Autism, RCCX, C4b Deficiency & Synaptic Pruning

RCCX is a copy number variation (CNV) gene cluster located on chromosome 6. Variations of the RCCX genotype leads to an increased susceptibility to numerous diseases. Significant research links together one of the RCCX genes, C4b with autism, associated cell danger signaling, and problems related to neuronal, synaptic pruning.

Last year I wrote an article on the remarkably complex and significant gene cluster located on chromosome 6, known as RCCX. In that article, I presented a theory of why I believe this anomalous gene cluster is likely involved in many complex diseases. Since the time of that article I have been engaged in more intensive research and review of the existing literature. As I’ve come to learn, there’s even more associations that exist. I’ve come away with a deeper understanding of what RCCX represents from an evolutionary perspective, and how and why it is so significant in chronic diseases.

One of the areas of research I’ve been engaged in is related to processes known as synaptic pruning and neuronal maintenance. These processes are highly significant as they relate to brain development, and central nervous system function. The ability for the CNS to prune and maintain axonal terminals is central to the function of neurotransmitter synapses. In humans, pruning processes begin after birth and continue through puberty. There are many genes, and physiological processes involved in the pruning process. These include: trophic factors, hormones, a maintenance process known as autophagy, regulation of mTOR, purinergic signaling, as well as important contributions from elements of the complement immune system.

The inability to adequately or appropriately maintain neurons through pruning is a significant contributing factor in mental illnesses such as schizophrenia. In autism research, it has been known for years that autistic children and adolescents have an excess of synapses in their brain (2, 3), and a reduction in pruning processes. Additional research in mice has found that deficient autophagy leads to the inability to prune synapses, which leads to social and behavioral defects (1).

The Complement Immune System & Synaptic Pruning

The complement immune system is a central part of the ancient, innate immune system. The activation of the innate immune system leads to the recruitment of intra and extracellular processes that invoke cell danger signaling, as well as the recruitment of pro-inflammatory cytokines, and coordination with the antibody system. Central to the complement immune system are complement proteins, C2, C3 and C4.

Complement C4 is a central gene located within the RCCX gene cluster. It is comprised of two isoforms, C4a and C4b. It has been postulated that C4 deficiency is likely the most common type of immune deficiency worldwide. The reason for this is likely related to its residence within the highly unstable RCCX gene cluster. As a copy number variation (CNV) gene sequence, RCCX will feature modular forms, and duplicates of its genes on the chromosome. This leads to genomic instability and various anomalous events to occur. The four possible modular forms of RCCX are determined by the number of C4 repeats. Based upon research, it seems that monomodular RCCX copy number variation phenotypes have an increased risk of autoimmune diseases, especially lupus (5) and type 1 diabetes. These are associated with C4 deficiency.

To make things more complicated is that an endogenous retrovirus, known as HERV-K is situated within the 9th intron of the C4 gene. Retroviral activation and reverse transcription of endogenous retroviruses is known to occur during certain inflammatory states of cell danger signaling (9), and is known to activate the innate immune response, likely through PAMP (pathogen associated molecular pattern) and DAMP (damage associated molecular pattern) formation (10).

Another critical function of Complement C4 is in synaptic pruning. The C4 protein has been found to be the most prevalent protein in the human brain that supports the connection between neurons (6). It has been identified that the strongest genetic risk factor for schizophrenia in the population is with C4 variations, specifically with longer forms of the C4a isoform (6). C4a genes produce 2-3 times the amount of RNA compared to C4b genes.

Null alleles of C4b have been observed in striking association with autism, with studies identifying carriers between 37-42% of autistic children (7, 8). In addition, it has been shown that roughly 40% of the family members of autistic children have autoimmune disease (7). More than 50% of the autistic children with null C4b alleles have been shown to have C4a duplications. This is significant because the C4a genes produce 2-3X the amount of RNA compared to C4b.

To add to this puzzle, it has been demonstrated that autism can feature the expression of endogenous HERV retroviruses, including HERV-K (11). HERV-K is situated within the 9th intron of the C4 gene. The trail of this research implicates the involvement between: RCCX genotype, including the C4 isoforms, synaptic pruning, the innate immune response, cell danger signaling and associated retroviral activation.

Clearly, the associations between autism, RCCX phenotype, C4, C4a, C4b, synaptic pruning and the constituent HERV-K retrovirus deserves deeper research.

Cell Danger Signaling, mTOR, Autophagy & Autism

Inside of cells, a regulatory protein known as mTOR activates numerous pathways that lead to increased growth processes. When mTOR activity is excessive, too much growth occurs. Excessive mTOR is associated with a reduced ability to prune neuronal synapses, and has been found to be excessive in autism (3, 4). Inhibiting mTOR has shown to activate the cell clean-up process known as autophagy, which leads to the pruning of neuronal synpases.

During cell danger signaling, a series of highly conserved intracellular events leads to the release of ATP molecules into the extracellular environment, in order to coordinate danger and inflammatory processes (9). Extracellular ATP processes are known as “purinergic signaling”. Remarkably, purinergic receptors are known to be the most abundant receptor type in mammalian tissues. Purinergic signaling is strongly linked to chronic diseases, and this is especially true of autism (5).

Pioneering research by Naviaux found that the administration of a hundred year old drug named suramin, inhibited purinergic signaling and corrected the core autism features in a small placebo-controlled trial (5).

One major mechanism with how suramin may work in autism is through the inhibition of mTOR. mTOR is known to be an important downstream effector protein that is activated by purinergic signaling, while the phosphorylation of the mTOR protein is required for ATP-activated cell proliferation (12).

In the instance of increased purinergic signaling in autism due to cell danger signaling that doesn’t recede, coupled with excessive neuronal synapses in the autistic brain and excessive mTOR, suramin may inhibit mTOR. This would lead to improvement in autophagy and other processes involved in synaptic pruning. The attenuation of these pathways should result in substantial improvement in brain development, behavior, language and other core, autism-associated symptoms.

Indeed, suramin was shown to inhibit mTOR, and multiple parts to the mTOR pathway (13).

Another probable mechanism of suramin’s effects in autism is likely related to its ability to function as an anti-retroviral, through inhibiting reverse transcriptase (14). Since HERV retroviral activation has been shown to occur in autism, and through this activation the innate immune system is engaged, it suggests the drug may inhibit these processes.

Future research in autism and synaptic function should focus on the complex series of interactions between: RCCX genotype, Complement C4, C4a, C4b, HERV-K retroviral activation, mTOR, autophagy, purinergic signaling and the cell danger response.