Pulmonary Outcome Measures in Long Term Survivors of Infantile Pompe Disease on Enzyme Replacement Therapy: A case series

Abstract

Objectives:

To report the respiratory function of school aged children with infantile Pompe disease (IPD) who started enzyme replacement therapy (ERT) in infancy and early childhood.

Study design:

This is a retrospective chart review of pulmonary function tests (PFT) of: 1) IPD patients 5 to 18 years of age 2) who were not ventilator dependent, and 3) were able to perform upright and supine spirometry. Subjects were divided into a younger (5-9 years) and older cohort (10-18 years) for the analysis. Upright and supine forced vital capacity (FVC), maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) were analyzed.

Results:

Fourteen patients, all cross-reactive immunologic material (CRIM)-positive, met the inclusion criteria and were included in this study. Mean upright and supine FVC were 70.3% and 64.9% predicted, respectively, in the 5-9 years old cohort; and 61.5% and 52.5% predicted, respectively, in the 10-18 years old group. Individual patient trends showed stability in FVC over time in six of the 14 patients. MIPs and MEPs were consistent with inspiratory and expiratory muscle weakness in the younger and older age group but did not decline with age.

Conclusion:

Data from this cohort of CRIM-positive IPD patients showed that ERT is able to maintain respiratory function in a subgroup of patients whereas others had a steady decline. There was a statistically significant decline in FVC from the upright to supine position in both the younger and older age groups of CRIM-positive ERT treated patients. Prior to ERT, patients with IPD were unable to maintain independent ventilation beyond the first few years of life.

Introduction:

Pompe disease is an autosomal recessive lysosomal storage disease caused by a deficiency of acid alpha-glucosidase (GAA) – an enzyme that hydrolyzes lysosomal glycogen. Deficiency of GAA results in lysosomal glycogen accumulation in many organ systems including the heart, skeletal muscle, smooth muscle, and central nervous system (1, 2). In the respiratory system, lysosomal glycogen accumulation occurs in the muscles of respiration as well as the motor neurons innervating these muscles and patients eventually develop respiratory failure (3). Pompe disease can be broadly classified into infantile-onset and late-onset forms. The Infantile-onset form is further sub-divided into classic and non-classic forms (2). Classic infantile-onset Pompe disease (IPD) occurs in infants with little to no GAA activity and results in severe cardiomyopathy and weakness within the first few days to weeks of life (2, 4). Non-classic IPD patients have less severe cardiac involvement in the first year of life and have a longer survival (2, 5). In contrast, patients with late-onset Pompe disease (LOPD) have a more variable presentation, absence of cardiomyopathy in first year of life and can present anywhere from the first year to sixth decade of life (6-8).

Respiratory insufficiency occurs in both IPD and LOPD patients as a result of progressive diaphragm and accessory muscle weakness (3, 9-11). As a result of this muscle weakness, hypoventilation initially occurs at night and then progresses to daytime hypoventilation. In addition, restrictive lung disease occurs as evidenced by a reduced total lung capacity secondary to the inability of the respiratory muscles to generate adequate lung volumes (9, 10). The progressive muscle weakness also manifests as a decreased forced vital capacity (FVC), a drop in FVC from the upright to supine position and also a decreased maximal inspiratory and expiratory pressures (MIP and MEP). A drop in supine FVC is often one of the initial indications of diaphragmatic weakness(10). Lingual weakness is also noted in patients with Pompe disease(12, 13). Furthermore, with progressive weakness of the tongue and respiratory muscles, there is an inability to protect the upper airway and coughing is impaired. This combination of factors can result in aspiration pneumonia and inadequate airway clearance.

