The principles of using saliva samples for SMA detection are mainly based on the following aspects:

The SMA saliva collection device itself does not directly perform the detection. Instead, it is used to collect saliva samples, providing a sample basis for subsequent SMA testing. 

• Principle of Gene Detection
  • DNA Extraction: Saliva contains cellular components such as oral mucosal epithelial cells. These cells contain cell nuclei, and the human genomic DNA is present within the cell nuclei. Through specific reagent kits and experimental methods, genomic DNA can be extracted from the cells in the saliva sample, providing a material basis for subsequent genetic analysis.
  • Detection of Gene Loci: SMA is mainly caused by the deletion or mutation of the survival motor neuron 1 (SMN1) gene. After extracting the DNA from the saliva, the polymerase chain reaction (PCR) technique can be used to specifically amplify specific fragments of the SMN1 gene. Then, methods such as gel electrophoresis, real-time fluorescent quantitative PCR, and gene sequencing are employed to analyze the amplified gene fragments and detect whether there are abnormalities such as deletions and mutations in the SMN1 gene. For example, the real-time fluorescent quantitative PCR technique can quantitatively analyze the copy number of the SMN1 gene by detecting the intensity of the fluorescent signal. If the copy number is lower than the normal range, it may indicate the presence of a deletion in the SMN1 gene, thus assisting in the diagnosis of SMA.
  • Linkage Analysis: For some samples with unclear SMN1 gene test results or complex genetic situations, the method of gene linkage analysis can be adopted. By detecting other genetic markers closely linked to the SMN1 gene, such as single nucleotide polymorphisms (SNPs) or short tandem repeats (STRs), and analyzing the transmission of these genetic markers within the family, the status of the SMN1 gene can be inferred indirectly, and it can assist in determining whether there is a genetic risk related to SMA.
• Principle of Protein Detection
  • Analysis of mRNA Transcription: Inside the cell, genes are transcribed into messenger ribonucleic acid (mRNA), which in turn directs the synthesis of proteins. The survival motor neuron (SMN) protein formed by the transcription of the SMN1 gene plays a crucial role in the survival and functional maintenance of motor neurons. By detecting the transcription level of the SMN1 gene mRNA in saliva cells, the expression of the SMN1 gene can be indirectly reflected. For example, the reverse transcription polymerase chain reaction (RT-PCR) technique is used. First, the mRNA in the saliva cells is reverse transcribed into complementary DNA (cDNA), and then the cDNA is amplified and quantitatively analyzed by PCR to detect the expression level of the SMN1 gene mRNA. If the expression level of the SMN1 gene mRNA is significantly reduced, it may be related to the onset of SMA.
  • Analysis of Protein Expression: Although the content of the SMN protein in saliva is relatively low, through some highly sensitive protein detection techniques, such as enzyme-linked immunosorbent assay (ELISA) and Western blotting, the SMN protein in saliva can also be qualitatively or quantitatively analyzed. These methods use specific antibodies to bind to the SMN protein, and by detecting indicators such as the signal intensity after the antibody binds to the protein, the expression level of the SMN protein in saliva and the presence of abnormal protein structures can be determined, providing a basis for the diagnosis of SMA.