ACSF3 encodes an
acyl-CoA synthetase, which is localized in the
mitochondrial matrix and has a high
specificity for malonic acid and methylmalonic acid. These
substrates are activated by ACSF3 through an
ATP-dependent reaction, linking them to
coenzyme A (CoA) and generating the
thioesters
malonyl-CoA and
methylmalonyl-CoA. A major proposed source is the non-enzymatic
hydrolysis of cytosolic malonyl-CoA generated during
de novo fatty acid synthesis, whose levels correlate with
lipogenic activity. Propionic acid arises from
bacterial fermentation in the
gut and from dietary intake, being naturally present in certain cheeses or added as a
preservative, especially in baked goods.
Propionyl-CoA carboxylase forms D-
methylmalonyl-CoA, which is
epimerized to L-methylmalonyl-CoA and converted by
methylmalonyl-CoA mutase to
succinyl-CoA for entry into the
citric acid cycle, a reaction that requires the
coenzyme adenosylcobalamin. However, D-methylmalonyl-CoA may also be
hydrolyzed by
D-methylmalonyl-CoA hydrolase, releasing
coenzyme A and generating methylmalonic acid, which represents a
by-product of this pathway. But in CMAMMA, methylmalonic acid mainly derives from threonine metabolism, as shown in
Acsf3 knockout mice. Moreover, methylmalonic acid was found to impair
osteogenesis by inhibiting
osteoblast differentiation and reducing
mineralization, providing a mechanistic link to the reduced body length observed in these mice.
Product deficiencies Defective ACSF3 leads not only to accumulation of its substrates but also to reduced levels of its mitochondrial products, malonyl-CoA and methylmalonyl-CoA.
Malonyl-CoA Malonyl-CoA is an intermediate that cannot cross
membranes and therefore requires local synthesis within mitochondria. Partial compensation of defective ACSF3 by mtACC1 could explain the broad clinical heterogeneity of CMAMMA. Mitochondrial malonyl-CoA is required for
lysine malonylation,
mitochondrial fatty acid synthesis, acetyl-CoA synthesis and incorporation into cellular lipids. This can influence protein
conformation, enzyme activity, and
protein–protein interactions and has been linked to the regulation of energy metabolism, in particular
glycolysis and
β-oxidation. ACSF3 expression, tightly coupled to feeding cycles, controls the extent of mitochondrial lysine malonylation by regulating the availability of malonyl-CoA, which serves as the donor of malonyl groups. In
ACSF3 and
Acsf3 knockout models, mitochondrial lysine malonylation was shown to be markedly reduced, confirming that ACSF3-derived malonyl-CoA is required for this modification.
Mitochondrial fatty acid synthesis (mtFAS) has been described as a nutrient-responsive
signaling pathway linked to acetyl-CoA utilization,
respiratory chain function,
iron–sulfur cluster biogenesis, mitochondrial
translation, and l
lipid-mediated signaling processes. It generates acyl-ACP species of different chain lengths, which fulfill distinct functions:
Octanoyl-ACP (C8) is one such mtFAS product and a direct precursor of
lipoic acid biosynthesis, which serves as a
cofactor for several mitochondrial
multienzyme complexes involved in energy metabolism, including the
pyruvate dehydrogenase complex (PDH), the
α-ketoglutarate dehydrogenase complex (α-KGDH), the
branched-chain α-ketoacid dehydrogenase complex (BCKDH), the
2-oxoadipate dehydrogenase complex (OADH), and the
glycine cleavage system (GCS). In humans, this network comprises at least 12 proteins and regulates mitochondrial translation, iron–sulfur cluster biogenesis, and the assembly of
electron transport chain complexes. The mitochondrial methylmalonyl-CoA pool, however, is primarily provided via the propionate metabolism pathway, where it is synthesized from
propionyl-CoA by
propionyl-CoA carboxylase. The importance of this anaplerosis varies with tissue type and metabolite levels and is particularly pronounced in the
brain, where maintaining the
α-ketoglutarate pool supports the production of
GABA and
glutamine. In CMAMMA, mitochondrial methylmalonyl-CoA does not accumulate, a major distinction from
isolated methylmalonic acidemias that may explain the absence of acute metabolic decompensation events. In line with this, the pathological post-translational modification
lysine methylmalonylation, for which methylmalonyl-CoA serves as the donor, is reduced in
Acsf3 knockout mice and is even lower than in healthy controls. This is reflected in reduced levels of the fusion mediators
mitofusin-1 (MFN1) and
mitofusin-2 (MFN2) and abnormal
phosphorylation of the fission mediator
dynamin-related protein 1 (DRP1), resulting in
mitochondria that are smaller, more numerous, and fragmented rather than elongated. As in other diseases with disturbed mitochondrial dynamics, CMAMMA is also associated with alterations of the
endosomal–
lysosomal system, reflected by an overrepresentation of proteins in the endosomal (15-fold) and lysosomal
lumen (10-fold). Their increased levels are consistent with the reduced mitochondrial spare respiratory capacity and may reflect the compensatory up-regulation of
Complex IV. Beyond these bioactive lipids, structural membrane lipids are also altered, with
phosphatidylglycerins decreased as precursors of cardiolipin synthesis,
phosphatidylcholines reduced as major components of cellular membranes, and
triacylglycerides, the main storage lipids, are increased about two-fold with altered chain length and odd-chain species arising from propionyl-CoA, accompanied by increased expression of
CD36.
Tertiary effects Patients with CMAMMA often develop
neurological symptoms later in life, including
seizures,
psychiatric problems,
memory impairment, and progressive
cognitive decline. These tertiary effects suggest a link between disturbed energy metabolism and
neurodegeneration. Although
neural cells have a high energy demand, they cannot efficiently rely on fatty acids for energy, with the exception of certain
hypothalamic neurons and astrocytes. In CMAMMA, a fibroblast study demonstrated a compensatory shift toward mitochondrial β-oxidation, a process associated with higher
oxygen consumption,
hypoxia, and
oxidative stress. It is therefore hypothesized that chronic reliance on β-oxidation in
neural tissue, together with dysregulated mitochondrial dynamics and impaired lysosomal clearance of
misfolded proteins or toxic products, may drive the gradual progression toward neurodegeneration observed in these patients. == Diagnosis ==