Enzyme replacement therapy (ERT) with recombinant human acid alfa-glucosidase was granted FDA approval for patients with Pompe disease in 2006 and has significantly improved prognosis in both IPD and LOPD patients (14-17). Prior to ERT, patients with IPD became ventilator dependent or died within their first 2 years of life (6, 18, 19). ERT has significantly changed the course of the disease in IPD with children surviving ventilator-free well beyond infancy (14-16). However, as these children are living longer, new long-term medical issues are unmasked (20). One of these issues is respiratory muscle strength weakenss and respiratory insufficiency. ERT slows the decline of pulmonary function in LOPD patients who are not ventilator dependent (21). However, response to treatment is variable in IPD patients and there are several factors that can impact the response to ERT including age at ERT start, stage of the disease, and cross-reactive immunologic material (CRIM) status (18, 22, 23). ERT is less effective in patients that are CRIM-negative, due to the antibody response against the ERT (18, 19, 24). Finally, ERT cannot cross the blood-brain barrier to treat respiratory motor neuron pathology (25, 26). Although there have been reports of long-term survivors of IPD patients on ERT, the pulmonary status of those patients who are ventilator-free into childhood is unclear. To date, no study has examined the impact of ERT on respiratory function in long-term surviving IPD patients. The purpose of this study is to report the respiratory function of school-aged IPD children 5 years of age and older who were diagnosed and initiated on ERT in infancy and early childhood.

Methods:

Subjects:

This study was approved by the Duke Institutional Review Board (Protocol #: Pro00010830). Patients were included based on the following criteria; 1) a confirmed diagnosis of IPD as previously described (27), 2) five to 18 years of age, who were diagnosed in the first year of life with cardiomyopathy, and 3) able to perform spirometry as well as maximal inspiratory pressure and maximal expiratory pressure (Figure 1). Patients were excluded if they had no pulmonary function testing (PFT) available on file, were unable to perform PFT and if they were ventilator dependent. Five years of age was chosen as the lower age cut off because with appropriate coaching, at this age they are often able to perform acceptable spirometry (28, 29). PFT results were interpreted by a pediatric pulmonologist (RMK) at the time of the visit, and confirmed by a pediatric pulmonologist (MKE) during the time of the retrospective chart review. Study relevant clinical data including demographics, GAA variants, CRIM status, ACE genotype, ERT dosing, and motor status were extracted from patient’s health records.

Figure 1:

Eligible patients for analysis

Pulmonary Function Tests:

FVC, MIPs, and MEPs were reviewed for each patient per year. If a patient performed spirometry more than once in a given year, the best spirometry values were used and spirometry was not included if the patient had an illness. In addition, PFTs that were performed during an illness were not used for this analysis. Data was then analyzed in groups and were combined into a younger age group – 5-9 year old where the last upright and supine PFT in that age group was used. Another group from 10-18 years old was also analyzed and the latest upright and supine PFTs recorded were used for each patient in this age group. The last available data point for any of the evaluated patients was at 17 years of age so the data is presented as “10-17 years”. Six patients had PFTs that spanned the two age groups and were included in both the 5-9 year age group and the 10-17 year age group. In order to take a closer look at individual patient trends and to eliminate the bias of group data, we also graphed the best FVC per year in each patient.

Statistics:

Normality testing was performed using the D’Agostino & Pearson normality test. Since a subset of FVC data did not pass the normality test, we used non-parametric testing to ensure uniform statistical analysis was used on all the data. All data were analyzed using Graph Pad Prism version 7.04. The upright and supine FVC were analyzed using the Wilcoxon test. MIPs for the older age group passed the normality test (p=0.16), but the MIPs did not (p=0.007). MEPs in the older age group did not pass normality testing (p=0.03) whereas MEPs in the younger age group passed the normality test (p=0.96)therefore MIPs and MEPs were also analyzed using a non-parametric test – the a Mann Whitney U test. Differences were considered statistically significant if p<0.05.

Results:

A retrospective chart review of 35 IPD patients who were seen at Duke Pompe disease clinic between 1999 and 2018 was conducted. Out of these 35 IPD patients, 14 patients, all CRIM-positive, met the inclusion criteria (Table 1). The median age of diagnosis was 2 months (range: prenatal to 18 months). The patient diagnosed at 18 months of age had non-classic IPD. The median age of ERT initiation was 3 months (range: 18 hours of life to 42 months). All patients, were on 40 mg/kg/week of ERT at the time of their latest PFT (Table 1). The changes in ERT doses can be seen in the individual patient plots (Figure 3). During their initial PFT, patients were on varying doses of ERT (Table 1). Forced vital capacity (FVC) was decreased in all patients in both the upright and supine position in both age groups. First, the group data was examined by looking at the latest recorded upright and supine FVC for each patient in the 5-9 year and the 10-18 year age group. The mean FVC in the 5-9 year old age group (n=13) was 70.3% predicted and 64.9% predicted in the upright and supine positions, respectively (Figure 2A, p=0.005). In the 10-17 year age group (n=7), the mean FVC was 61.5% predicted upright and 52.5% predicted in the supine position (Figure 2B, p=0.016). The decline in FVC between upright and supine spirometry was statistically significant in both age groups as can be expected in a child with myopathy or neuromuscular involvement (5-9: p=0.005 and 10-17:p=0.016, respectively).

