FABP4: A preschool-aged biomarker for autism

ASD is a developmental disorder that begins in early childhood and affects learning, communication, and social behavior. The severity of symptoms falls on a broad spectrum, which researchers believe is related to genetic and environmental factors that interact during brain development. Because young children with ASD are at particular risk for being overweight, the RIKEN CBS group hypothesized a link between ASD and fat-cell metabolism.

Fat cells make hundreds of important biomolecules called adipokines, some of which regulate brain activity. The researchers took blood samples from preschool-aged children with and without ASD and compared their adipokine levels. The team examined adipokines known to be associated with ASD as well as the protein FABP4. “We previously found lower levels of FABP4 in the hair follicles of patients with schizophrenia,” explains first author Motoko Maekawa. “Although the disorders themselves are very different, we knew that FABP4 was an adipokine that can modulate brain function, especially during development.”

Dendritic spines of cortical neurons in layers II and III (left) and layer V (right) in 4-week-old wildtype (top) and FABP4 KO mice (bottom). Like postmortem brains of people with autism spectrum disorders, the number and density of dendritic spines was greater in the FABP4 KO mice than in the control mice.

The researchers found that preschool-aged children with ASD had much lower levels of FABP4 in their blood than other children did, but that other adipokines did not differ between groups. A second test in two other groups of children confirmed these results. This makes FABP4 a potential early biomarker for ASD. “The identification of FABP4 as a biomarker that can detect ASD in four- to -six-year old children is good news,” says Maekawa, “especially because early diagnosis and intervention can lead to better long-term prognosis.”

Further analysis showed that the story is a little more complex. Similar comparisons in older children and in postmortem brains showed equal levels of FABP4 between ASD and non-ASD groups. This means that FABP4 levels differ during a critical period during brain development, making it more than just a biomarker. Its lack could be a factor that leads to ASD, rather than being just a byproduct.

To confirm the importance of FABP4, the researchers created knockout mice that lacked the FABP4 gene. Compared with wildtype mice, behavior testing showed that these mice interacted less with unknown mice and had more difficulty with spatial learning and memory, all reminiscent of difficulties shared by those with ASD. Additionally, when the team examined the neurons in the mouse brains, they found shape and structural characteristics that match those found in postmortem brains from people with ASD.

A model for FABP4 pathogenesis of ASD. (Top, left) In typically developed children, fat cells make many important adipokines, of which Fatty Acid Binding Protein 4 is one. (Top, center) After binding to a fatty acid, FABP4 takes it through the blood and then releases it so the fatty acid can cross over to the brain. (Top, right) Once in the brain, the fatty acids enter neurons or astrocytes (not shown here), and proceed to trigger specific gene expression, which regulates synaptic function and ultimately behavior. (Bottom, left) In children with ASD, genetic and environmental factors (such as a mother’s nutrition during pregnancy) disrupt fat tissue, which then fail to produce normal FABP4. (Bottom, center) This reduces the amount in the blood (the biomarker), which in turn reduces the amount of fatty acid that gets to the brain. (Bottom, right) The change in fatty acid levels leads to altered gene expression, which affects synaptic function (for example, through the increase in spines, see first figure above) and thus behavior.

Looking forward, there are three questions that the researchers hope to answer. “We hope to replicate our findings in a larger group, which will allow us to determine whether specific ASD symptoms or their severity are related to low levels of FABP4,” says Maekawa. “We also hope to conduct a prospective cohort study of newborns to determine if FABP4 levels at birth can predict the future manifestation of ASD.” Such an early indication of ASD would mean that the contributing environmental factors were related to the mother’s condition during pregnancy. In fact, Maekawa has already shown in a previous study that low fatty acid intake during pregnancy (in mice) leads to pups with symptoms of schizophrenia, and the in ASD might be similar.

