The goal of this clinical study is to learn about an investigational gene therapy product called AVB-101, which is designed to treat a disease called Frontotemporal Dementia with Progranulin Mutations (FTD-GRN). FTD-GRN is an early-onset form of dementia, a progressive brain disorder that affects behavior, language and movement. These symptoms result from below normal levels of a protein called progranulin (PGRN) in the brain, which leads to the death of nerve cells (neurons), affecting the brain's ability to function. The main questions that the study aims to answer are: 1. Is a one-time treatment with AVB-101 safe for patients with FTD-GRN? 2. Does a one-time treatment with AVB-101 restore PGRN levels to at least normal levels? 3. Could AVB-101 work as a treatment to slow down or stop progression of FTD-GRN? In this study there is no placebo (a dummy pill or treatment used for comparison purposes), so all participants will receive a one-time treatment of AVB-101 delivered directly to the brain, with follow-up assessments for 5 years.