Topographic variabilities of immunoreactivity to subunit c of mitochondrial ATP synthase and lectin binding in late infantile neuronal ceroid-lipofuscinosis

Neuronal P/Q-type calcium channel dysfunction in inherited disorders of the CNS

The past two decades have witnessed the emergence of a new and expanding field of neurological diseases--the genetic ion channelopathies. These disorders arise from mutations in genes that encode ion channel subunits, and manifest as paroxysmal attacks involving the brain or spinal cord, and/or muscle. The voltage-gated P/Q-type calcium channel (P/Q channel) is highly expressed in the cerebellum, hippocampus and cortex of the mammalian brain. The P/Q channel has a fundamental role in mediating fast synaptic transmission at central and peripheral nerve terminals. Autosomal dominant mutations in the CACNA1A gene, which encodes voltage-gated P/Q-type calcium channel subunit α(1) (the principal pore-forming subunit of the P/Q channel) are associated with episodic and progressive forms of cerebellar ataxia, familial hemiplegic migraine, vertigo and epilepsy. This Review considers, from both a clinical and genetic perspective, the various neurological phenotypes arising from inherited P/Q channel dysfunction, with a focus on recent advances in the understanding of the pathogenetic mechanisms underlying these disorders.

Cellular pathology and pathogenic aspects of neuronal ceroid lipofuscinoses

Lysosomal accumulation of autofluorescent, ceroid lipopigment material in various tissues and organs is a common feature of the neuronal ceroid lipofuscinoses (NCLs). However, recent clinicopathologic and genetic studies have evidenced that NCLs encompass a group of highly heterogeneous disorders. In five of the eight NCL variants distinguished at present, genes associated with the disease process have been isolated and characterized (CLN1, CLN2, CLN3, CLN5, CLN8). Only products of two of these genes, CLN 1 and CLN2, have structural and functional properties of lysosomal enzymes. Nevertheless, according to the nature of the material accumulated in the lysosomes, NCLs in humans as well as natural animal models of these disorders can be divided into two major groups: those characterized by the prominent storage of saposins A and D, and those showing the predominance of subunit c of mitochondrial ATP synthase accumulation. Thus, taking into account the chemical character of the major component of the storage material, NCLs can be classified currently as proteinoses. Of importance, although lysosomal storage material accumulates in NCL subjects in various organs, only brain tissue shows severe dysfunction and cell death, another common feature of the NCL disease process. However, the relation between the genetic defects associated with the NCL forms, the accumulation of storage material, and tissue damage is still unknown. This chapter introduces the reader to the complex pathogenesis of NCLs and summarizes our current knowledge of the potential consequences of the genetic defects of NCL-associated proteins on the biology of the cell.

High prevalence of CACNA1A truncations and broader clinical spectrum in episodic ataxia type 2

OBJECTIVE

To characterize the nature of CACNA1A mutations in episodic ataxia type 2 (EA2), to search for mutations in sporadic cases, and to delineate better the clinical spectrum.

BACKGROUND

EA2 is an autosomal dominant disorder characterized by recurrent acetazolamide-responsive attacks of cerebellar ataxia. The mutated gene, CACNA1A, located on chromosome 19, encodes the alpha1A subunit of a voltage-dependent calcium channel. So far, only three CACNA1A mutations have been identified-in two EA2 families and in one sporadic case. These three mutations disrupted the reading frame and led to truncated proteins. Interestingly, distinct types of CACNA1A mutations have been identified in familial hemiplegic migraine (missense mutations) and spinocerebellar ataxia type 6 (SCA-6) progressive cerebellar ataxia (expanded CAG repeats). However, except for SCA-6, these genotype-phenotype correlations relied on the analysis of very few families.

METHODS

To characterize CACNA1A mutations, eight familial and seven sporadic EA2 patients were selected. All 47 exons of CACNA1A were screened by a combination of single-strand conformer polymorphism and sequencing analysis. In addition, the length of the CAG repeat has been determined in all patients.

RESULTS

Seven new mutations were detected in four multiple case families and three sporadic cases. Six of them lead most likely to truncated or aberrant proteins. CAG repeat sizes were in the normal range.

CONCLUSION

These data clearly establish the specificity of EA2 mutations compared with SCA-6 and familial hemiplegic migraine. Detailed clinical analysis of the mutation carriers showed the highly variable penetrance and expression of this disorder: Several of the carriers did not show any clinical symptom; others displayed atypical or permanent neurologic symptoms (such as recurrent, transient diplopia or severe, permanent, and isolated cerebellar ataxia).

De novo mutation in CACNA1A caused acetazolamide-responsive episodic ataxia

With the recent report of mutations in the calcium channel gene CACNA1A in two families with episodic ataxia type 2, we investigated a patient with nonfamilial episodic vertigo and ataxia responsive to acetazolamide for similar mutations. Single-strand conformation polymorphism (SSCP) analysis of exon 23 identified an extra band in the patient that was not present in other relatives or in normal controls. Exon 23 of the patient showed a spontaneous C to T substitution at position 4410 resulting in an early stop codon. Patients with nonfamilial episodic ataxia may respond to acetazolamide and may have mutations in CACNA1A.

