Fukuyama-type congenital muscular dystrophy (FCMD), one of the most common autosomal recessive disorders in Japan (incidence is 0.7-1.2 per 10,000 births), is characterized by congenital muscular dystrophy associated with brain malformation (micropolygria) due to a defect in the migration of neurons. We previously mapped the FCMD gene to a region of less than 100 kilobases which included the marker locus D9S2107 on chromosome 9q31. We have also described a haplotype that is shared by more than 80% of FCMD chromosomes, indicating that most chromosomes bearing the FCMD mutation could be derived from a single ancestor. Here we report that there is a retrotransposal insertion of tandemly repeated sequences within this candidate-gene interval in all FCMD chromosomes carrying the founder haplotype (87%). The inserted sequence is about 3 kilobases long and is located in the 3' untranslated region of a gene encoding a new 461-amino-acid protein. This gene is expressed in various tissues in normal individuals, but not in FCMD patients who carry the insertion. Two independent point mutations confirm that mutation of this gene is responsible for FCMD. The predicted protein, which we term fukutin, contains an amino-terminal signal sequence, which together with results from transfection experiments suggests that fukutin is a secreted protein. To our knowledge, FCMD is the first human disease to be caused by an ancient retrotransposal integration.

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant, neurodegenerative disorder that affects the cerebellum and other areas of the central nervous system. We have devised a novel strategy, the direct identification of repeat expansion and cloning technique (DIRECT), which allows selective detection of expanded CAG repeats and cloning of the genes involved. By applying DIRECT, we identified an expanded CAG repeat of the gene for SCA2. CAG repeats of normal alleles range in size from 15 to 24 repeat units, while those of SCA2 chromosomes are expanded to 35 to 59 repeat units. The SCA2 cDNA is predicted to code for 1,313 amino acids-with the CAG repeats coding for a polyglutamine tract. DIRECT is a robust strategy for identification of pathologically expanded trinucleotide repeats and will dramatically accelerate the search for causative genes of neuropsychiatric diseases caused by trinucleotide repeat expansions.

Hereditary progressive dystonia with marked diurnal fluctuation (HPD) (also known as dopa responsive dystonia) is a dystonia with onset in childhood that shows a marked response without any side effects to levodopa. Recently the gene for dopa responsive dystonia (DRD) was mapped to chromosome 14q. Here we report that GTP cyclohydrolase I is mapped to 14q22.1-q22.2. The identification of four independent mutations of the gene for GTP cyclohydrolase I in patients with HPD, as well as a marked decrease in the enzyme's activity in mononuclear blood cells, confirms that the GTP cyclohydrolase I gene is a causative gene for HPD/DRD. This is the first report of a causative gene for the inherited dystonias.

Autosomal dominant guanosine triphosphate cyclohydrolase I (GCH-I) deficiency (Segawa disease) is a dopa-responsive dystonia caused by mutation of the GCH-I gene located on 14q22.1-q22.2. Neurohistochemical examination revealed a decrease of the tyrosine hydroxylase protein as well as its activity in the striatum and decrease of dopamine content, particularly in its ventral portion rich in D1 receptors (striatal direct pathways). Neuroimaging, clinical neurophysiological, and biochemical studies showed preservation of the structure and function of the terminal of the nigrostriatal DA neuron. Clinical neurophysiological studies showed no progressive decrement of DA activities. As the enzymatic activity of pteridine metabolism is highest in the early developmental course, it may modulate dopamine receptors maturing early in the developmental course. Its product, tetrahydrobiopterin, has higher affinity to tyrosine hydroxylase among hydroxylases. Thus, partial deficiency of tetrahydrobiopterin caused by heterozygous mutation of the GCH-I gene decreases dopamine activity rather selectively. This affects the DA receptors that mature early and demonstrates characteristic symptoms age-dependently along with the developmental decrement of the tyrosine hydroxylase activities at the terminals and the maturational processes of the projecting neurons of the basal ganglia. A difference in the ratio of mutant/wild-type GCH-I mRNA that depends on the locus of mutation may explain intrafamilial and interfamilial variation of phenotype.

Hereditary progressive dystonia with marked diurnal fluctuation or the strictly defined dopa-responsive dystonia (HPD/DRD) is an autosomally dominantly inherited dystonia caused by abnormalities of the gene of the GTP cyclohydrolase I (GCH 1) located on the 14q22. 1-q22.2. The heterozygotic gene abnormality induces partial decrement of tetrahydrobiopterin (BH4) and affects synthesis of tyrosine hydroxylase (TH) rather selectively. The reduction of TH exists at the terminals of the nigrostriatal (NS) dopamine (DA) neuron, predominantly in the ventral area of the striatum and disfacilitates the D1 receptor-striatal direct pathway. This consequently disinhibit the inhibitory efferent pathways and develops postural dystonia via the particular descending pathways to the reticulospinal tract and postural tremor via the ascending pathways to the ventralis lateralis (VL) nucleus of the thalamus. This also inhibits the efferents to the superior colliculus, and affects voluntary saccade but spares that to the pedunculo-pontine nucleus (PPN) preserving locomotive movement clinically. The DA-D2 receptors, the striatal indirect pathways or the efferent connecting to these pathways are not involved in the pathophysiology of HPD/DRD. So parkinsonian plastic rigidity, parkinsonian resting tremor, cogwheel rigidity or levodopa induced dyskinesia are not observed. In some patients, particularly in compound hetereozygotes, there are symptoms suggesting the involvement of serotonergic neurons or those thought to be caused by exaggeration of DA-D2 receptors. Neuropathologically there is no degenerative changes. Clinical laboratory examinations suggest that levels of TH and DA activities are around 20% of the normal values throughout the course of illness. Therefore, the age-dependent clinical course, marked progression in the first one and one half decades, its subsiding in the third decade and almost stationary course from the fourth decade are just the reflection of age-related decremental variation of the TH activities at the terminal of the normal NS-DA neuron. The diurnal fluctuation is also the reflection of circadian oscillation of the TH activities at the terminal. Functional maturation of the striatal indirect pathways in the first one and one half decades and developmental decremental variation of the DA-D2 receptor in the first three decades also reflect in the age-dependent variation of symptoms by modulating the background tone of muscle. The later functional development of the ascending efferents of the basal ganglia to the thalamus, may cause the postural tremor which appears in the second decade and becomes predominant in the fourth decade. Early decrease of TH due to deficiency of BH4 in HPD/DRD also affects the DA-D4 receptor of the tuberoinfundibular DA neuron and cause stagnation of increase of body length in childhood. With normal preservation of the fundamental function of the NS-DA neuron, levodopa, by replacing the DA content at the terminal, alleviates the motor symptoms completely and the effects sustain wit...