Diagnostic Testing and the CTSF Gene

Reaching an accurate diagnosis of CLN13 requires careful clinical evaluation combined with advanced genetic analysis. Because this condition is so rare and its symptoms overlap with many other adult-onset disorders, comprehensive molecular genetic testing has become the most reliable tool for diagnosis ^[1][2].

The Role of the CTSF Gene

The CTSF gene acts as a blueprint for a lysosomal enzyme called cathepsin F ^[2][3]. This enzyme operates inside the lysosome—the part of the cell responsible for breaking down and recycling waste ^[3][4].

When a person has CLN13, they typically have pathogenic (disease-causing) variants in both copies of their CTSF gene ^[1]. This leads to a deficiency in cathepsin F, preventing the cell from properly clearing out waste materials known as ceroid and lipofuscin ^[2][5]. Over time, the buildup of these materials contributes to cell damage and the progressive loss of neurons ^[6][7].

Understanding the Genetic Diagnosis

For a diagnosis to be genetically supported, the laboratory must find two disease-causing variants. Crucially, these variants must be “in trans”—meaning one variant sits on the chromosome inherited from the mother, and the other variant sits on the chromosome inherited from the father.

Genetic testing has limitations. A single pathogenic variant, or a “Variant of Uncertain Significance” (VUS)—a genetic change where it is unknown if it causes disease—does not by itself confirm CLN13. Furthermore, a negative genetic panel does not entirely exclude all forms of NCL, as some structural changes or deep genetic variants may be missed by standard tests. In complex cases, advanced testing like whole exome or genome sequencing, or later reanalysis of the data, may be needed ^[8][9].

Why Genetic Testing Usually Supersedes Biopsies

In the past, doctors relied heavily on taking a small piece of skin or tissue (a biopsy) to look for storage material under a microscope ^[2]. While expert pathology can still provide supportive clues, medical consensus has shifted toward genetics for adult-onset cases due to several challenges with biopsies:

• Sensitivity Issues: Biopsies can be negative in people who actually have the disease, because the storage material is not uniformly present in all tissues outside the brain ^[1][10].
• The Aging Factor: As humans age, normal cells naturally accumulate a substance called age-related lipofuscin. This normal aging process can look very similar to the abnormal storage seen in CLN13 under a microscope, making interpretation difficult and sometimes leading to misdiagnosis ^[11][12].

When a biopsy and a thorough clinical picture are paired with confirmed pathogenic variants in the CTSF gene, the molecular diagnosis provides the definitive answer ^[2][11].

Inheritance and Family Impact

CLN13 follows an autosomal recessive inheritance pattern ^[1][9]. This means:

• The affected person inherited one mutated CTSF gene from each parent ^[1].
• The parents are typically carriers—they have one mutated gene and one healthy gene, so they do not show symptoms of the disease ^[13][1].
• When both parents are carriers, there is a 25% chance with each pregnancy that a child will be affected, a 50% chance they will be a carrier, and a 25% chance they will not carry the mutation at all ^[1].

For the affected person’s children: Each child of a person with CLN13 will inherit one variant, meaning all their children will be carriers. However, their children will only be at risk of having the disease if the affected person’s partner is also a carrier of a CTSF variant.

Because of these complexities, meeting with a genetic counselor is highly recommended ^[14]. They can explain the specific lab results, discuss reproductive options, and guide the testing of relatives ^[14][15].