TABLE 1:

Dose of enzyme replacement therapy in classic IPD patients during the time that pulmonary function testing was performed.

ID/Gender	Complementary DNA change		ACE Genotype	Age at diagnosis (months)	Age at ERT initiation (months)	Current age (years)a	Cough assist device or NIPPV	Mobility
	GAA Variant 1	GAA Variant 2
1/M	c.1438-1G>T	c.1655T>C	I/I	Prenatal	2	19	NONE	Ambulatory with difficulties. Uses scooter
2/M	c.1933G>A	c.1933G>A	N/A	3	3	17	NONE	Bilateral AFOs
3/M	c.1327-2A>G	c.1327-2A>G	N/A	Prenatal	0.03	11	NONE	Bilateral AFOs
4/F	c.1802C>T, c.1726G>A, c.2065G>A	c.1099T>C	I/I	6	6	15	Cough assist device	Wheelchair
5/M	c.525delT	c.1642G>T, c.1880C>T	I/D	Prenatal	0.5	12	NONE	Bilateral AFOs
6/M	c.1933G>A	c.1933G>A	I/D	2	2	14	Cough assist device	abnormal gait and uses wheelchair
7/F	c.1293_1312del20	c.1716C>G	N/A	1.2	2.1	9	NONE	Ambulates independently with AFOs
8/M	c.307C>T	c.2219_2220delTG	I/D	1	2	10	NONE	Ambulate independently with frequent falls
9/M	c.2297A>C	c.2297A>C	N/A	6	6	15	NONE	Wheelchair
10/F	c.655G>A	c.655G>A	N/A	7	7	13	Cough assist device	Wheelchair
11/M	c.2481+102_2646+331del	c.947A>G	N/A	0.4	0.4	7	Cough assist device	Uses Chipmunk shoe inserts wheelchair for long distance
12/F	c.1447G>A	c.2560C>T	D/D	13	13	6	Cough assist device	Uses bilateral hinged AFO’s
13/M	c.1408_1410delAAC	c.925G>A	D/D	7	8	6	Cough assist device	Unable to bear weight on legs.
14/F	c.3G>A	c.923A>C	N/A	18	42	19	Cough assist device; BiPAP	Motorized wheelchair

Abbreviations: ACE, angiotensin-converting enzyme; AFO, ankle-foot orthoses; BiPAP, bilevel positive airway pressure; ERT, enzyme replacement therapy; GAA, acid alpha-glucosidase; F, female; IPD, infantile Pompe disease; M, male; mo, month; N/A, not available; NIPPV, nasal intermittent positive pressure ventilation; wk, week; y, year.

^aAll patients are currently receiving ERT at a dose of 40 mg/kg/wk.

Figure 3:

Panels A & B: Upright and Supine Forced Vital Capity per patient over time shows the variable progression of IPD pulmonary function over time and the response to different doses of ERT. Of note, patients did not always undergo supine spirometry when they performed upright spirometry. For example, patient 14 did not have any supine spirometry measurements and patient 4 did not have any supine spirometry measures beyond 12 years of age. Green # and * depict the change in ERT dose to 30mg/kg/week and 40mg/kg/week, respectively.

Figure 2:

Forced Vital Capacity (FVC), Maximal Inspiratory Pressure (MIP) and Maximal Expiratory Pressure (MEP) in classic IPD patients on Enzyme Replacement therapy. Panels A and B shows the latest upright and supine FVC data point in the 5-9 year old and 10-17 year old patients. Panel C shows MIPs and MEPs at 5-9 years and again at 10-17 years. Data represented as mean ± SEM; paired t-test analysis; *p<0.05. Table 2 shows the age and the ERT dose for these patients during the time that the upright and supine FVC graphed in Figure 1 were measured. Patient 14 did not have a supine PFT so was not included in this table.

