Heterogeneity of X-linked recessive (spino)cerebellar ataxia with or without spastic diplegia

Hereditary ataxias

There are many causes of hereditary ataxia. These can be grouped into categories of autosomal recessive, autosomal dominant, and X-linked. Molecularly, many of them are due to trinucleotide repeat expansions. In Friedreich ataxia, the trinucleotide repeat expansions lead to a "loss of function." In the dominant ataxias, the expanded repeats lead to a "gain of function," most likely through accumulation of intranuclear (and less commonly cytoplasmic) polyglutamine inclusions. Channelopathies can also lead to ataxia, especially episodic ataxia. Although phenotypic characteristics are an aid to the clinician, a definitive diagnosis is usually made only through genotypic or molecular studies. Genetic counseling is necessary for the testing of symptomatic and asymptomatic individuals. No effective treatment is yet available for most ataxic syndromes, except for ataxia with isolated vitamin E deficiency and the episodic ataxias.

Multi-system signs and symptoms in X-linked ataxia carriers

Neurological, auditory, vestibular and ocular motor examinations were performed on 3 definite and 3 possible heterozygous carriers of a previously described X-linked multi-system disorder with early childhood onset, rapid progression and a fatal outcome (Arts et al., 1993). The symptoms, i.e., delayed motor development, ataxia, hearing loss and subnormal intelligence, were so evident in 2 of the possible carriers that they could be redesignated as probable carriers. Other symptoms in the definite and probable carriers were clubfeet, dysarthria, intention tremor and abnormal gait, while their signs included dysdiadochokinesia, ataxic paraplegia, abnormal muscle tendon reflexes and extensor plantar responses. All the symptomatic carriers developed moderate-to-severe sensorineural hearing loss with normal stapedial reflexes and brain stem auditory evoked potentials (BAEPs) in those in whom this could be evaluated. Speech discrimination was disproportionally poor unilaterally in one case from whom no BAEPs could be obtained because of her degree of hearing loss. Various combinations were found of high gain of the vestibulo-ocular reflex, spontaneous nystagmus and directional preponderance of vestibularly evoked nystagmus, slowing, hypometria or multi-stepping of saccades, saccadic intrusions of eye movements (macro square wave jerks, double saccadic pulses), impairment of smooth pursuit eye movements and optokinetic nystagmus, and failure of visual fixation suppression of vestibularly evoked nystagmus. Such findings indicate major involvement of the (vestibulo)cerebellum and the vermis. MRI in one carrier showed mild cerebellar atrophy.

X-linked nonprogressive congenital cerebellar hypoplasia: clinical description and mapping to chromosome Xq

We examined a large family in which an X-linked recessive congenital ataxia manifested in 7 males from three generations. The affected boys first exhibited a marked delay of early developmental motor milestones. A neurological syndrome became evident by 5 to 7 years of age and included cerebellar ataxia, dysarthria, and external ophthalmoplegia; there were no symptoms of mental retardation, spastic paraparesis, or sensory loss. Neuroimaging studies revealed hypoplasia of cerebellar hemispheres and vermis. The disease showed no progression beyond early childhood. The unique heredity and clinical features clearly distinguish this new entity from a variety of previously described familial ataxias. Pairwise linkage analysis and haplotype reconstruction allowed us to map the gene responsible for this disorder to a 38-cM interval on chromosome Xp11.21-q24 flanked by the loci DXS991 and DXS1001. Upon multipoint linkage analysis, the disease gene was determined to be located most likely in the proximal part of chromosome Xq, with the maximal lod score of 4.66 at the locus DXS1059 (Xq23). This is the first example of the genetic mapping of a pure congenital cerebellar hypoplasia syndrome.

X-linked spastic paraplegia and Pelizaeus-Merzbacher disease are allelic disorders at the proteolipid protein locus

Three forms of X-linked spastic paraplegia (SPG) have been defined. One locus (SPG 1) maps to Xq28 while two clinically distinct forms map to Xq22 (SPG2). A rare X-linked dysmyelinating disorder of the central nervous system, Pelizaeus-Merzbacher disease (PMD), has also been mapped to Xq21-q22, and is caused by mutations in the proteolipid protein gene (PLP) which encodes two myelin proteins, PLP and DM20. While narrowing the genetic interval containing SPG2 in a large pedigree, we found that PLP was the closest marker to the disease locus, implicating PLP as a possible candidate gene. We have found that a point mutation (His139Tyr) in exon 3B of an affected male produces a mutant PLP but a normal DM20, and segregates with the disease (Zmax = 6.63, theta = 0.00). It appears, therefore, that SPG2 and PMD are allelic disorders.

