F002 How many people have a Huntington disease expansion: a population-based prevalence study in Northern Scotland

<h3>Background</h3> Previous work has demonstrated a prevalence of 14.6 per 100,000 manifest Huntington’s disease (HD) patients and 8.3 per 100,000 identified pre-symptomatic gene carriers (IPGEC) in Northern Scotland. Many individuals at high risk of having the HTT gene expansion remain untested, with the exact number unknown. <h3>Aims</h3> To estimate how many people in Northern Scotland are at 50% risk of having the HTT gene expansion. These data would help with HD clinic service planning and in calculating how many people could access an effective treatment if available. <h3>Methods</h3> Clinical and pedigree records from the North of Scotland Genetic Clinic were examined to estimate numbers of manifest HD patients, IPGEC, and untested relatives at 50% risk. <h3>Results</h3> The prevalence of those at 50% risk living in Northern Scotland was 45.2 per 100,000. Every manifest HD patient has 4.4 relatives at 50% risk, and every patient with an HTT gene expansion has 2.9 relatives at 50% risk. There are around 415 (46.2 per 100,000) adults who could access an effective treatment if available, but this number is likely higher given there will be many individuals in our region who are unknown to us. <h3>Conclusions</h3> Despite high predictive testing rates, at least 2.2 adults are living with the <i>HTT</i> gene expansion for every one of the 14.5 per 100,000 manifest HD patients in our region. The variation in rates regionally, as well as modes of ascertainment, need to be considered in future service planning, including genetic counselling and testing, management, and treatment delivery.