Pompe disease (OMIM 232300), also known as glycogen storage disease type 2 (GSD2) is a rare, progressive, autosomal recessive disorder caused by a deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA; OMIM 606800), leading to lysosomal glycogen accumulation in many tissues, particularly respiratory, skeletal and cardiac muscles (1–3). Patients' age at the onset of Pompe disease and the rate of deterioration can vary considerably. First symptoms can present in infants, children, and adults. In clinical practice, Pompe disease has been classified in two main subtypes: infant-onset Pompe disease (IOPD), which is rapidly progressive with a typical hypertrophic cardiomyopathy, and the late-onset Pompe disease (LOPD), characterized by a milder disease progression and free from cardiac involvement (4, 5). The “classic” form applies to those infants who die within the first year of life, while the term “atypical” defines patients with milder myopathy who typically have respiratory failure between 12 and 18 months of age but can live longer (6).

Given the low prevalence of Pompe disease, very few cases of Pompe disease have been reported in China (4). There are relatively rare data describing the distribution of GAA mutations, phenotypes, disease course, and treatments for this disorder among individuals from China. The GAA gene is located on chromosome 17q25.2–q25.3 and contains 20 exons (1).

As of May 2023, more than 900 different variants had been listed in the Pompe Disease Mutation Database (http://www.Pompecenter.nl/). These gene mutations exhibit race specificity. In Thai patients, Amarinthnukrowh et al. reported that mutation c.1935C > A accounted for 80% of the mutant alleles and is the most common mutation in the Thai population (7). The c.1726G > A causes low GAA activity in normal individuals and is relatively common in Asian populations (8). While in Caucasian and Belgian patients, the most common mutation is c.-32-13T > G (4).

However, despite the increasing awareness of Pompe disease, data on PD in patients from mainland China are still limited. The objective of the present study was to identify the clinical manifestations, echocardiographic imaging and genetic characteristics of classic and atypical PD.

We described 15 patients with typical presentation of PD and fatal outcomes early in life. In this study, the cardiac characteristics of PD patients were comprehensively evaluated by analyzing the myocardial function data obtained from clinical, conventional echocardiography, serological biomarkers and GAA gene analysis. All patients showed a typical clinical course of infantile onset, characterized by hypertrophic cardiomyopathy and severe systemic weakness in the first few months of life, usually rapid progression and death at 1 year old.

Due to the low prevalence of Pompe disease, there is a scarcity of data focused on clinical characteristics, phenotype, and genetic characterization in China. Early recognition and diagnosis of Pompe disease is important. The clinical spectrum of PD depends on the age of onset. The severity of clinical manifestations, tissue impairment and age of onset correlate with the nature of mutations and the residual enzymatic activity levels (20). Classic infantile-onset Pompe disease (IOPD) is the most aggressive and life-threatening form of the disease.In classical IOPD, early manifestations of muscle weakness are often seen, as well as cardiomyopathy or cardiac hypertrophy (21). In contrast, late onset Pompe disease (LOPD) is generally associated with a wider range of age of onset and clinical symptoms. In our study, 80% (12 of 15 cases) patients presented with a typical clinical course of the infantile-onset form, which was characterized by hypertrophic cardiomyopathy and profound generalized weakness presenting in the first few months of life, with rapid progression and death usually occurring by one year of age. As IOPD patients continued to grow, they developed previously unrecognized complications of Pompe disease, such as ptosis, hyper-nasal speech, osteopenia, sensorineural and/or conductive hearing loss and gastroesophageal reflux (22). In our study, two patients were of late-onset form. The age of late onset was between 2 and 3 years old. They were complicated with hypertrophic cardiomyopathy, progressive lower limb weakness, and patient 11 had intellectual and hearing impairment.

The natural course of infantile Pompe disease has also been reported in a large retrospective chart review of 168 cases, including patients with atypical and classic infantile-onset form (23). In the large review study, the median age at symptom onset was 4 months, diagnosis at 4.7 months, and death occurred at 8.7 months. The average delay of diagnosis is 5.84 months in IOPD newborns with cardiomyopathy and other symptoms developed during the first 12 months of life. In PD patients with the onset of symptoms during the first 12 months of life, the longest delay of 109 months, was reported. A similar delay was observed in PD patients with the symptom onset between 12 months and 12 years (18, 20). In our study, the median age at symptom onset was 5.07 months and diagnosis at 19.53 months. All patients died during follow-up, and the median age at death was 24.80 months. The age at diagnosis was considerably older than that in the previous studies. Lack of awareness and qualified diagnostic tests might be the reason for the late diagnosis in Northwest of China. In this study, we found that the delay between symptom onset and diagnosis for Chinese patients with PD was similar to that in patients from the rest of the world. We feel that even with the use of blood-based assays there is still room for improvement in the delay of symptom onset and diagnosis. With the availability of specific tests, physicians can now diagnose Pompeii disease without invasive procedures. Improving understanding of the early symptoms and applying diagnostic tests of Pompe disease can prevent unnecessary invasive examinations such as muscle biopsy and achieve early diagnosis.

