The benefit of animal models in RCDP research
The use of animal models is essential in research to understand the molecular, cellular, physiological, and pathological manifestations of human diseases. This is made possible by the high degree of anatomical and physiological similarities between humans and animals. As such, researchers can use animal models to investigate the mechanisms behind disorders, learn more about progression, and the genetic components underlying the phenotypes, before applying these hypotheses to humans. Especially with ultra-rare diseases such as, rhizomelic chondrodysplasia puncata (RCDP), animal models also provide a greater sample size to determine the consistent symptoms and manifestations associated with the disorder. RCDP is a class of recessive genetic disorders caused by plasmalogen deficiency. RCDP presents as a very complex and typically severe phenotype consisting of shortening of the proximal limbs, punctate calcifications in epiphyses, cataracts, recurrent respiratory illness, cardiac abnormalities, and cognitive deficits, but the direct cause behind a reduction in plasmalogens and these effects are still being determined and with the help of animal models. This short review will discuss three examples of how researchers are using the Gnpat (glycerophosphate O-acyltransferase; an enzyme involved in plasmalogen synthesis) knock out (KO) model to expand our understanding of RCDP.
Using a Gnpat KO mouse model, Dorninger et al (1) analyzed neurotransmitter levels in plasmalogen deficient mice. Neurotransmitters are chemicals released from a presynaptic axon into a synapse where they can bind to receptors on the dendrites of a postsynaptic neuron, resulting in a signal being transmitted from one cell to another. In the presynaptic neuron, neurotransmitters are packaged into vesicles which will attach and fuse to the membrane of the presynaptic axon, releasing the neurotransmitters into the synaptic cleft. In addition to many proteins involved in this process, the lipid composition of the neuronal membrane also has an important role, and it was hypothesized that reducing plasmalogens would also reduce neurotransmitter signaling. In the brain of Gnpat KO mice dopamine, norepinephrine, serotonin, and gamma aminobutyric acid (GABA) were reduced by 24%, 28%, 23%, and 20%, respectively, compared to wildtype mice. Dorninger et al also determined that there was reduced vesicle binding in the plasmalogen deficient mice. Together these findings would suggest that neurotransmitter signaling is affected by a plasmalogen deficiency. These neurotransmitters are important for many voluntary and involuntary behaviours and processes, therefore these reductions could be responsible for the cognitive deficits and seizures seen in RCDP patients.
Malheiro et al (2) examined the role of plasmalogen deficiency in myelin, an insulating layer on neuronal axons that allows for more efficient transmission of signals between the neurons. Their work demonstrated that demyelination was evident on the spinal cord, optic nerve, corpus callosum, internal capsule, and cerebellum in the Gnpat KO mice compared to wildtype mice. Extensive demyelination could certainly alter axonal function and the ability of neurons to transmit signals, affecting development and function.
Clinical studies of 18 and 14 patients (Huffnagel et al, 2013 (3); Duker et al, 2015 (4)), demonstrated that cardiac abnormalities can be present in patients with RCDP. Based on these findings, Todt et al (5) used the Gnpat KO mouse model to determine if plasmalogen level has any influence on cardiac transduction. It was found that the mutant mouse had a ~40% reduction of connexin43 (Cx43), a broadly expressed transmembrane protein required for the formation of gap junctions. This is significant because gap junctions are essential in cardiac ventricles for conduction of electrical pulses. As well, electrocardiograms (ECG) of the hearts in these mice showed abnormal rhythms. It is important to note that the clinical studies demonstrated developmental differences resulting in anatomical abnormalities, so the results by Todt et al cannot be directly translated to that seen in people with RCDP. However, as plasmalogens are an important component of cell membranes and signal transduction, an alteration in their levels could reasonably alter cardiac impulse conduction. With the increasing diagnoses of cardiac abnormalities in RCDP patients, the more knowledge we can obtain about what specifically causes this, whether it is anatomically or due to deficient signal transduction, the better care these patients can receive.
Research utilizing animal models has demonstrated how a severe plasmalogen deficiency alters neurotransmitter release, myelination, and cardiac impulse conduction, as well as many other discoveries. In addition to providing more information about the full pathology of RCDP, these animal models have offered methods to determine the chemical and molecular alterations present and deliver an easier way to observe organ changes. Although this review only discussed one model, there many Pex7 (peroxisomal biogenesis factor 7) KO for RCDP type 1 models based on the magnitude of enzyme reduction, as well as Gnpat KO for RCDP type 2, and Agps (alkylglycerone phosphate synthase) KO for RCDP type 3), and surely others in development. With more awareness of RCDP comes more research into this disease and a better understanding of what these patients experience and how best to help them. We are already seeing an increase in RCDP animal models, with many studies being published in the last couple years.
References:
1) Dorninger F, Kӧnig T, Scholze P, Berger ML, Zeitler G, Wiesinger C, Gundacker A, Pollak DD, Huck S, Just WW, Forss-Petter S, Pifl C, and Berger J. (2019) Disturbed neurotransmitter homeostasis in ether lipid deficiency. Human Molecular Genetics
2) Malheiro AR, Correia B, Ferreira da Silva T, Bessa-Neto D, Van Veldhoven PP, and Brites P. (2019) Leukodystrophy caused by plasmalogen deficiency rescued by glyceryl 1-myristyl ether treatment. Brain Pathology.
3) Huffnagel IC, Clur SB, Bams-Mengerink AM, Blom NA, Wanders RJA, Waterham HR, and Poll-The BT. (2013) Rhizomelic chondrodysplasia punctata and cardiac pathology. Journal of Medical Genetics
4) Duker AL, Eldridge G, Braverman NE, and Bober MB. (2015) Congenital heart defects common in rhizomelic chondrodysplasia puncata. American Journal of Medical Genetics
5) Todt H, Dorninger F, Rothauer PJ, Fischer CM, Schranz M, Bruegger B, Lüchtenborg C, Ebner J, Hilber K, Koenig X, Erdem FA, Gawali VS, and Berger J. (2020). Oral batyl alcohol supplementation rescues decreased cardiac conduction in ether phospholipid-deficient mice. Journal of Inherited Metabolic Dis.