Frontotemporal dementia (FTD) is caused by the buildup of abnormal proteins in the brain’s frontal and temporal lobes, which damages and kills nerve cells in the regions that control personality, behavior, and language. About 10 to 20% of cases are directly inherited through gene mutations, while the majority arise without a clear family history. It is the most common form of dementia in people younger than 60.
Protein Buildup in the Brain
At the cellular level, FTD comes down to proteins behaving badly. In a healthy brain, proteins fold into specific shapes to carry out their jobs. In FTD, certain proteins misfold and clump together inside neurons, disrupting normal cell activity and eventually killing the cells. As neurons die, the frontal and temporal lobes physically shrink, which is why these areas show visible atrophy on brain scans.
Two proteins account for nearly all cases. About half of people with FTD have an abnormal form of tau, a protein that normally helps stabilize the internal structure of nerve cells. When tau misfolds, it forms tangles that choke the neuron from the inside. The other half have accumulations of a protein called TDP-43, which normally helps regulate how genes are read and used. A small percentage, roughly 5%, have buildup of a third protein called FUS.
Which protein is involved shapes how the disease looks clinically. People with tau-based FTD tend to develop personality and behavioral changes first, while those with TDP-43 accumulation are more likely to show early problems with thinking and speech. One specific form of tau-based FTD, called Pick’s disease, produces distinctive swollen neurons and dense clumps of tau known as Pick bodies, both visible under a microscope after death.
Genetic Mutations Behind FTD
Roughly 10 to 20% of FTD cases are considered genetic, meaning they’re caused by inherited mutations passed from parent to child. In familial FTD, multiple people across consecutive generations on the same side of the family develop the condition or a related disorder. Three genes are responsible for the vast majority of inherited cases.
The most common genetic cause is an expansion in the C9orf72 gene, identified in 2011. Normally, a short sequence of DNA in this gene repeats a handful of times. In people with the mutation, that sequence repeats hundreds or even thousands of times. This expansion accounts for 12 to 25% of familial FTD cases and even 6 to 7% of cases with no known family history. The expansion causes harm in multiple ways: it reduces the gene’s normal function, it produces toxic RNA molecules that trap essential proteins, and it generates abnormal small proteins that accumulate in neurons.
Mutations in the MAPT gene, which provides the blueprint for tau protein, directly cause the tau tangles seen in many FTD cases. Symptoms in people with MAPT mutations can begin as early as 17 or as late as the 80s. Mutations in the GRN gene, which makes a protein called progranulin involved in cell survival and inflammation, represent the third major genetic cause. People with GRN mutations tend to have TDP-43 buildup rather than tau.
The Link Between FTD and ALS
FTD and amyotrophic lateral sclerosis (ALS), the motor neuron disease that causes progressive muscle weakness, share a surprising genetic overlap. The C9orf72 expansion is the single most common genetic cause of both diseases, responsible for about 40% of familial ALS in addition to its role in FTD. Some people with this mutation develop FTD alone, some develop ALS alone, and some develop both simultaneously.
This connection means that families carrying the C9orf72 expansion may see different members affected in different ways. One relative might lose the ability to speak or behave appropriately, while another loses the ability to move. Scientists still don’t fully understand why the same mutation targets different parts of the nervous system in different people, but the shared molecular pathway, particularly TDP-43 accumulation, links the two diseases at a fundamental level.
Sporadic Cases and Unknown Risk Factors
The majority of FTD cases are sporadic, meaning only one person in a family is affected and no clear genetic cause can be identified. For these cases, the honest answer is that the cause remains unknown. Unlike Alzheimer’s disease, where risk factors like cardiovascular health, diabetes, and low physical activity are well established, researchers have not yet pinpointed reliable environmental or lifestyle factors for FTD.
Head trauma, thyroid conditions, and autoimmune disorders have been explored as potential contributors, but none has been confirmed as a definitive risk factor. Research into non-genetic causes is ongoing, and the National Institute on Aging continues to fund studies examining environmental influences. For now, the absence of a known trigger is one of the more frustrating realities of the disease for families seeking answers.
Why FTD Strikes Younger Than Other Dementias
Most people associate dementia with old age, but FTD frequently begins between ages 40 and 65. It is the leading cause of dementia in people under 60. Among those with genetic mutations, the age of onset varies widely depending on which gene is involved. MAPT mutations can cause symptoms anywhere from the late teens to the 80s, while C9orf72 and GRN mutations show a range from the 20s into the 90s.
This younger onset means FTD often hits people in the middle of their careers and while they’re still raising families. Early symptoms like personality changes, poor judgment, or difficulty finding words are frequently mistaken for depression, a midlife crisis, or a psychiatric condition, which can delay diagnosis by years.
How FTD Is Identified
There is no single blood test or brain scan that definitively confirms FTD during life. Diagnosis relies on a combination of clinical evaluation, neuropsychological testing, and brain imaging that shows characteristic shrinkage in the frontal and temporal lobes. Genetic testing can confirm a mutation in familial cases, providing a definitive answer for those families.
For non-genetic cases, doctors piece together the picture from symptoms, brain scans, and by ruling out other conditions. Some blood-based markers are showing promise. Neurofilament light chain, a protein released when nerve cells are damaged, rises in the blood years before symptoms appear and can help distinguish between genetic subtypes of the disease. For people with GRN mutations specifically, low levels of progranulin in the blood can identify carriers with over 95% sensitivity. But for most people with sporadic FTD, these tools remain limited, and a definitive diagnosis often comes only from examining brain tissue after death.
The difficulty in pinpointing the cause for any individual case reflects the broader reality of FTD: it is not one disease but a family of related disorders, each driven by different proteins, different genes, and likely different triggers that science is still working to understand.