Batten disease, also called neuronal ceroid lipofuscinosis (NCL), is a serious childhood epilepsy disorder. There are several types of Batten disease, all caused by mutations in genes encoding for proteins involved in the disposal of cellular wastes composed of fat and protein. For some types of Batten disease, enzyme assays can be performed as a diagnostic test.
What enzymes are affected by Batten disease?
In infantile Batten disease, mutations in the PPT1 gene cause an enzyme called palmitoyl-protein thioesterase 1 (PPT1) to be made incorrectly or not at all. This enzyme is active in cell compartments called lysosomes, which are sometimes referred to as the “trash collector” of the cell. Lysosomes digest and recycle different types of cellular waste. PPT1 removes fats from certain proteins, which helps to break them down. PPT1 also is thought to be involved in many other cellular functions.
Mutations in the PPT1 gene result in low or no PPT1 enzyme activity in the cell.
In late infantile Batten disease, mutations in the TPP1 gene cause an enzyme called tripeptidyl peptidase 1 (TPP1) to be made incorrectly or not at all. This enzyme also is found in lysosomes. TPP1 breaks down protein fragments into their individual building blocks, called amino acids.
Mutations in the TPP1 gene result in low or no TPP1 enzyme activity in the cell.
What is an enzyme assay?
An enzyme assay is a biochemical test in which the activity of a particular enzyme is measured (assayed). The results of this test, compared to control samples, gives an estimate of how severe the disease is and how fast it may progress. In both infantile and late infantile Batten disease, patients with no TPP1 or PPT1 enzyme activity seem to progress more rapidly, while those with mutations that do not completely inactivate the enzymes seem to progress more slowly.
How are enzyme assays performed?
A small blood or tissue sample from the patient is required to test enzyme activity. The tissue sample is usually a skin biopsy obtained from an arm or leg. It is first treated with a topical anesthetic, then a needle or metal punch is inserted into the tissue to collect a small piece of skin.
The biopsy procedure is very quick, usually less than 15 minutes. Recovery depends on how the biopsy is performed, but patients usually are completely recovered within a few days. The biopsy will cause a small scar.
Once collected, the skin or blood sample is taken to a diagnostic laboratory for enzymatic activity testing. First, a particular type of cells called fibroblasts is isolated from the sample. Fibroblasts are representative of many other tissues in Batten disease.
Enzyme including TPP1 and PPT1, if present, are isolated, and provided with a special substrate that becomes fluorescent once acted on by the enzyme. (A substrate is any substance on which an enzyme acts). The increase in fluorescence is related to the activity of the enzyme in the patient sample.
Results enzyme assay may take a few days to weeks, depending on where the tests are performed.
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