C9orf72-FTD/ALS is a repeat expansion disorder caused by an abnormally expanded hexanucleotide (G4C2) repeat sequence within the C9ORF72 gene on chromosome 9. The condition is inherited in an autosomal dominant pattern, meaning one expanded allele is sufficient to confer disease risk, and each child of an affected individual carries a 50% probability of inheriting the expansion. GeneReviews documents that C9orf72-FTD/ALS is associated with age-dependent reduced penetrance: penetrance is approximately 0% at age 35 years, approximately 50% at age 58 years, and near 100% by age 80 years, meaning that many individuals who carry the expansion will not develop symptoms until middle or later adulthood, and a small proportion may remain asymptomatic throughout their lives. GeneReviews further notes that clinical features cannot predict the size of the C9ORF72 repeat expansion, nor can expansion size reliably predict the disease course in any given individual, reflecting the high degree of phenotypic variability associated with this condition.

According to GeneReviews, C9orf72-FTD/ALS is diagnosed by identifying a pathogenic C9ORF72 G4C2 repeat expansion through molecular genetic testing. GeneReviews describes suggestive clinical findings including FTD with prominent neuropsychiatric features such as hallucinations and delusions, motor neuron disease with both upper and lower motor neuron involvement, or a combination of these features—particularly in the context of a family history consistent with autosomal dominant inheritance. GeneReviews notes that pathogenic C9ORF72 repeat expansions are substantially more frequent in individuals with a family history of FTD and/or ALS than in those without such history, and that this expansion accounts for a high proportion of individuals who manifest both FTD and ALS phenotypes concurrently. Neuroimaging and neuropsychological evaluation contribute to the clinical assessment, as documented in the GeneReviews diagnostic framework for this condition.

No specifically approved treatments for C9orf72-FTD/ALS as a distinct condition are identified in this knowledge packet. GeneReviews notes that consensus clinical management guidelines specific to this syndrome have not been published. Management, as described in GeneReviews, centers on a multidisciplinary team approach addressing individual manifestations across multiple domains. GeneReviews documents that comprehensive care encompasses neurology, pulmonology, speech therapy, physical therapy, occupational therapy, respiratory therapy, nutrition, and psychology, among other disciplines. Physical and occupational rehabilitation, assistive devices for mobility and communication, and pharmacologic approaches addressing individual manifestations—including spasticity, muscle cramps, dysarthria, and behavioral and psychiatric symptoms—are documented in GeneReviews management tables. Respiratory function and nutritional status are described in GeneReviews as areas addressed within comprehensive multidisciplinary care for this condition. Investigational approaches targeting the C9ORF72 repeat expansion, including antisense oligonucleotide and RNA-based therapeutic strategies, are in early-stage clinical development as documented in GeneReviews.

The prognosis in C9orf72-FTD/ALS is variable and closely associated with the clinical phenotype. According to GeneReviews, life expectancy is highly variable and primarily determined by the specific clinical manifestations present. GeneReviews documents that the presence of motor neuron disease correlates with an earlier age of onset and a worse overall prognosis compared to pure FTD presentations. As a progressive neurodegenerative disorder, C9orf72-FTD/ALS is characterized by clinical decline across affected domains, with the pace and pattern of progression varying substantially among individuals. GeneReviews notes that age-dependent penetrance means some expansion carriers remain asymptomatic into their nineties, reflecting the variability in disease expression associated with this repeat expansion. The condition is described by GeneReviews as having a highly heterogeneous clinical course that may differ substantially between and within families.

Several active clinical trial records are certified for C9orf72-FTD/ALS on ClinicalTrials.gov. Active studies include a longitudinal observational cohort study (NCT04363684) tracking frontotemporal lobar degeneration disease progression across multiple sites, sponsored by Mayo Clinic with completion planned through 2030. A Phase 1 study (NCT07204977) evaluating a pharmacological approach in C9ORF72-associated ALS is active, sponsored by the National Institute of Neurological Disorders and Stroke (NINDS). A Phase 1 gene transfer study (NCT04747431) targeting C9ORF72-associated frontotemporal dementia is active, sponsored by Passage Bio. Additional active studies include a biomarker surveillance project (NCT04516499) and natural history and family registry investigations (NCT00317616, NCT03865420). GeneReviews documents that antisense oligonucleotide therapy targeting the C9ORF72 repeat expansion has shown preclinical promise and has entered Phase 1 clinical investigation; RNA-based modalities targeting C9ORF72 transcripts are also under investigation. The research literature for this condition encompasses 301 classified publications with a predominance of basic science and preclinical research, alongside substantial biomarker and gene therapy publication activity. Active clinical trials for this condition are listed on ClinicalTrials.gov.