China nationwide landscape of 16 types inherited metabolic disorders: a retrospective analysis on 372,255 clinical cases

Inherited Metabolic Disorders (IMDs), which is also termed “Inborn Errors of Metabolism”, is a class of inborn genetic diseases. Usually, it begins with defects on genes encoding enzymes. Resultant defected enzymes fail to normally catalyze a variety of in vivo chemical reactions, causing metabolic malfunctions. Consequently, a large amount of substance may accumulate, causing toxic effects or malfunctions in vivo [1]. Decades ago, diagnosis of IMDs was difficult and many types of IMDs were untreatable. Nowadays, thanks to the advancement of technologies, diagnosis of IMDs is facilitated and improved by using mass spectrometry (MS)-based technologies [2]. And parts of previously untreatable IMDs now become treatable. Conventional therapies for several IMDs are dietary restriction, dietary supplementation, etc. Recently, advanced therapies such as gene therapy and enzyme replacement are available [3].

Urine organic acids could be used as specific diagnostic indicators for certain kinds of IMDs. E.g., a part of amino acid disorders and organic acidemias. And hence hospitals or clinical laboratories measure the concentrations of organic acids to indicate people’s conditions of relevant IMDs [4]. Currently, most hospitals and medical laboratories measure amino acids and acylcarnitines in dried blood spot samples for early screening of IMDs, including amino acid disorders, organic acidemia and fatty acid oxidation disorders [5]. While urine could also be a useful and reliable source for diagnosing several types of IMDs because concentrations of a variety of organic acids and other components could also be quantitated as well [6]. For example, using urine for clinical tests, the increase of homogentisic acid could indicate people’s conditions of Alkaptonuria (AKU, OMIM# 203,500). The increase of isovalerylglycine could indicate people’s conditions of Isovaleric Acidemia (IVA, OMIM# 243,500). The increase of Glutaric acid, Glutaconic acid, 3-hydroxyglutaric acid could indicate people’s conditions of Glutaric Acidemia Type I (GA-I, OMIM# 231,670). Specifically, using blood samples alone for clinical tests, it is difficult to distinguish between Methylmalonic Acidemias (MMA, OMIM# 251,000, 277,400, 277,410, 251,100, 251,110, 277,380, 309,541, 613,646, 614,265 and 614,857) and Propionic Acidemia (PA, OMIM# 606,054). While quantitation of methylmalonic acid and methylcitric acid from people’s urine samples could indicate people’s conditions of MMA [7‐9]. Moreover, together with the measurement of concentration of citrulline in people’s blood sample, measuring concentrations of organic acids in people’s urine, e.g., 4-hydroxy phenyllactic acid, 4-hydroxy phenylpyruvic acid, orotic acid, and uracil, could indicate other disorders such as Citrullinemia Type I (CIT-I, OMIM# 215,700), Citrin Deficiency disease (CD, OMIM# 605,814 and 603,471) and Ornithine Transcarbamylase Deficiency (OTCD, OMIM# 311,250) [10]. Therefore metabolites in urine are also valuable for IMDs diagnosis.

Several studies have been conducted to estimate the prevalence of IMDs in preliminary screening and high-risk screening, as shown in Table 1. Epidemiological data of the above studies indicate that IMDs seemed have a relatively low prevalence in general compared with other more commonly seen diseases. Despite such data from above studies, IMDs are by no means less important than other diseases. Instead, IMDs should be paid high attention to because they are closely associated with early neonatal death and abnormal growth and development.

However, to our best knowledge, there lacks such comprehensive analysis and up-to-date study, which can provide IMDs testing data of a large population and reflect the global IMDs landscape of a nation. For regional IMDs information, the aforementioned studies could be referred to some extent [11‐17] (Table 1). While in terms of IMDs information covering greater areas, the above works had limited sample sizes, and hence have limited reference value for greater areas. For instance, Han et al. described an analysis on 18,303 patients’ organic acid assays and IMDs data collected from February 2002 to June 2012. Though this work covered most provinces of China, yet several provinces such as Henan, Tibet, and Inner Mongolia regions were not included, and thus it was not a nationwide study [13]. Therefore, the objective of the study is to evaluate IMDs incidence and regional distributions in China at a national and province level. To do so, we analyzed the age and regional distributions of different IMDs. This work revealed the latest IMDs information of both the provincial domestic situation and the nationwide situation in China, it will provide a reference for promoting policy implementation and resource allocation for IMDs.

