Caspase-8, association with Alzheimer's Disease and functional analysis of rare variants.

The accumulation of amyloid beta (Aß) peptide (Amyloid cascade hypothesis), an APP protein cleavage product, is a leading hypothesis in the etiology of Alzheimer's disease (AD). In order to identify additional AD risk genes, we performed targeted sequencing and rare variant burden association study for nine candidate genes involved in the amyloid metabolism in 1886 AD cases and 1700 controls. We identified a significant variant burden association for the gene encoding caspase-8, CASP8 (p = 8.6x10-5). For two CASP8 variants, p.K148R and p.I298V, the association remained significant in a combined sample of 10,820 cases and 8,881 controls. For both variants we performed bioinformatics structural, expression and enzymatic activity studies and obtained evidence for loss of function effects. In addition to their role in amyloid processing, caspase-8 and its downstream effector caspase-3 are involved in synaptic plasticity, learning, memory and control of microglia pro-inflammatory activation and associated neurotoxicity, indicating additional mechanisms that might contribute to AD. As caspase inhibition has been proposed as a mechanism for AD treatment, our finding that AD-associated CASP8 variants reduce caspase function calls for caution and is an impetus for further studies on the role of caspases in AD and other neurodegenerative diseases.

Missense mutations in the regulatory domain of PKC gamma: a new mechanism for dominant nonepisodic cerebellar ataxia.

We report a nonepisodic autosomal dominant (AD) spinocerebellar ataxia (SCA) not caused by a nucleotide repeat expansion that is, to our knowledge, the first such SCA. The AD SCAs currently comprise a group of > or =16 genetically distinct neurodegenerative conditions, all characterized by progressive incoordination of gait and limbs and by speech and eye-movement disturbances. Six of the nine SCAs for which the genes are known result from CAG expansions that encode polyglutamine tracts. Noncoding CAG, CTG, and ATTCT expansions are responsible for three other SCAs. Approximately 30% of families with SCA do not have linkage to the known loci. We recently mapped the locus for an AD SCA in a family (AT08) to chromosome 19q13.4-qter. A particularly compelling candidate gene, PRKCG, encodes protein kinase C gamma (PKC gamma), a member of a family of serine/threonine kinases. The entire coding region of PRKCG was sequenced in an affected member of family AT08 and in a group of 39 unrelated patients with ataxia not attributable to trinucleotide expansions. Three different nonconservative missense mutations in highly conserved residues in C1, the cysteine-rich region of the protein, were found in family AT08, another familial case, and a sporadic case. The mutations cosegregated with disease in both families. Structural modeling predicts that two of these amino acid substitutions would severely abrogate the zinc-binding or phorbol ester-binding capabilities of the protein. Immunohistochemical studies on cerebellar tissue from an affected member of family AT08 demonstrated reduced staining for both PKC gamma and ataxin 1 in Purkinje cells, whereas staining for calbindin was preserved. These results strongly support a new mechanism for neuronal cell dysfunction and death in hereditary ataxias and suggest that there may be a common pathway for PKC gamma-related and polyglutamine-related neurodegeneration.

Autosomal dominant sensory/motor neuropathy with Ataxia (SMNA): Linkage to chromosome 7q22-q32.

The autosomal dominant (AD) spinocerebellar ataxias (SCAs) and hereditary sensory neuropathies (HSN) are heterogeneous disorders characterized by variable clinical, electrophysiological, and neuropathological profiles. The SCAs are clinically characterized by slowly progressive incoordination of gait often associated with poor coordination of hands, speech, and eyes. Peripheral neuropathy is not a frequent part of the SCA syndrome. In contrast, the HSNs are primarily characterized by progressive sensory loss. There is substantial clinical overlap between the various SCAs and the various HSNs, and they often cannot be differentiated on the basis of clinical or neuro-imaging studies. We have identified a five-generation American family of Irish ancestry with a unique neurological disorder displaying an AD pattern of inheritance. There was variable expressivity and severity of symptoms including sensory loss, ataxia, pyramidal tract signs, and muscle weakness. Nerve conduction studies were consistent with a sensory axonal neuropathy. Muscle biopsy revealed neurogenic atrophy and brain MRI showed mild cerebellar atrophy. To identify the responsible locus we pursued a whole genome linkage analysis. After analyzing 114 markers, linkage to D7S486 was detected with a two point LOD score of 4.79 at theta = 0.00. Evaluation of additional markers in the region provided a maximum LOD score of 6.36 at theta = 0.00 for marker D7S2554. Haplotype analysis delimited an approximately 14-cM region at 7q22-q32 between markers D7S2418 and D7S1804 cosegregating with the disease. Because this disorder does not easily fall into either the SCA or HSN categories, it is designated sensory/motor neuropathy with ataxia (SMNA).