The working principle of the SMA (Spinal Muscular Atrophy) – test saliva collector is mainly based on the following aspects:

1. Saliva Sample Collection

The saliva collector is usually a specially designed container, which is convenient for users to collect an adequate amount of saliva. The collector generally has a relatively large – opening collection port, making it easy for people to spit saliva into it. Some collectors may also be equipped with tools to stimulate saliva secretion, such as citric – acid – impregnated cotton tablets. When the cotton tablet is chewed in the mouth, citric acid stimulates the salivary glands to secrete more saliva, ensuring that a sufficient amount of sample can be collected to meet the testing requirements.

2. Cell Capture and Preservation

Saliva contains various cells, including oral epithelial cells, etc., which contain the individual’s complete genomic DNA. Inside the collector, there is a special preservation solution. When saliva enters the collector, the preservation solution protects the cells, preventing cell rupture and DNA degradation. The components of the preservation solution generally include buffers, which can maintain a stable pH environment, keep the enzymes in the cells stable, and avoid DNA damage caused by abnormal enzyme activity due to pH changes. At the same time, the preservation solution may also contain preservatives to inhibit the growth and reproduction of microorganisms, as enzymes or metabolites produced by microorganisms may damage the DNA in the sample.

3. DNA Extraction

In the laboratory, extracting DNA from the saliva sample is a crucial step. First, the sample is centrifuged. The centrifugal force causes the cells to precipitate to the bottom of the tube, enabling the preliminary separation of cells from impurities in the saliva (such as food residues and bacteria). Then, the cells are treated with a lysis solution. The lysis solution can break the cell membrane and nuclear membrane, releasing the DNA into the solution. The lysis solution usually contains surfactants, which can dissolve the lipid bilayer to rupture the cell membrane, and may also contain proteases to break down the proteins bound to DNA, allowing the DNA to be fully released.

4. DNA Purification and Concentration

The released DNA also needs to be purified. Materials that can specifically adsorb DNA, such as silica membranes or magnetic beads, are often used. When the sample passes through a column containing these materials or is mixed with magnetic beads, the DNA will be adsorbed onto the materials, while other impurities (such as proteins and polysaccharides) can be washed away. Subsequently, the pure DNA is eluted from the adsorption material with an eluent, and the eluted DNA can be concentrated to a concentration suitable for subsequent detection.

5. Gene Detection

For the detection of SMA, the analysis is mainly focused on specific gene loci. The commonly used method is the Polymerase Chain Reaction (PCR) technique. The PCR technique can specifically amplify the gene fragments related to SMA. In a reaction system containing a DNA template (DNA extracted from saliva), primers (short DNA sequences that can specifically bind to both ends of the SMA – related gene locus), DNA polymerase (used to synthesize new DNA strands), dNTPs (raw materials for DNA synthesis), and buffer, through multiple cycles of temperature changes, the target gene fragments are continuously replicated and amplified.
The amplified gene fragments can be analyzed by various methods. For example, in real – time quantitative PCR, the copy number of the gene is determined by detecting the intensity of the fluorescent signal. Or gene sequencing can be carried out to directly read the gene sequence to determine whether there are gene mutations related to SMA, thus achieving the detection of SMA.