Two New GAA Mutations Linked to Stroke in Late-onset Pompe Disease, Study Finds
The study, “GAA compound heterozygous mutations associated with autophagic impairment cause cerebral infarction in Pompe disease,” was published in the journal Aging.
PD is caused by mutations in the GAA gene that provides instructions to make the acid alpha-glucosidase (GAA) enzyme. The abnormal GAA gene leads to either the production of a non-functional enzyme, or blocks its production entirely. Without functional GAA, glycogen (a complex sugar molecule) accumulates in tissues. Also, lack of GAA activity impairs cells’ natural cleaning system — called autophagy.
Despite its genetic cause, PD’s onset can range from infancy to adulthood. The late-onset form is the most common and mild, and although muscle weakness is pointed to as the most frequent symptom, patients’ symptoms are highly variable.
Stroke is rarely described in PD cases, yet reports on its presence in these patients has been increasing. It is speculated that glycogen accumulation in the inner vascular wall may be responsible for the blockage of blood vessels (infarction) in the brain.
A team of researchers in China studied two siblings from a consanguineous family (related by blood from a common ancestor) with late-onset PD who experienced stroke.
The first patient showed signs of leg weakness and complained of constant fatigue at age 14. Brain computerized tomography (CT scan) revealed he had left cerebral infraction, while magnetic resonance imaging (MRI) showed many small bleedings (micro-hemorrhages) in both brain hemispheres.
Researchers observed the patient had an abnormal carotid artery (a major blood vessel in the neck that supplies blood to the brain) and calcified plaques accumulated in brain vessels. They also detected an aneurysm (a bulge that can rupture a blood vessel) in the basilar artery, one of the vessels supplying the brain with oxygen-rich blood.
At examination, the team detected enlargement of spleen and liver, as well as thickening of the mitral valve — located between the two left chambers of the heart.
MRI scan showed multiple bleedings in brain areas such as the cerebellum and brainstem. The patient also had an ischemic lesion (death of brain tissue due to insufficient blood supply) that affected the brain’s white matter (which connects the gray matter via nerve fibers).
Researchers then sequenced the GAA gene in both siblings, which revealed two mutations — one inherited from the father and the other from the mother. The first mutation (p.Trp746Cys) is known to affect the function of the GAA enzyme. In turn, the second mutation (p. Arg463fs) causes a deletion within the gene that leads to early termination of protein production and a non-functional GAA enzyme.
GAA activity levels were measured in the first patient and the child of the second patient. While the first patient had a GAA activity markedly below the reference range, the child’s enzyme activity was slightly below normal.
Next, researchers investigated how the novel mutations affected cells. To do so, they used human cells grown in the lab to express the two mutated GAA forms.
They observed that autophagy was impaired, as shown by accumulation of a protein called LC3. These results seem to indicate that autophagy deficiency and glycogen accumulation caused by the two mutations are the mechanisms causing cerebral infarction in PD patients.
Researchers also examined variations in genes correlated to stroke risk and cardiovascular disease. They found that the two patients and six other family members had more than 50% of the risk alleles (gene copies) in seven genes related with hemorrhagic stroke. Also, two alleles associated to abnormal autophagy and cardiovascular disease were both present in the PD patients. So, these risk factors could not be excluded as causes for stroke.
Finally, researchers tried to understand why the second patient’s child had abnormal blood tests. They evaluated the gut microbiome (the community of microbes living in the gut), since its composition has been linked to a variety of chronic diseases. Results showed similar alterations in mother and child. Although further research is needed, this suggests that mothers might pass down their microbiome alterations to children and impact body metabolism, the scientists said.
Overall, these findings highlight that different GAA mutations can help explain the diverse symptoms of PD, including stroke.
“Our finding enriches the gene mutation spectrum of Pompe disease, and identified the association of the two new mutations with autophagy impairment,” the researchers wrote.