Frontotemporal dementia (FTD) is caused by the buildup of abnormal proteins that kill nerve cells in the frontal and temporal lobes of the brain, the regions that govern personality, behavior, and language. About 20% of cases trace to a specific inherited gene mutation, while roughly 60% occur in people with no family history of the disease at all. Unlike Alzheimer’s, which typically strikes after age 65, FTD is the most common form of dementia in people younger than 60.
Toxic Proteins That Destroy Brain Cells
At the cellular level, FTD comes down to proteins that misfold and clump together inside neurons. Two proteins account for the vast majority of cases: a protein called TDP-43 and the tau protein. A third, called FUS, is responsible for a smaller subset. Which protein is involved shapes the specific symptoms a person develops, but all three cause damage the same basic way: they form toxic clumps that interfere with normal cell function and eventually kill the neuron.
TDP-43 normally lives in the nucleus of a cell, where it helps manage genetic instructions. In FTD, it gets displaced into the surrounding cell body, where it forms insoluble clumps. These clumps are directly toxic to neurons, damaging axons (the long fibers cells use to communicate) and triggering cell death. What makes TDP-43 pathology particularly aggressive is its seeding behavior. Once clumps form, they can spread from cell to cell in a pattern sometimes compared to prion diseases, recruiting healthy copies of the protein into the growing aggregate.
Tau protein normally stabilizes the internal scaffolding that gives neurons their shape. When tau misfolds, it forms tangled fibers that disrupt that scaffolding and bind to critical structures inside the cell, including mitochondria (the cell’s energy source) and the machinery neurons use to send signals. Like TDP-43, misfolded tau can seed further aggregation and spread to distant brain regions over time. Tau pathology underlies several FTD subtypes, including Pick disease, progressive supranuclear palsy, and corticobasal degeneration.
As these proteins accumulate and neurons die, the affected brain regions physically shrink. This atrophy is visible on brain imaging and progresses over months to years, tracking closely with the worsening of symptoms.
The Genetic Mutations Behind FTD
About 40% of people diagnosed with FTD have at least one blood relative with FTD, ALS, a related movement disorder, or a psychiatric condition like depression. But having a family history doesn’t necessarily mean a specific gene mutation is present. When researchers look for identifiable mutations, they find one in roughly 20% of all FTD cases. Three genes account for most of those.
The C9orf72 gene carries the most common genetic cause of FTD. A short segment of DNA in this gene repeats itself, and when the number of repeats exceeds about 30 copies, it becomes pathogenic. In large studies, C9orf72 expansions appear in roughly 8% of all FTD cases, 15% of familial cases, and about 3% of cases with no family history. People with C9orf72 mutations develop symptoms at an average age of 58, though onset can range from the 20s to the 90s. This mutation also causes ALS, and some people develop both conditions simultaneously.
Mutations in the GRN gene, which provides instructions for making progranulin (a protein involved in cell survival and inflammation control), are considered the most prevalent single-gene cause. When progranulin production drops, neurons become more vulnerable to damage. The average age of symptom onset for GRN carriers is 61.
Mutations in the MAPT gene, which encodes the tau protein directly, tend to cause the earliest symptoms, with an average onset of 49.5 years. Some carriers develop symptoms as young as 17. MAPT mutations lead directly to tau pathology, causing the protein to misfold more readily.
How Different Causes Produce Different Symptoms
FTD isn’t a single disease. It’s an umbrella term covering several syndromes, and the underlying protein or gene involved helps determine which syndrome develops.
Behavioral variant FTD, the most common form, primarily affects personality and social behavior. It can be driven by either tau or TDP-43 pathology. People with this variant may lose empathy, act impulsively, develop compulsive behaviors, or show dramatic personality shifts years before anyone suspects dementia.
Primary progressive aphasia (PPA), which primarily affects language, breaks into three subtypes with distinct biological underpinnings:
- Nonfluent/agrammatic PPA: People struggle to form grammatically correct sentences and may have difficulty coordinating the mouth movements needed for speech. Most cases involve tau pathology, and familial cases often carry MAPT mutations.
- Semantic dementia: People lose the meaning of words and can no longer recognize familiar objects or faces. This subtype usually involves TDP-43 pathology, and many familial cases have GRN mutations.
- Logopenic PPA: People have trouble finding words and repeating sentences. This variant most often turns out to be a focal presentation of Alzheimer’s disease rather than a true FTD pathology, involving amyloid plaques rather than tau or TDP-43.
Traumatic Brain Injury as a Risk Factor
For the majority of FTD cases with no identifiable genetic mutation, the triggers remain poorly understood. The strongest non-genetic association identified so far is traumatic brain injury. In a study of veterans, those with FTD were roughly four times more likely to have a history of TBI than people with other types of dementia (12.7% versus 3.5%). After adjusting for other factors, TBI carried an odds ratio of 4.4 for developing FTD.
Common vascular risk factors like high blood pressure, diabetes, high cholesterol, and smoking showed no difference between people with FTD and those with other dementias. Interestingly, heart disease was actually less common in the FTD group, though the reasons for this are unclear. The overall picture suggests that FTD, unlike vascular dementia or Alzheimer’s, is driven less by cardiovascular health and more by genetic vulnerability and possibly physical brain injury.
How the Biological Cause Is Identified
Pinpointing which protein or gene is responsible during a person’s lifetime has historically been difficult, since definitive identification often required examining brain tissue after death. That’s changing with advances in blood and spinal fluid testing.
One of the most promising tools is a blood test measuring neurofilament light chain (NfL), a protein released when neurons are damaged. People with active FTD have NfL levels in their blood roughly ten times higher than people who carry FTD gene mutations but haven’t developed symptoms yet (31.5 versus 3.5 picograms per milliliter). In spinal fluid, the difference is even more dramatic, with levels jumping three to fourfold once symptoms begin. This test can identify active disease with about 77% sensitivity and 98% specificity from a simple blood draw.
NfL testing doesn’t reveal which specific protein is accumulating, but it can flag when neurodegeneration is underway, which is especially valuable for people from families with known FTD mutations who want to monitor their status. Genetic testing can identify C9orf72, MAPT, and GRN mutations directly, and a 2025 NIH report noted that researchers have now identified additional molecular signatures, including abnormal assemblies of a protein called TAF15 and a newly discovered pairing of two proteins (ANX11 and TDP-43) that aggregate together in certain FTD subtypes. These discoveries are opening the door to more precise biological classification of individual cases.
Age and Who Is Affected
FTD can appear at almost any point in adulthood, but the typical window is between the mid-40s and early 60s. Among people aged 40 to 64, the prevalence is roughly 13 per 100,000 people. After 65, that number climbs to about 33 per 100,000, challenging the older assumption that FTD is exclusively a “young-onset” dementia.
The age you develop symptoms depends heavily on genetics. MAPT mutation carriers tend to develop symptoms around age 50, with some cases appearing in the late teens. C9orf72 carriers average 58 years, and GRN carriers average 61. For sporadic cases with no identified mutation, onset typically falls somewhere in this same range, but the variability is enormous, spanning decades even within the same family carrying the same mutation.