Lastly, the researchers will continue studying the FABP4 mouse model of ASD to understand exactly how the FABP4 protein affects the developing brain. ✅

ASD is a developmental disorder that begins in early childhood and affects learning, communication, and social behavior. The severity of symptoms falls on a broad spectrum, which researchers believe is related to genetic and environmental factors that interact during brain development. Because young children with ASD are at particular risk for being overweight, the RIKEN CBS group hypothesized a link between ASD and fat-cell metabolism.

Fat cells make hundreds of important biomolecules called adipokines, some of which regulate brain activity. The researchers took blood samples from preschool-aged children with and without ASD and compared their adipokine levels. The team examined adipokines known to be associated with ASD as well as the protein FABP4. “We previously found lower levels of FABP4 in the hair follicles of patients with schizophrenia,” explains first author Motoko Maekawa. “Although the disorders themselves are very different, we knew that FABP4 was an adipokine that can modulate brain function, especially during development.”

Dendritic spines of cortical neurons in layers II and III (left) and layer V (right) in 4-week-old wildtype (top) and FABP4 KO mice (bottom). Like postmortem brains of people with autism spectrum disorders, the number and density of dendritic spines was greater in the FABP4 KO mice than in the control mice.

The researchers found that preschool-aged children with ASD had much lower levels of FABP4 in their blood than other children did, but that other adipokines did not differ between groups. A second test in two other groups of children confirmed these results. This makes FABP4 a potential early biomarker for ASD. “The identification of FABP4 as a biomarker that can detect ASD in four- to -six-year old children is good news,” says Maekawa, “especially because early diagnosis and intervention can lead to better long-term prognosis.”

Further analysis showed that the story is a little more complex. Similar comparisons in older children and in postmortem brains showed equal levels of FABP4 between ASD and non-ASD groups. This means that FABP4 levels differ during a critical period during brain development, making it more than just a biomarker. Its lack could be a factor that leads to ASD, rather than being just a byproduct.

To confirm the importance of FABP4, the researchers created knockout mice that lacked the FABP4 gene. Compared with wildtype mice, behavior testing showed that these mice interacted less with unknown mice and had more difficulty with spatial learning and memory, all reminiscent of difficulties shared by those with ASD. Additionally, when the team examined the neurons in the mouse brains, they found shape and structural characteristics that match those found in postmortem brains from people with ASD.

A model for FABP4 pathogenesis of ASD. (Top, left) In typically developed children, fat cells make many important adipokines, of which Fatty Acid Binding Protein 4 is one. (Top, center) After binding to a fatty acid, FABP4 takes it through the blood and then releases it so the fatty acid can cross over to the brain. (Top, right) Once in the brain, the fatty acids enter neurons or astrocytes (not shown here), and proceed to trigger specific gene expression, which regulates synaptic function and ultimately behavior. (Bottom, left) In children with ASD, genetic and environmental factors (such as a mother’s nutrition during pregnancy) disrupt fat tissue, which then fail to produce normal FABP4. (Bottom, center) This reduces the amount in the blood (the biomarker), which in turn reduces the amount of fatty acid that gets to the brain. (Bottom, right) The change in fatty acid levels leads to altered gene expression, which affects synaptic function (for example, through the increase in spines, see first figure above) and thus behavior.

Looking forward, there are three questions that the researchers hope to answer. “We hope to replicate our findings in a larger group, which will allow us to determine whether specific ASD symptoms or their severity are related to low levels of FABP4,” says Maekawa. “We also hope to conduct a prospective cohort study of newborns to determine if FABP4 levels at birth can predict the future manifestation of ASD.” Such an early indication of ASD would mean that the contributing environmental factors were related to the mother’s condition during pregnancy. In fact, Maekawa has already shown in a previous study that low fatty acid intake during pregnancy (in mice) leads to pups with symptoms of schizophrenia, and the in ASD might be similar.

Lastly, the researchers will continue studying the FABP4 mouse model of ASD to understand exactly how the FABP4 protein affects the developing brain. ✅