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New insights into genetic white matter disorders

Leukodystrophies and leukoencephalopathies, a heterogeneous set of conditions now often referred to as ‘genetic white matter disorders’ (GWMDs), have been a problematic set of conditions for diagnosis and treatment. Three major phases have marked studies into GWMDs. First, with the advent of widespread use of magnetic resonance imaging (MRI) in the 1990s and early 2000s, GWMDs were recognized as distinct clinical and disease entities. The second and third overlapping phases have occurred over the past 15 years: the field has focused on improving diagnosis of these conditions including newborn infant screening; and on using epidemiological and database approaches to determine the incidence, prevalence, and disease burden of GWMD. Knuutinen et al. provide important insights into GWMDs. In this long-term, longitudinal, populationbased study of pediatric GWMDs, the authors report on 80 patients with 30 years of data. They report a birth incidence of 30 per 100 000 live births (1 in 3333 live births), using natural history and data collection from northern Finland. Impressively, this incidence corresponds very closely to two other recent estimates. Thus, using different approaches and in different populations, and considering both ‘classic’ GWMDs (such as adrenoleukodystrophy) and more recently identified conditions (e.g. tRNA synthetase gene mutations), the field has reached a relative consensus of a birth incidence of close to 1 in 3000 live births for GWMDs. Knuutinen et al. did note an increased rate of GWMD diagnosis in the third decade of their study, potentially suggesting that increasing familiarity with GWMDs, and use of more diagnostic testing (MRI, genetic testing), could have a significant effect on incidence determination. An important implication arising from this and related publications is that GWMDs may occur at a relatively standard incidence in any given population. This arises from the closely matching incidences, despite the different methods of calculation and different distribution of GWMD disease types. For example, in this study, there were no cases of Pelizaeus-Merzbacher disease or leukoencephalopathy with vanishing white matter. The similar incidences suggests that mutation rates and risk of GWMD is a relative constant. With the study from Knuutinen et al. and others, I believe we are ending the earlier, descriptive phases of GWMD study. This new phase will be of developing and delivering therapies, a goal we can all be excited about.