Overlapping and distinct features of cardiac pathology in inherited human and murine ether lipid deficiency.

Dorninger F, Kiss A, Rothauer P, Stiglbauer-Tscholakoff A, Kummer S, Fallatah W, Perera-Gonzalez M, Hamza O, Kӧnig T, Bober MB, Cavallé-Garrido T, Braverman NE, Forss-Petter S, Pifl C, Bauer J, Bittner RE, Helbich TH, Podesser BK, Todt H, and Berger J. (2023) Overlapping and distinct features of cardiac pathology in inherited human and murine ether lipid deficiency. International Journal of Molecular Sciences.

Plasmalogens are a unique class of lipids that contain a vinyl-ether bond at the sn-1 position, giving this class important characteristics. This double bond causes plasmalogens to have a more compact formation than other lipids making them important for membrane fluidity and structure. As well, plasmalogens are essential for vesicular fusion, lipid raft formation, and they have antioxidative properties due to the double bond scavenging radical oxygen species. When people are unable to synthesize plasmalogens because of mutations in genes that encode the enzymes involved in the plasmalogen biosynthetic pathway, it results in Rhizomelic chondrodysplasia punctata (RCDP). RCDP is an ultra-rare genetic disease that causes proximal shortening of the limbs, developmental delays, congenital cataracts, seizures, respiratory issues, and often cardiac problems. To be able to better research the disease, mouse models of plasmalogen deficiency have been developed, and although they do not perfectly model the full pathology seen in humans, they are very useful. Dorninger et al were interested in determining the cardiac phenotype in the glyceronephosphate O-acyltransferase (Gnpat; encodes the enzyme responsible for the first step in the plasmalogen biosynthetic pathway) knockout (KO) mouse model and compared this to the cardiac pathology seen in humans with RCDP.

Due to septal defects being very common in RCDP patients, the authors first wanted to analyze this feature in the Gnpat KO mice. They evaluated heart morphology using histology in embryos at E13.5, because at this point ventricular septation is expected to be completed in a mouse fetus, and at E14.5 for an additional timepoint. It was found that at E13.5 all the wild-type animals had completed ventricular septation. However in the Gnpat KO mice, 8 out of 11 had completed septation with a thinner septum than the wild-type mice and in 3 out of 11 there were ventricular septal defects that would have allowed the exchange of blood between the ventricles. On E14.5 all Gnpat KO animals evaluated had completed ventricular septation, however again it was much thinner than the wild-type counterparts. Because of these findings, Dorninger et al suggest that the plasmalogen deficiency caused a developmental delay since the defect was not present at a later embryonic stage.

Although ventricular septation eventually occurs in the Gnpat KO mice, ensuring proper cardiac function of the heart was also investigated in 12-14 month-old mice using MRI and echocardiography. Both procedures showed that the Gnpat KO mice had diminished stroke volume compared to the wild-type mice, but the ejection fraction (the volumetric fraction of fluid ejected from a heart chamber with each contraction) was not different. As well, when looking at 13-16 month-old mice the Gnpat KO animals had statistically increased left ventricle end-systolic and end-diastolic pressure. There was also increased wall stress on the myocardium in the Gnpat KO mice.

Cardiac pathology in patients with RCDP is complex, but also quite common and the presence of congenital heart disease in severe or less severe RCDP patients occurs frequently. Because of this, the authors reviewed echocardiography data from a registry of RCDP patients and confirmed a range of pathologies including ventricular and atrial septal defects similar to the mouse model, conotruncal anomalies, and congenital mitral valve prolapse. They also evaluated blood pressure and heart rate in the patients and both were found to be mostly normal across all the RCDP patients. To specifically observe whether the defective cardiac transmission seen in their mouse model was also found in RCDP type 1 patients, data from 10 patients was collected. Their cardiac function was found to be largely normal but all had another R wave following the S wave in the QRS complex which may indicate the presence of an atrial septal defect.

As defective cardiac function is a known pathology in disorders caused by plasmalogen deficiency, such as RCDP, Dorninger et al was interested in further characterizing this pathology. They used a mouse model of RCDP Type 1 to identify the morphological and functional differences between these mice and wild-type controls and compare this to what is seen in patients with RCDP. The Gnpat KO mice had much thinner septal walls compared to the control mice and ~25% had septal defects. MRI and echocardiography was used to evaluate cardiac function in the mice and both demonstrated that the plasmalogen-deficient mice had reduced stroke volume, increased left-ventricle end-systolic and end-diastolic pressure, as well as increased myocardial wall stress. However, the ejection fraction was not different in these animals. When looking at patients with RCDP, blood pressure, heart rate, and most other assessments of cardiac function were in normal ranges, but all had an QRS complex pattern that indicated a possible presence of a septal defect. Further work with this model could help clarify exactly what mechanism is causing the cardiac pathology in humans with RCDP and provide a route to determine treatment.