Reduced penetrance in genetic ALS/FTD spectrum disorders: implications for genetic counseling, predictive testing and treatment

As the offer of genetic testing for people with ALS/FTD becomes standard of care, clinicians and affected individuals should have accurate and balanced information regarding the clinical and familial implications of test results, including the penetrance of identified variants. Published estimates of the penetrance of specific ALS/FTD variants, including the C9orf72 repeat expansion, have varied widely. However, it is now apparent that most pathogenic variants identified in clinical testing exhibit reduced penetrance. Although data on the disease risk of many variants is limited and likely to evolve in the coming years, the challenges of estimating penetrance should not preclude transparent discussion of these issues with affected individuals and their families. Here, we review published penetrance data and highlight genetic counseling considerations to support the clinician in discussing disease risk and facilitating decision-making in genetic testing and patient care.