The main two macromolecular types of SCID are X-SCID and ADA-SCID. X-SCID is a severe immune disorder caused by mutation of the interleukin (IL)-2 receptor gamma chain gene (IL2RG) located on the X chromosome. The dysfunction of IL-2 Rγ chain may lead to severe T cells deficits and B cells dysfunction, and furtherly lead to the decreased copy quantity of TRECs. ADA-SCID is a rare hereditary disease affecting purine metabolism, the morbidity rate is estimated to be 1:200,000~1:1000,000, which accounts for 10–15% of all SCID [18]. ADA-SCID differs from X-SCID which may be due to a large number of ADA gene mutations. ADA produces several kinds of costimulatory signal in different cells as associated with CD26. ADA-CD26 interaction stimulates the activation of T helper 1 (Th1) and secretion of proinflammatory cytokines IFN-γ, TNF-α, and IL-6 during T cell receptor (TCR)/ CD3 engagement [19]. The dysfunction of ADA may lead to the decrease of proinflammatory cytokines and the performance lose of Th1, and furtherly lead to the decreased copy quantity of TRECs.
For SCID, what puzzled us are the characteristics of lacking of recognizable features on physical test, non-typical symptom in early life. NBS for SCID based on measuring TREC copy quantities allows early identification, protection from infection which is characterized by simple collected and low charge. TREC is the toroidal and free DNA characterized by the nature of stable during the gene rearrangement of T lymphocytes cell receptors and appears in the late differentiation of T lymphocytes cells in thymus [20]. TREC could not be duplicate and could be dilute during the proliferate of T cells. TREC only exist in the αβT cells with initial phenotypes. Because more than 95% T cells in peripheral blood is αβT cells, the quantities of the TRECs in peripheral blood mononuclear cells could represent the quantities of native T cells as TCR genes rearrange initially and form functional TCR genes. Mei W. Baker reviewed that the quantity of TRECs is related to CD45RA: CD45RO ratio [21]. However, several previous studies reported that the copy quantities of TRECs in patients with CD40 ligand deficiency was identical with those with normal CD40 ligand in the cell differentiation process. So, it is unreliable to diagnosis SCID based on sole analysis CD marker [22]. Molecular TREC could be taken as one of the measurable indicators for the initial SCID screening in newborns with clinical appearance.
According to analysis of the included studies, we find TREC cut-off values varied from 24 to 300 copies/ul. The mean TREC content is 53 copies/ul, and median 35 copies/ ul. The main reason of the difference might be due to the different definition method or laboratory method. In the studies (cut-off value > 100 copies/µL), Beth H. Vogel chose ten percent of the normal value or 2-3 times of the patients' value as cut-off value. Besides SCID, TREC is also the biomarker for other immunologic diseases. So, even George S. Amatuni chose the cut-off value (300 copies/µL) which is about determining SCID and excluding other diseases. In the studies (cut-off value≤100 copies/µL), the most studies chose 0.5th or 1st percentiles all the collected sample as the cut-off value. Ana Argudo-Ramírez and Huang chose the cut-off value based on retest rate, referral rest, missed diagnosis rate and positive detection rate. Besides, only two studies chose cut-off value according to Receiver Operating Characteristic (ROC) curve and the sudden dropped copy quantities of samples. The population and sample size also could be the reasons result in different cut-off value. Lowering the cut-off value will reduce the amount of referrals and other TCL cases and the followed pressure on the health care system without decreasing the sensitivity of NBS for SCID. However, if the cut-off value is much higher, it may have a higher FPR in premature infants. So, it is necessary to formulate an opt and consistent cut-off value according to the different regions and population [23]. To ensure a reference cut-off value, we systhesised all the included studies containing different regions and population. Finally, we recommend 90 and 40 copies/ul as the primary and secondary cut-off value, respectively [24]. What is more, we hope that this recommend reference value could be furtherly confirmed and practiced in Chinese population, even all over the world.
In terms of heterogeneity, one of the primary causes of heterogeneity in test accuracy studies is threshold effect, which is arised according to the different cut-off value to define a positive or negative test result and furtherly lead to different sensitivities and specificities. We used the Spearman correlation coefficient to analyze the threshold. The result showed P>0.05, indicating that there is no specific heterogeneity in the threshold. We found the detecting methods for TREC are all based on PCR analysis or TREC kit of which the essence is also PCR. However, there are no unified primers which may have different augmentational efficiency, different operational technique for PCR analysis; different sensitivity of operating machine; and inconsistent standard for systhsising standard curve. Therefore, these may all produce difference in quantifying the copy quantities of TRECs, then furtherly result in the heterogeneity.
Actually, SCID could be cured if diagnosis early. So, seeking the pre-detecting biomarker is important to predict SCID. Besides the simple collected, taking the copy quantity as the biomarker could save the cost and furtherly reduce the economic burden of families and nation. Also, our meta-analysis systematically analyzed the relationship between the TREC copy quantity and SCID. The result showed the TREC copy quantity as a biomarker is reliable. Besides, we integrated the published data and produced this meta-analysis. The relative study has not been reported before. We also need consider some limitations in this analysis. Firstly, our meta-analysis included data from America, Europe and Asia without Australia and Africa. Also, the sample could lose to follow-up before determining whether SCID occurred or not [25]. If we own the comprehensive data contained different continents, the result will be more representative and persuasive. Secondly, we find the cut-off value is not consistent in the included studies. The different detecting assays with different clinical recognition lead to different sensitivity and specificity. The more consistent the detecting method is, the more persuasive and application prospect detecting the copy quantity of SCID is. If the consistence could be achieved, we could have fixed cut-off value and standard case definition all over the world.
In conclusion, this meta-analysis supports the notion that the TREC copy quantity could be used as an biomarker to screen for newborns' SCID. However, the data we extracted are all in developed countries apart from the underdeveloped regions. So, larger-scale studies comprising different regions and population are still necessary to confirm more practicing value about this detecting method. Besides, in terms of TREC detecting method, the Real Time PCR is the main method to detect TREC. With the development of modern technology, digital PCR is a more precise method to detect TREC copy quantity and furtherly ensure a more appropriate cut-off value.