To report this study, the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement was used [18].

AKU is an autosomal recessive IMD. In theory, it affects both men and women in equal probability. Nonetheless, the disease was found to be more severe for men [28]. In our dataset, the number of positive case of AKU by gender (Male: Female) was 6:1 (Table 2), which is different from the theory. While another observation on 339 cases of AKU patients also reported an imbalanced ratio (about 2:1) of Male vs. Female number of positive AKU [29]. Unfortunately, the cause of the difference in disease severity is unknown so far.

So far, AKU is difficult to be diagnosed via public program of new-born screening of China because GC–MS measurement of urine homogentisic acid (which is the gold-standard diagnosis method for AKU) is currently not covered by the public screening program [28]. Another reason why AKU is hard to be diagnosis might be that, due to AKU has association with multiple types of clinical symptoms. Patients and clinicians may consider patient has other types of diseases rather than AKU [30]. Therefore, for people who have multiple types of symptoms, e.g., the arthritis and renal calculi/kidney stones, it is better not to forget to take an examination of AKU.

In result section, we mentioned that, after using 30 degree north latitude was used to divide China’s provinces, we observed provinces with lower latitude had significant higher positive rates of CD that those with higher latitude. Interestingly, another reports also compared positive CD data between Guangdong province and Shaanxi province, and stated that latitudinal gradient existed for CD, with a higher prevalence reported in lower latitudes [31, 32].We also analyzed the provincial latitudinal data and our own CD positive data. Nevertheless, we did not observe aforementioned latitudinal gradient in our dataset. At least, in our dataset, the positive rates of CD were not always increase along the increase of provinces’ degree number of latitude.

For clinical diagnosis, clinical signs and symptoms between MMA and PA are nonspecific, and hence it is not easy to distinguish between this pair of diseases [33]. Therefore, we suggested that there might be similarities between MMA and PA. Using the positive rate data of MMA and PA in different provinces in China, we conducted the Student’s test. As a result, the p-value was found to be smaller than 0.05, indicating that a significant difference existed between MMA’s and PA’s positive case provincial distribution. Therefore, in our dataset, we did not observe the association of distribution between MMA and PA. Moreover, a rough comparison between Figs. 2 and 5 could also support this conclusion.

The age ranges set for statistics become wider as the age increases. Most diseases show a trend of decreasing incidence rates with increasing age, indicating that IMDs usually occurs in childhood, except for some mild/late-onset IMDs such as OTCD and MMA. From a perspective of age, 1–6 months’ cases showed highest number of positive IMDs. i.e., 1441 cases. Age stage of younger than 28 days old was with the second highest number of IMDs positive. i.e., 1121 cases. High number of IMDs positive case of new-born infants might be because of the advancement and wider application of diagnostic technology based on tandem mass spectrometry and gas chromatography-mass spectrometry. Compared with DNA sequencing-based IMDs diagnosis, combination of LC–MS/MS and GC–MS for IMDs diagnosis generally takes shorter waiting time and is more economic. And thus the combination of LC–MS/MS and GC–MS technologies allow precise and quicker diagnosis of IMDs, which for IMDs patients, is the prerequisite of the life-saving early therapy.

Another reason why 1–6 months’ cases was peak age period might be that, several IMDs were failed to be screened in public programs for new-born screening. After the public screening program, children displayed abnormal symptoms found by parents and clinicians, and through IMDs clinical tests, children were confirmed to have IMDs.

We investigated the differences in IMDs incidence among provinces. The incidence distribution of most diseases in this study was consistent with previous studies. As aforementioned, the incidence of HPA is higher in northern China, mainly concentrated in northwestern regions such as Qinghai, Gansu, and Ningxia [34]. In contrast, CD showed a high incidence in southern China (Guangxi, Fujian) [31, 32] which might result from the heritability of IMDs. Most IMDs are autosomal recessive. And the primary cause of onset is whether the parents carry the pathogenic genes, so it is region-dependent and not affected by the environment. Furthermore, the study relying on Hospital Quality Monitoring System showed that one possible reason for the high incidence of MMA in Shanghai, Beijing, and Chongqing might be that patients are concentrated in these large cities with better medical resources [35]. Moreover, this study indicated that MMA concentrated in the eastern region because the major specimens were obtained from grassroots medical units in underdeveloped areas, which could reflect the practical disease distribution. Moreover, we did not find any previous studies reporting the incidence of GA-1, and our study indicated a high incidence of GA-1 in southeast China.