Next, we examined the individual upright and supine FVC for each of the patients in the study (Figure 3) and correlated this with the dose of ERT. Nine of the fourteen patients had spirometry performed over several years. Of these nine patients, three (patients 2, 3, and 6) had improving upright FVC over time and this was enhanced by increasing the ERT dose to 40 mg/kg/week. Patient 2 had an improvement of 74% at 5 years of age to 91 % at 17 years of age. Patient 3 had an improvement of FVC from 55% at 6 years of age to 63% at 10 years of age. Patient 6 had an improvement of FVC from 88% at 5 years of age to 91 % at 13 years of age. However, there was a decline in upright FVC for patient 1, patient 4, and patient 5. All the patients with an improvement in their FVC had ERT commenced in the first 3 months of life. Patient 14 was diagnosed at a later age and started ERT much later than the rest of the IPD patients described here which may explain why the FVC of this patient is low. However, her FVC remained stable without further deterioration.

We also analyzed maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) (30, 31). MIPs and MEPs are less reliable because they depend on a patient’s motivation and are subject to more variability (32-34). MIPs and MEPs in the younger age group were consistent with significant inspiratory and expiratory strength weakness. These values did not decline with age, but instead, they seemed to improve slightly (mean MIP improved from −45mmHg to −66mmHg; mean MEP improved from 48mmHg to 67mmHg). However, a Mann-Whitney U test shows that MIPs did not significantly decrease with age (U=46.5, p=0.44); similarly MEPs did not significantly deteriorate with age (U=29.5; p = 0.57).

Discussion

Historically, patients with IPD were either deceased or ventilator dependent within the first two years of life (18, 19). The advent of ERT has resulted in ventilator-free survival in these CRIM-positive Pompe disease patients. Despite the significant improvement in ventilator-free survival in CRIM positive patients with Pompe disease, there continues to be evidence of respiratory insufficiency. The respiratory system is affected in Pompe disease with glycogen accumulation occurring in the diaphragm (3), the tongue (35), the tracheal smooth muscle (36), and in the respiratory motor neurons (35, 37). In this current case series, there is a variable course with some patients showing an annual decline in FVC and others showing an improvement. It is unclear whether the improvement is secondary to improved diaphragmatic strength but nonetheless it shows that there is stability of lung function in these patients who historically would have been ventilator dependent if they were not on ERT.

A few of our IPD patients continued to have a decline in FVC despite increasing the dose of ERT to 40 mg/kg/week. This deterioration occurred in some patients despite an early diagnosis and early ERT initiation (17). For example, patient 8 was diagnosed at 1 month of life and started ERT at 2 months of life but his FVC was significantly lower than the other patients who started ERT later. Patient 4 had a significant decline in FVC over time despite increasing ERT dose and she is the only patient in this study who is also BiPAP dependent. She was diagnosed at 6 months of age and so had a significant delay in ERT initiation. In addition, patients 1 and 4 had a decline in FVC despite the fact that patient 1 was diagnosed in utero and started ERT at 2 months. In addition, both patients 1 and 4 have an I/I ACE genotype and were the only patients in this group with I/I ACE genotype. The I/I ACE genotype has previously been correlated with a delay in muscle weakness and disease onset but in these two patients it does not appear to be protective and their FVC continued to decline (22, 38, 39). Thus, in this case series, age at ERT initiation and ACE genotyping have only a slight correlation with the rate of FVC decline. When all the data were grouped and averaged into younger and older spirometry groups, there was a significant drop in supine FVC at both time points. This is expected with the neuromuscular phenotype of these patients. However, when averaged upright spirometry in the younger age groups was compared to that of the older age group FVC did not significantly decline. This can possibly be explained by the optimization of ERT dosing with 40 mg/kg/week effectively which blunted further respiratory decline (14, 40, 41).

However, ERT did not prevent a decline in respiratory function in all of our patients. This persistent decline may be due to inefficient targeting and uptake of ERT into the respiratory muscles such as the diaphragm and intercostal muscles (42). In addition, these patients most likely also had persistent respiratory neuropathology (25, 26, 43) since ERT does not cross the blood brain barrier.

