Multispectral Optoacoustic Tomography for Translational Molecular Imaging in Pompe Disease

Pompe disease (PD) is a rare, autosomal-recessive disorder caused by deficiency of the lysosomal acid alpha-glucosidase enzyme (GAA), leading to generalized build-up of glycogen, especially in the heart, muscle, liver and nervous system. Among the glycogen storage diseases, PD is the only one with a defect in lysosomal metabolism.

PD is considered as a progressive disease with variation by age of onset, severity of organ involvement and degree of myopathy. This great phenotypic variability has led to the creation of types based on the age of onset and degree of organ involvement. They all have in common, that symptoms of affected patients are expected to worsen over time if left untreated. The classification is generally based on the age of onset as infantile (infantile onset Pompe disease, IOPD) when it presents during the first 12 months of life and late-onset (LOPD) when first symptoms appear after 12 months of age. If cardiomyopathy is present, IODP is generally referred to as classic Pompe disease (however there may be variably classification in the literature with the infantile or childhood forms). Clinically, infants with classic PD present during the first few months of life with rapidly progressive disease characterized by prominent hypertrophic cardiomyopathy, hepatomegaly, hypotonia, generalized muscle weakness, macroglossia, feeding difficulties and respiratory insufficiency. Mortality rate is high by one year of age if untreated. Patients with non-classic PD will usually present within the first year of life with motor developmental delay and weakness, but without clinically relevant cardiac involvement. The rate of clinical progression is slower in these children and without treatment, death will usually occur in childhood as a result of respiratory insufficiency. LODP include childhood and adult-onset PD. These patients generally present with slowly progressive limb girdle type weakness and respiratory insufficiency without significant cardiomyopathy. The diagnosis of PD is usually established by the typical clinical presentation, followed by confirmation of GAA deficiency in dried blood spots, e.g. through new-born screening. Further (confirmatory) methods include GAA activity measurement in lymphocytes, muscle or skin fibroblasts, as well as GAA mutation testing. All of them are invasive techniques. Early identification is important as it will likely significantly improve the outcome for all patients with PD as treatment can be initiated earlier. Treating the underlying cause of PD involves the replacement of the missing enzyme GAA via enzyme replacement therapy (ERT) with alglucosidase alfa (recombinant human GAA, rhGAA). Currently, this is the only specific treatment approved for PD. In classical IOPD, treatment significantly lengthens survival and improves motor development and respiratory and cardiac function. The sooner ERT begins, the better are the results. With ERT being one very important aspect of care, patients will also need a multidisciplinary approach to ensure that all aspects of the disease are addressed.Regardless of age of onset and severity, all patients with PD should be monitored prospectively. However, there is lack of standardization across centers. A variety of clinical evaluations and tests are currently used for monitoring Pompe's disease, which may include laboratory tests including CK, AST, ALT, and LDH, cardiologic tests including electrocardiogram and echocardiogram and respiratory tests including sleep studies and breathing tests to measure lung capacity. To quantify muscle involvement electromyography is an option as well as clinical tests including 6 minutes walking test or timed to up and go test. Muscle MRI of affected patients often show fatty degeneration of muscles. One study showed that muscle MRI correlates with muscle function in patients with adult-onset Pompe disease. Another study suggested that muscle imaging data in late-onset Pompe disease reveal a correlation between the pre-existing degree of lipomatous muscle alterations and the efficacy of long-term enzyme replacement therapy. For small children, however, there is always a need for sedation for MRI's, limiting its use. Therefore, ultrasound is another option to examine children's muscles.

At the moment there are no prospective biomarkers available to detect muscle degeneration at an early age and/or to follow up disease progression or ERT-treated patients. Within the last years our multidisciplinary research team (Medical Department 1, Department of Pediatrics and Adolescent Medicine, University Hospital Erlangen) published a novel non-invasive imaging modality to be able to detect subcellular tissue composition in vivo. Multi-spectral optoacoustic tomography (MSOT), an imaging technology comparable to ultrasound, allows quantitative imaging in patients of all ages (including the non-sedated child).