Amyotrophic lateral sclerosis (ALS) with frontotemporal dementia (FTD) is a rare clinical entity, in which both disorders are variably associated in the same patient or within the family. This adult-onset disorder, which is rapidly fatal, occurs in some families with autosomal dominant (AD) transmission and age-dependant penetrance. Two studies have provided evidence for linkage of this condition to chromosomes 15 (in a single family) and 9 (in five families). However, none of these loci have been yet confirmed. Through a national network of 10 centres with specialists for FTD and/or ALS, we have identified 35 probands with ALS-FTD, including 13 with a family history consistent with AD inheritance.

Mutations in the SOD1 and tau genes, respectively responsible for autosomal dominant forms of ALS and FTD, will be excluded by direct sequencing. We will then extend the pedigree of the 13 autosomal dominant families to all consenting first, second and eventually third degree relatives, using well defined criteria for FTD and ALS. The same strategy will be applied to newly identified families during the course of the project (at least, seven families with AD inheritance expected). Linkage studies will be performed in the 20 families using markers from the two candidate regions on chromosomes 9 and 15. Then, refinement of the candidate region will be obtained by analyzing the linked families with a high density of microsatellite markers. This should lead to the refinement of the candidate regions, allowing to search for mutations in candidate genes. Genes located within the critical regions will be prioritized for their analysis by sequencing, according to their expression in the nervous system and to their function.

Once the responsible gene(s) will be identified, it will then possible to define its spectrum of mutations and to establish genotype/phenotype correlations. Alternatively, if none of the candidate regions is confirmed, a genome wide search will be performed, allowing to identify one or more loci for ALS-FTD. The same strategy would then be applied to identify the corresponding gene(s). This project should contribute for identifying the molecular basis of this devastating disorder with practical consequences for genetic counselling in ALS-FTD families, and with the perspective of elucidating the pathophysiology of this disorder.