Several preclinical studies from Pompe disease animal models (including the Gaa^−/− mice and the acid-maltase deficient Japanese quail) reveal substantial glycogen accumulation in the motor neurons (44-46), and specifically the respiratory motor neurons (25, 35, 37, 47). In fact, when muscle was rescued in a muscle-specific GAA mice that only had CNS pathology, measurements of respiratory function revealed significantly blunted breathing at baseline and during a challenge with hypercapnia compared to wild type (25) This neurological pathology is also noted in patients with Pompe disease. Clinically, autopsy findings of patients with Pompe disease confirm anterior horn motor neurons and brain stem motor neuron glycogen storage(48-50). In addition, autopsy reports of Pompe patients that were on ERT also show persistent glycogen accumulation in the motor neurons of the cervical spinal cord where the phrenic motor neurons are located (25, 26, 51). These motor neuron findings are reported in both IPD (25, 51) and LOPD patients(26) and highlight the importance of treating the respiratory motor neurons in order to correct the respiratory pathology.

Nonetheless, although ERT does not cross the blood-brain barrier, its impact on the respiratory muscles appears to preserve some respiratory function in CRIM-positive patients. Our findings are consistent with those findings in LOPD patients which demonstrate a variable evolution of pulmonary function tests and respiratory strength (10, 52). Pellegrini et al reported that LOPD patients have a gradual decline in respiratory function over time with a weak correlation between respiratory and locomotor function (52). A recent study from the Pompe registry also reported stability in FVC over five years in 379 LOPD patients (53).

The limitations of this case series include a small sample size. In addition, since this study was mostly retrospective in nature, aspects of the data were missing for the older children in the study. As stated above, we also only evaluated patients that were able to perform pulmonary function testing which makes the study biased towards those patients with better respiratory function at 5 years of age. One point that needs to be made is we only looked at the patients who are currently alive and others may have died of respiratory failure or other causes. Furthermore, there were no CRIM-negative patients at the time of this study that had evaluable PFTs and therefore were not included in this study. However, recent advances in immune modulation with ERT have enhanced survival in CRIM-negative patients and these patients have a new and emerging phenotype but data on pulmonary function tests has yet to be collected (54). We will need to evaluate more of the IPD patients over time to assess the impact of genotype, ACE genotype, CRIM status, and dose escalation on the rate of decline of FVC (19, 22, 41). Finally, because these children had variable efforts with their maximal inspiratory and expiratory strengths measurements, these data were difficult to interpret. The higher MIPs and MEPs at the older age groups are most likely a reflection of improved effort rather than improved muscle strengths. A decrease in FVC from the upright to the supine position is a more sensitive indicator of diaphragm weakness than MIPs and MEPs (30, 31). MIPs and MEPs are difficult to perform reliably because they depend on effort and may be altered if there is any bulbar weakness (30).

In conclusion, this study is the first to report the respiratory function of CRIM-positive IPD patients who commenced ERT in the first year of life. In preparation for upcoming clinical trials involving novel ERTs and gene therapy (55, 56), this respiratory function information provides a baseline so we can better evaluate the impact of future therapies.

Table 2.

Age and ERT Dose for patients included in Figure 2.

Patient	Age at PFT 1 (years)	ERT dose at PFT 1	Age at PFT 2	ERT dose PFT 2
1	7.1	20 mg/kg qoweek	17.4	40 mg/kg/week
2	9.0	20 mg/kg/qoweek	16.3	30 mg/kg/week
3	7.8	30 mg/kg/qoweek	10.1	40 mg/kg/week
4	9.0	30 mg/kg/week	12.8	40 mg/kg/week
5	8.1	30 mg/kg/qoweek	11.5	40 mg/kg/week
6	8.2	40 mg/kg/qoweek	12.9	40 mg/kg/week
7	7.8	40 mg/kg/week
8	8.7	40 mg/kg/week
9	7.8	30 mg/kg/qoweek	12.3	40 mg/kg/week
10	8.9	40 mg/kg/week	11.2	40 mg/kg/week
11	6	40 mg/kg/week
12	5.9	40 mg/kg/week
13	5.9	40 mg/kg/week