The disease is linked to be found in the PRNP gene, which produces a protein called prion protein, or PrP. In FFI the variant gene, the PrP that is produced develops an abnormal 3-dimensional shape that is described as misfolded. This is toxic to the body, especially to cells of the nervous system. In FFI, misfolded PrP is primarily found in the thalamus, which is a structure deep within the brain that helps to regulate many functions of the body including sleep, appetite, and body temperature. As the misfolded PrP builds up in the thalamus, it results in a progressive destruction of nerve cells (neurons), which leads to the symptoms of the disorder.
FFI is classified as a transmissible spongiform encephalopathy (TSE) or a prion disease. Prion diseases are caused by the accumulation of misfolded prion proteins in the brain. This disorder is very rare, and it degenerates the brain. The characteristic symptom in FFI is progressive insomnia. Some people may start having insomnia suddenly and from being mild it then becomes progressively worse until an affected individual gets very little sleep. The lack of sleep leads to physical and mental deterioration and the disease ultimately progresses to coma and death.
This disease can be a product of two unaffected parents and in rare instances, a variation in the PRPN gene in individuals with FFI occurs spontaneously, without a family history of the disease. However, a person that is affected with this disease could pass on the variant gene to their offspring in an autosomal dominant manner. Unfortunately, there is no cure for this disease, but researchers are finding a way to help treat the disease.
https://rarediseases.org/rare-diseases/fatal-familial-insomnia/ Fatal Familial Insomnia
https://ghr.nlm.nih.gov/gene/PRNP PRNP gene