blood test.

Because of the heavy emotional weight of this disease, medical professionals strongly recommend undergoing genetic counseling before pursuing genetic testing. A genetic counselor can help you and your family navigate the complex psychological impacts of testing and understand what the results mean for you.

Additionally, if you are planning to have children, genetic anticipation does not mean you are without options. A genetic counselor can discuss family planning alternatives, such as Preimplantation Genetic Testing (PGT) used during In Vitro Fertilization (IVF). This procedure allows parents to ensure they are passing on an embryo that does not carry the Huntington’s disease gene expansion.

Common questions in this guide

Why does Huntington's disease start earlier in some generations?
Hunting's disease can start earlier in successive generations because the genetic mutation that causes it can expand when passed down. A higher number of these gene repetitions, known as CAG repeats, typically leads to symptoms beginning at a younger age.
What is the difference between inheriting Huntington's from my mother versus my father?
The gene mutation is highly unstable when produced in sperm, meaning fathers are much more likely to pass down a significantly expanded gene. This large expansion can cause children to develop symptoms much earlier than their fathers, whereas maternal inheritance usually keeps the gene more stable.
What are the CAG repeat numbers for Huntington's disease?
A person with 26 or fewer CAG repeats will not develop Huntington's disease, while those with 40 or more repeats will almost certainly develop the condition. Repeat counts between 27 and 39 carry varying degrees of risk for the individual and their future children.
How can I find out my exact CAG repeat count?
The only way to determine your exact CAG repeat count is through a specialized blood test. Medical professionals strongly recommend meeting with a genetic counselor before testing to help you prepare for the emotional impact of the results.
Can I prevent passing Huntington's disease to my children?
Yes, family planning options are available to prevent passing the gene to the next generation. Procedures like Preimplantation Genetic Testing (PGT) combined with In Vitro Fertilization (IVF) can be used to ensure an embryo does not carry the gene expansion.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What does my exact CAG repeat count mean for my potential age of symptom onset?
  2. 2.If we are considering having children, what family planning options (such as PGT-IVF) are available to prevent passing on the expanded gene?
  3. 3.Are there specific clinical trials available testing therapies aimed at stabilizing CAG repeats?
  4. 4.How often should I be monitored for early symptoms based on my genetic results?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

Related questions

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References

References (7)
  1. 1

    CAG repeat instability in embryonic stem cells and derivative spermatogenic cells of transgenic Huntington's disease monkey.

    Khampang S, Parnpai R, Mahikul W, et al.

    Journal of assisted reproduction and genetics 2021; (38(5)):1215-1229 doi:10.1007/s10815-021-02106-3.

    PMID: 33611676
  2. 2

    Somatic CAG Repeat Stability in a Transgenic Sheep Model of Huntington's Disease.

    Handley RR, Reid SJ, Burch Z, et al.

    Journal of Huntington's disease 2024; (13(1)):33-40 doi:10.3233/JHD-231516.

    PMID: 38393920
  3. 3

    Risk factors for the onset and progression of Huntington disease.

    Chao TK, Hu J, Pringsheim T

    Neurotoxicology 2017; (61()):79-99 doi:10.1016/j.neuro.2017.01.005.

    PMID: 28111121
  4. 4

    Behavioural changes in an adopted teenager.

    Soni A, Booysen G, Heckmann JM

    Practical neurology 2022; (22(2)):158-159 doi:10.1136/practneurol-2021-003236.

    PMID: 34824155
  5. 5

    Biological and clinical manifestations of juvenile Huntington's disease: a retrospective analysis.

    Fusilli C, Migliore S, Mazza T, et al.

    The Lancet. Neurology 2018; (17(11)):986-993 doi:10.1016/S1474-4422(18)30294-1.

    PMID: 30243861
  6. 6

    DNA mismatch repair in trinucleotide repeat instability.

    Guo J, Chen L, Li GM

    Science China. Life sciences 2017; (60(10)):1087-1092 doi:10.1007/s11427-017-9186-7.

    PMID: 29075942
  7. 7

    Genetic Contributors to Intergenerational CAG Repeat Instability in Huntington's Disease Knock-In Mice.

    Neto JL, Lee JM, Afridi A, et al.

    Genetics 2017; (205(2)):503-516 doi:10.1534/genetics.116.195578.

    PMID: 27913616

This page provides educational information about Huntington's disease inheritance and genetic anticipation. It is not intended as medical advice; always consult a genetic counselor or neurologist regarding genetic testing and family planning.

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