At 36 to 39 repeats, incomplete or reduced penetrance allele (RPA) may cause symptoms, usually later in the adult life. The maximum risk is 60% that a person with an RPA will be symptomatic at age 65, and 70% at 75. At 27 to 35 repeats, intermediate allele (IA), or large normal allele, is not associated with symptomatic disease in the tested individual, but may expand upon further inheritance to give symptoms in offspring. With 26 or fewer repeats, the result is not associated with HD.Testing before the onset of symptoms is a life-changing event and a very personal decision. The main reason given for choosing to test for HD is to aid in career and family decisions. Predictive testing for Huntingtons disease has been available via linkage analysis (which requires testing multiple family members) since 1986 and via direct mutation analysis since 1993. At that time, surveys indicated that 50–70% of at-risk individuals would have been interested in receiving testing, but since predictive testing has been offered far fewer choose to be tested. Over 95% of individuals at risk of inheriting HD do not proceed with testing, mostly because it has no treatment. A key issue is the anxiety an individual experiences about not knowing whether they will eventually develop HD, compared to the impact of a positive result. Irrespective of the result, stress levels are lower two years after being tested, but the risk of suicide is increased after a positive test result.
This causes the number of repeats to change in successive generations, such that an unaffected parent with an "intermediate" number of repeats (28–35), or "reduced penetrance" (36–40), may pass on a copy of the gene with an increase in the number of repeats that produces fully penetrant HD. The earlier age of onset and greater severity of disease in successive generations due to increases in the number of repeats is known as genetic anticipation. Instability is greater in spermatogenesis than oogenesis; maternally inherited alleles are usually of a similar repeat length, whereas paternally inherited ones have a higher chance of increasing in length. Rarely is Huntingtons disease caused by a new mutation, where neither parent has over 36 CAG repeats.In the rare situations where both parents have an expanded HD gene, the risk increases to 75%, and when either parent has two expanded copies, the risk is 100% (all children will be affected). Individuals with both genes affected are rare. For some time, HD was thought to be the only disease for which possession of a second mutated gene did not affect symptoms and progression, but it has since been found that it can affect the phenotype and the rate of progression. Mechanisms
Huntingtin protein interacts with over 100 other proteins, and appears to have multiple functions. The behavior of the mutated protein (mHtt) is not completely understood, but it is toxic to certain cell types, particularly brain cells.