The increasing incidence of T1DM12
has been recognized as an urgent public health issue.13
The most important genetic locus, HLA-D, works in conjunction with other genes and nongenetic factors to influence T1DM risk. Thus most people with even the highest risk HLA-D haplogenotypes do not develop T1DM, while some people with lower risk haplogenotypes do. Most diabetologists believe that environmental factors play a major role in triggering T1DM, and many believe that exposures early in life (even in utero
) are important. However, the exact nature of such exposures remains unknown. In addition to specific exposures, other environmental influences may be important, including birth by C-section,14
maternal stress during pregnancy,15
and vitamin D.16
Type 1 diabetes mellitus is generally caused by the autoimmune destruction of the insulin-producing beta cells in pancreatic islets.17
About half of the risk for T1DM can be attributed to genetic predisposition, and much of that risk resides in the MHC. The MHC contains genes that code for transplantation antigens, which, in humans, are known as the HLAs. The HLA genes are highly polymorphic, and certain combinations of HLA alleles are associated with increased or decreased risk for T1DM. While other genetic loci that impart increased risk have been identified,18,19
the HLA locus is by far the largest single contributor.20
The difference between susceptible and protective HLA haplogenotypes can result in as much as a 20-fold difference in the risk for T1DM.21
Moreover, the highest risk HLA haplogenotypes are those associated with an earlier age at onset, which increases the chance of diabetic ketoacidosis (DKA) and hospitalization.22
Identification of increased risk lowers the probability of hospitalization and DKA23
and therefore provides benefit even in the absence of a preventative treatment.
From a research perspective, early recognition of higher T1DM risk is required for natural history studies1
and for clinical trials of therapies that could prevent or delay autoimmune destruction of islet cells. From a public health perspective, early recognition can reduce the chance of DKA in incident cases, a major contributor to morbidity and health care expenses. The most effective public health program for early recognition of congenital risk is NBS. A number of T1DM research efforts have used NBS to identify higher risk individuals for recruitment into natural history studies and early intervention trials. Results suggest that early identification and intervention can preserve insulin production for up to five years.24,25
If the onset of T1DM can be prevented or significantly delayed, NBS for T1DM genetic risk could become a standard public health practice.
Determining the exact haplogenotypes in the HLA complex, which is required to match tissues for organ transplantation, is complex and expensive. The extensive polymorphism in the HLA results in a vast array of individual haplogenotypes, as illustrated in and . This allows identification of higher risk haplogenotypes without the necessity of identifying every allele. Still, the complexity of the HLA loci demands careful attention to specificity of allele identification. For instance, the haplogenotypes in the second and third rows of differ only by the DQB1 alleles on the second chromosome (0301 versus 0302), but this difference in closely related alleles discriminates a protective haplogenotype from a higher risk haplogeno-type. Conversely, the two haplogenotypes designated as eligibility code “H,” differ only in the DRB1 locus of chromosome 1 (0401 versus 0404), a difference that does not affect the risk category as defined by the TEDDY study.
The T1DM research programs that use HLA-based risk assessment algorithms have identified technical approaches that make use of the known allelic associations within the HLA complex. In addition, protective alleles are dominant, so a tiered approach that first culls the general population to remove those with protective alleles can provide more cost-effective selection of higher risk individuals. Since TEDDY protocol includes only a small set of eligible haplogenotypes, screening algorithms can be designed to exclude individuals in whom any ineligible allele is detected.1
Proficiency testing is an effective method for laboratories to ensure quality control and quality assurance. It provides laboratory personnel with an objective benchmark with which to measure their accuracy and also to compare their score with other laboratories conducting similar testing. While the two PT programs discussed here use different methodologies, they both achieve the goal of objectively measuring the performance of the laboratories. To do so, the challenge panels had to include a wide variety of lower risk haplogenotypes as well has the higher risk (or TEDDY-eligible) haplogenotypes.
The difference in algorithms used to assign T1DM genetic risk presents challenges for PT. Laboratories will not necessarily test for the same genetic markers, and the tests that they employ may differ in the extent to which they can resolve closely related HLA alleles. The complex and inconsistent use of nomenclature to identify these alleles presents additional problems. Assignment of risk levels can also vary, depending on the population being tested and the reference data used by the laboratory. Taken together, these factors hamper the ability to make valid comparisons between laboratories.
The TEDDY study circumvents the problem of assessing laboratory comparability by specifying a small set of higher risk HLA haplogenotypes for study eligibility. Even though screening laboratories can use their own methods, they must all identify the eligible HLA haplogenotypes properly. All participants screened as eligible are confirmed by higher resolution testing in a single HLA reference laboratory to assure accurate identification.3
The TEDDY screening laboratories have, to date, exceeded the required accuracy of 98%, a remarkable achievement given the demand for low-cost analysis, the large sample volume, and need for rapid turn-around time.
Results from the VQPT also support the technical feasibility of newborn DBS screening in multiple distinct laboratory locations. Although the error rate was at first much higher than in the TEDDY PT, most of the errors occurred in the first year of the VQPT. Performance in the VQPT program improved over time, and in several of the latest quarterly challenges, all the laboratories received a perfect score. Combined with the highly accurate PT results from TEDDY, our experience documents the technical feasibility of population-based public health newborn screening, which could provide the early identification of T1DM risk essential for subsequent prediction and intervention strategies.25