We also briefly reviewed the locations of hospitals that sent samples to KingMed. All samples of KingMed went through the commercial IMDs diagnosis program based on mass spectrometry, and the diagnosis program was paid by patients themselves. To make it simple, all samples of KingMed were 72% of the samples came from developed cities, and 8% samples were from poorer areas. This indicated that a significant difference of healthcare resource distribution across China. Another report also support this viewpoint [35]. Therefore, obviously, current policies and programs for healthcare have a lot to be improved. Current public new-born screening programs only cover limited types of IMDs diagnosis and the testing methods are immunoassays. Considered aforementioned advantages of gas chromotography and mass spectrometry technologies, and high IMDs positive cases and rates for infant who were younger than 6 months old, we recommend healthcare authorities to optimize healthcare policies based on data of our work. In light of aforementioned advantages of mass spectrometry-based IMDs diagnosis, e.g., lower costs, shorter waiting time, and coverage of diagnosis of more types of IMDs than immunoassay methods, we recommend that, the public new-born screening programs could include mass spectrometry-based assays for diagnosis of multiple types of IMDs. Also the screening programs would be better to cover infants who are less than 6 months old. Moreover, the programs should be spread to wider areas, especially those poorer areas with less healthcare resources.

There are several limitations of this works. First, a part of gender information is missing from the assay record due to the issue of database. Second, IMDs naturally have low prevalence compared with other types of common diseases. Consequently it is not easy to discover the patients (or positive cases) of IMDs. In our dataset, several types of IMDs had very low number of positive cases, making us difficult to further analyze and study these diseases. For example, the HMGCLD only had 5 positive cases and MGA only had 6 positive cases in our dataset. Thirdly, this work focused on organic acid disorders and a part of disorders of amino acid metabolism that, whose diagnostic indicators were associated with urine organic acids. Instead, this work did not included the analysis of fatty acid oxidation disorders and their relevant diagnostic indicators. Fourthly, since the samples in this research showed a geographical heterogeneity, and the uneven total number of cases in different regions, which might hurt representativeness of this study.

Though our dataset is large enough, above limitations limited us from doing further and deeper investigations on IMDs. While in the future, we plan to extend this work by overcoming above limitations. E.g., we could seek for other complementary resources so as to carry out further studies on IMDs. In addition, another large-scale dataset of LC–MS/MS-based China nationwide clinical blood testing for IMDs is also available from KingMed databases. We might integrate current dataset with that for further analysis, which might be able to depict better and more detailed global landscape of IMDs conditions in China. What is more, we might be able to explore and discovery better diagnostic indicators for IMDs, or use state-of-art artificial intelligence / machine learning methods to construct better and smarter diagnostic models.

In this study, we collected and comprehensively analyzed 372,255 Chinese peoples’ clinical test data and IMDs diagnostic information (Table 2). Through statistical analyses, we characterized differences of IMDs’ distribution patterns by region, gender (Table 3), age etc. As a result, we discovered the unique distribution patterns of different IMDs. For example, the OTCD tended to progress on male infants who were less than 28 days old (Table 4). The MMA had the highest number of positive case among 16 types of IMDs (Table 2), and it had an imbalanced distribution pattern in China and its positive rate was significant higher in North China than South China (Fig. 2), and so on.

Results of our analyses provided most up-to-date information of IMDs of China and different provinces. Such information is valuable in all kinds of aspects. For instance, the provincial information informs domestic hospitals and clinicians about local status of IMDs. And provincial and nationwide information of IMDs also provide useful insights to the works of epidemiologists and workers of public health. Moreover, such information could also inspire medical policy makers via offering solid data and evidences for policy-making. Currently, regions of China are suffering from various kinds of medical and healthcare issues including but not limited to imbalanced medical resources distributions and insufficient budgets. Thus, we strongly recommend medical authorities of China to make better healthcare policies by referring to this comprehensive analytic study.