In our cohort, 6.7% (1 of 15 cases) of the patients was found having sensorineural hearing loss and mental retardation. Currently, hearing loss is recognized as an important cause of morbidity in IOPD patients. The auditory dysfunction of IOPD patients has been described in several studies; the most common type of hearing impairment is the sensorineural type (24–26). Chien Y-H et al. found that hearing loss and cognitive developmental delays were present in long-term survivors with IOPD (26). Van Capelle et al. reported on 11 IOPD patients who received Enzyme replacement therapy (ERT), 90.9% of them had hearing impairment (25). This may be explained by the less severe mutations and higher levels of residual α-glucosidase activity in juvenile patients (25). Martin et al. identified the central and peripheral nervous system dysfunction caused by glycogen storage for the first time through anatomopathological study (27). Hearing deficits were present shortly after birth in patients with IOPD. The extent of hearing loss did not decrease over time. This suggests that storage of glycogen in the cochlea had already commenced during pregnancy, and that enzyme therapy has no effect on it (25). It was also reported that weakness of the tensor veli palatini muscle might have caused a pressure drop in the middle ear, and may very well be the cause of the high incidence of conductive hearing loss found in infantile patients with Pompe disease (25, 28).

GAA sequencing is used to confirm PD diagnosis and identify the pathogenic variants. More than 900 different mutations have been identified in the GAA gene to date. Investigations have shown that the clinical course of Pompe disease is mainly established by the nature of the mutations in both GAA alleles, leading to different degrees of enzyme deficiency. In PD, as well as other genetic disorders, it is not easy to find a close correlation between genotype and phenotype. Up to 20% of mutations reported in GAA variant database are described without a strict correlation genotype/phenotype. PD patients with severe infantile form carry mutations that alter all forms of GAA causing low expression and enzymatic activity (18, 29, 30). The same mutations can be found in both infantile and late onset patients often with different incidence. GAA mutation analysis is used to confirm the diagnosis, assess the genotype−phenotype correlation, identify carriers within families, and provide genetic counseling (especially in IOPD families). In our study, we made a clear diagnosis of three children in a family, one of whom was diagnosed by postmortem umbilical cord blood (Patient 14), one was normal by amniocentesis, and one was identified as a heterozygous carrier of GAA-G615V/G802V mutation. In this paper, we characterized four novel mutations: c.2102T > C (p.L701P), c.2006C > T (p.P669l), c.766T > A (p.Y256N), and c.2405G > T (p.G802V). These affected amino acids were conserved in vertebrate genomes, making these mutations likely to be directly related to disease status. Nevertheless, the impact of new mutations should be assessed by functional studies.

According to previous studies, the frequency of gene mutations varies from country to country. PD is considered to be a pan-racial disease. However, the prevalence of the disease is very low in some countries. For example, in Finland, only one Pompe patient has been diagnosed with PD (31, 32). The c.-32-13T > G variant was the most common pathogenic mutation reported in California. The c.-32-13T > G splice mutation is very common in Caucasian patients with the childhood/adult form of the disease, with an allelic frequency ranging from 40% to 70% in different populations (33–36). Pompe disease-positive babies of Asian and Pacific Islander (API) heritage have a significant prevalence of the c.[1726G > A;2065G > A] variation (8, 37). The recent study found that there may be a potential high birth rate of Pompe disease in African Americans, but more data are needed to confirm (36). The c.525 del T and c.2481 + 102 _ 2,646 + 31 del mutations are highly prevalent in the Dutch population (35% and 31%, respectively) (38, 39). In contrast, the allele frequency of c.525 del T was 13.8% in Italian infants. In China, the most common pathogenic mutation in infants in northern China is c.2G > T (p. Glu888 *) mutation, accounting for 23.1% (40) of the total mutant alleles. The most common mutation in patients from southern China was c.1C > A (p.Asp 645Glu), accounting for 20%–25% of the total mutant alleles (20).

Our study is a retrospective, single-center cohort with a relatively small sample size. In future studies, a large number of patients with Pompe disease will be important to verify the results of our survival analyses. The cut-off values determined in the current investigation may not be appropriate when utilizing a system different from our study since the parameter GLS is not yet standardized. In addition, this was a preliminary study and the number of cases included in the study was limited. More conclusions need to be answered by further multicenter prospective studies.

This study is one of the largest single-center studies regarding phenotypes as well as genotypes in Pompe disease in China. Infantile Onset Pompe Disease course was dominated by hypertrophic cardiomyopathy in patients with classic infantile disease. Its clinical spectrum strongly correlated with residual acid alpha-glucosidase activity. Residual activity of this enzyme is primarily determined by the severity of the pathogenic mutations on both GAA alleles and likely controlled by yet unknown modifying factors. Our study resulted in the discovery of 4 novel mutations. We emphasize the apparent benefits of early diagnosis. Finally, our study illustrated natural courses of PD patients in our cohort who were not treated by enzyme replacement therapy due to resource limitation at the time of our study.