Adult-onset leukoencephalopathy involving the white matter of the brain is a heterogeneous disorder that exhibits a wide range of clinical manifestations. Recent advances in molecular genetics enable gene-based diagnosis of some forms of adult-onset leukoencephalopathy. In this review, the classification of adult-onset leukoencephalopathy based on molecular genetic findings is proposed. The autosomal dominant forms of adult-onset leukoencephalopathy include hereditary diffuse leukoencephalopathy with spheroids (HDLS), autosomal dominant adult-onset leukoencephalopathy (ALDL), cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), and Alexander disease. The autosomal recessive forms of adult-onset leukoencephalopathy include cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL), vanishing white matter (VWM) with leukoencephalopathy, Nasu-Hakola disease, and metachromatic leukodystrophy (MDL). X-chromosome-linked disorders include fragile X-associated tremor and ataxia syndrome (FXTAS) and adrenoleukodystrophy (ALD). Identification of the genes responsible for adult-onset leukoencephalopathy provides an important clue for elucidation of molecular pathophysiology underlying white matter disorders. One example is the identification of mutations in colony stimulating factor 1 receptor (CSF-1R) in patients with HDLS. Missense and splice site mutations have been found in the tyrosine kinase domain of CSF-1R. CSF-1R is highly expressed in microglia in the brain. It has been demonstrated that mice depleted of CSF-1R exhibit loss of microglia in the brain. In addition, stimulation of IL-34, a ligand of CSF-1R, induces proliferation and activation of microglia. These findings raise an intriguing possibility that dysfunction of microglia may play a role in the pathogenesis of white matter lesions occurring in patients with HDLS.

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By reviewing and collating data in a 2-step postal survey sent to all of the institutions for individuals with dementia in Ibaraki prefecture requesting information on early-onset dementia (EOD) cases, 617 subjects with EOD were identified. The estimated prevalence of EOD in the target population was 42.3 per 100,000. Of the illness causing EOD, vascular dementia was the most frequent followed by Alzheimer's disease, head trauma, dementia with Lewy body/Parkinson's disease with dementia, frontotemporal lobar degeneration, and other causes. On the other hand, hereditary diffuse leucoencephalopathy with spheroids (HDLS) is an autosomal-dominant central nervous system white matter disease with variable phenotypes. The onset of symptoms is usually in the fourth or fifth decade, progressing to dementia with death within 6 years. Recently, several mutations of the colony stimulating factor 1 receptor encoded by CSF1R segregating HDLS were identified. Since clinical presentations varied substantially within and across families with HDLS, CSF1R mutation carriers may be present in clinical series of Alzheimer's disease, frontotemporal lobar degeneration, corticobasal syndrome, multiple sclerosis, CADASIL, Parkinson's disease and ischemic stroke with additional white matter changes, all causing EOD. In the differential diagnosis of EOD, we should always consider HDLS and if necessary perform CSF1R gene analysis.

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The understanding of the pathomechanism of myotonic dystrophy (DM) has been greatly improved since the recognition as an mRNA disease. Pre-mRNA containing repeats sequesters or activates proteins such as MBNL and CELF that bind to an mRNA motif similar to repeat. Consequently, the regulation of mRNA splicing, a normal function of these proteins, is perturbed. Over 30 miss-splicing events have been documented including muscle chloride channel which is responsible for myotonia. Such molecular events might serve as a target for therapeutic intervention.
An important genetic feature of DM is the instability of expanded repeats between generations and organs. Since pathogenesis is connected to repeat length, manipulation of the repeat expansion size (somatic instability) might be also a potential therapeutic strategy.
With accumulation of pathomechanistic studies, clinical trials are highly expected. Our recent survey in Osaka Japan revealed the need of standardized management employing currently available therapies, which should be prerequisite for trials. Clinical trials for DM will face challenges including lack of reliable outcome measures and enrolment of highly restricted cohort. Global initiative to form international DM registry have been taken to facilitate natural history studies and trials. In Japan, the development of DM registry has just started.

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