Palmitoyl-protein thioesterase deficiency in a novel granular variant of LINCL

Typically, late infantile neuronal ceroid-lipofuscinosis (LINCL) patients present between the ages of 2 and 4 years with progressive dementia, blindness, seizures, and motor dysfunction. Curvilinear profiles are seen on electron microscopic examination of tissues derived from those patients. Data were collected on 122 LINCL cases, representing 81 independent families, diagnosed on the basis of age of onset, clinical symptomatology, and pathologic findings. Careful analysis of our data has revealed that 20% of these cases (24 of 122) show either an atypical clinical course or atypical pathologic findings and may represent variants of LINCL. Recent progress in the biochemistry and molecular genetics of NCL has led us to reevaluate these atypical cases. Five atypical LINCL cases (representing three independent families) manifested granular inclusions when examined by electron microscopy, a finding normally associated with the infantile form of NCL. In addition, these five cases did not show elevated subunit c levels in urine (typically seen in LINCL). In these five cases, palmitoyl-protein thioesterase activity was found to be deficient (less than 10% normal activity), suggesting that these cases represent INCL, presenting at a later age of onset. These findings suggest that palmitoyl-protein thioesterase deficiency is not restricted to infantile onset cases, and they raise the possibility that milder forms of INCL may result from less deleterious mutations.

Toward understanding the molecular pathology of Huntington's disease

Huntington's Disease (HD) is caused by expansion of a CAG trinucleotide beyond 35 repeats within the coding region of a novel gene. Recently, new insights into the relationship between CAG expansion in the HD gene and pathological mechanisms have emerged. Survival analysis of a large cohort of affected and at-risk individuals with CAG sizes between 39 and 50 repeats have yielded probability curves of developing HD symptoms and dying of HD by a certain age. Animals transgenic for the first exon of huntingtin with large CAG repeats lengths have been reported to have a complex neurological phenotype that bears interesting similarities and differences to HD. The repertoire of huntingtin-interacting proteins continues to expand with the identification of HIP1, a protein whose yeast homologues have known functions in regulating events associated with the cytoskeleton. The ability of huntingtin to interact with two of its four known protein partners appears to be influenced by CAG length. Caspase 3 (apopain), a key cysteine protease known to play a seminal role in neural apoptosis, has also been demonstrated to specifically cleave huntingtin in a CAG length-dependent manner. Many of these features are combined in a model suggesting mechanisms by which the pathogenesis of HD may be initiated. The development of appropriate in vitro and animal models for HD will allow the validity of these models to be tested.

Autosomal dominant cerebellar ataxia (SCA6) associated with small polyglutamine expansions in the alpha 1A-voltage-dependent calcium channel

A polymorphic CAG repeat was identified in the human alpha 1A voltage-dependent calcium channel subunit. To test the hypothesis that expansion of this CAG repeat could be the cause of an inherited progressive ataxia, we genotyped a large number of unrelated controls and ataxia patients. Eight unrelated patients with late onset ataxia had alleles with larger repeat numbers (21-27) compared to the number of repeats (4-16) in 475 non-ataxia individuals. Analysis of the repeat length in families of the affected individuals revealed that the expansion segregated with the phenotype in every patient. We identified six isoforms of the human alpha 1A calcium channel subunit. The CAG repeat is within the open reading frame and is predicted to encode glutamine in three of the isoforms. We conclude that a small polyglutamine expansion in the human alpha 1A calcium channel is most likely the cause of a newly classified autosomal dominant spinocerebellar ataxia, SCA6.

Familial episodic ataxia: clinical heterogeneity in four families linked to chromosome 19p

We describe the clinical and oculographic findings in 4 families with episodic ataxia and interictal nystagmus (EA-2) linked to chromosome 19p. Episodes varied from pure ataxia to combinations of symptoms suggesting involvement of the cerebellum, brainstem, and cortex. Some affected individuals exhibited a progressive ataxia syndrome phenotypically indistinguishable from the dominantly inherited spinocerebellar ataxia (SCA) syndromes. About one-half of the affected individuals had migraine headaches and several had episodes typical of basilar migraine. Oculographic findings were localizing to the vestibulocerebellum and posterior vermis. Additional genetic and environmental factors must account for the marked clinical heterogeneity in these families with an abnormal gene on chromosome 19p.

Rapid detection of subunit c of mitochondrial ATP synthase in urine as a diagnostic screening method for neuronal ceroid-lipofuscinoses

Using Western blot analysis and the ELISA technique, we showed previously significantly higher levels of subunit c in the urine of individuals with late-infantile neuronal ceroid lipofuscinosis (LINCL) and some patients with juvenile NCL (JNCL) [Wisniewski et al., J. Inherited Metab Dis 17: 205-210, 1994]. In an attempt to develop a diagnostic screening test for NCL based on detection of this biochemical marker in urine, we analyzed, using the blotting technique, urine from 7 infantile NCL (INCL), 17 LINCL, and 19 JNCL cases, 30 obligate heterozygotes, and 60 control cases. This analysis confirmed our former data showing significantly higher levels of subunit c in the urine from all LINCL and some JNCL cases. No false positive results were found. This simple analytical method may serve as a fast, non-invasive screening test for NCL.