X linked spastic paraplegia (SPG2): clinical heterogeneity at a single gene locus

X linked hereditary spastic paraplegia is a rare condition that has been divided into two forms (the pure spastic form and the complicated form) as a function of clinical course and severity. A gene for pure hereditary spastic paraplegia (SPG2) has been mapped to the proximal long arm of the X chromosome (Xq21) by linkage to the DXS17 locus, while a gene for a complicated form of the disease has been mapped to the distal long arm by linkage to the DXS52 locus (Xq28). Here we report on the mapping of a gene for complicated hereditary spastic paraplegia to the Xq21 region by linkage to the probe S9 at the DXS17 locus (Z = 5 at theta = 0.04) in a three generation pedigree. Multipoint linkage analysis supports the distal location of the disease gene with respect to the DXYS1-DXS17 block (cen-DXYS1-DXS3-DXS17-SPG2-tel). The observation of a complicated form of spastic paraplegia mapping to Xq21 raises the difficult issue of variable phenotypic expression, allelic heterogeneity, or even close proximity of two genes for hereditary spastic paraplegia in this region. However, since our study provides clinical evidence for intrafamilial heterogeneity in complicated X linked spastic paraplegia, the present data support the hypothesis of variable clinical expression of a single gene at the SPG2 locus, as previously suggested for SPG1. Finally, we report here what we believe to be the first evidence of clinical expression in heterozygous carriers, a feature that is relevant to genetic counselling in at risk females.

Congenital X-linked ataxia, progressive myoclonic encephalopathy, macular degeneration and recurrent infections

We report on 2 boys (maternal cousins), with severe congenital ataxia with generalized hypotonia, psychomotor retardation and recurrent bronchopulmonary infections. Later, they developed myoclonic encephalopathy and macular degeneration. Serial brain imaging investigations showed a cyst of the septum pellucidum, persistence of the cavum vergae, corpus callosum and cerebellar vermis hypoplasia without cortical atrophy. In the maternal pedigree, 5 males had recurrent bronchopneumonia associated with severe congenital hypotonia and died during the first years of life. Neurophysiological studies, including nerve conduction velocities, brainstem auditory evoked responses, somatosensory evoked potentials were normal. Electroretinogram showed normal wave morphology. Visual evoked potentials were mildly impaired. Extensive screening for metabolic disease gave normal results. Immunologic investigations showed normal T and B cell number, T cell function and immunoglobulin levels in both patients with a reduced level of IgG2 subclass in one.

Spastic paraplegia with iron deposits in the basal ganglia: a new X-linked mental retardation syndrome

We report on a family with X-linked mental retardation (XLMR) and severe spastic paraplegia. Appearance is normal but there is severe involvement of the lower limbs (affected relatives never walked), with minimal involvement of the upper limbs and unusual MRI findings including macrogyria, white matter hypoplasia, lack of myelination and a markedly increased paramagnetic signal suggestive of iron deposition. Linkage studies documented possible linkage, with no recombination, between the disease locus and DXS424. A 7-point linkage analysis yielded a maximum LOD score of 1.9, (theta = 0.00) for three loci spanning Xq22-q25. The combination of the unusual clinical and MRI findings and the tentative localization to a region different than other XLMR syndromes with spastic paraplegia, provide good evidence that this is a new XLMR syndrome.

MASA syndrome: clinical variability and linkage analysis

We report on a family with three males with MASA syndrome (mental retardation, aphasia, shuffling gait, and adducted thumbs). One patient demonstrated spastic paraplegia and psychomotor retardation but no adducted thumbs. The described family underlines the clinical variability in MASA syndrome. DNA studies confirm linkage to DNA markers of the Xq28 region. Analysis of published cases with hereditary spastic paraplegia (HSP), where linkage studies have been carried out, emphasizes the clinical variability in MASA syndrome and other types of HSP, thus making a definite diagnosis in single cases often impossible.