Intrauterine and Neonatal Levels of Neurotrophic Factors and Matrix Metalloproteinases-9 and Risk of Autism Spectrum Disorders

Background:

In concert with many neuropathological findings and accumulating lines of research suggesting that synaptic pathways are disrupted in some cases of autism spectrum disorders (ASD), abnormal formation of neuronal connections or elimination of inappropriate connections have been postulated to play a crucial role in the onset of ASD. Matrix metalloproteinases (MMP’s) and Neurotrophic Factors (NTFs) are two distinct families of secreted proteins with a pivotal function in a variety of forms of synaptogenesis and neuroplasticity.

Objectives:

To measure levels of MMP-9 and selected NTFs (brain-derived neurotrophic factor [BDNF], Neurotrophin-4 [NT-4], and transforming growth factor-β [TGF-β]) during pregnancy (in amniotic fluid [AF] samples), and after birth (in neonatal dried blood spot samples [n-DBBS]) in individuals diagnosed with ASD later in life and in controls.

Methods:

A total of 414 ASD cases and 820 control subjects, frequency-matched to cases on gender and year of birth, were included in the study. AF samples and n-DBBS were retrieved from a Danish Historic Birth Cohort (HBC) and the Danish Newborn Screening Biobank (DNSB), respectively. Both resources are maintained at Statens Serum Institute, Copenhagen, Denmark. Necessary administrative and ethical approvals were obtained accordingly. Using a unique identifier (CPR number) assigned for each citizen in Denmark, background information, psychiatric and somatic comorbidities and obstetric history of the study population were retrieved from different Danish nation-wide health registers. Measurements of the selected NTFs and MMP-9 were performed with an in-house assay panel using the multiplex flow cytometric assay system Luminex MultiAnalyte Profiling Technology. Associations were analyzed with continuous measures (tobit regression) as well as dichotomized at the lower and upper 10th percentiles cut-off points derived from the controls’ distributions (logistic regression). Supplementary analyses were performed on cases with infantile autism, cases diagnosed with ICD-10 only and after stratifying on gender.

Results:

In second trimester amniotic fluid samples, MMP-9 levels were elevated  in ASD cases compared to controls (Crude and adjusted tobit regression P values: 0.01 and 0.06). In n-DBBS, ASD cases were more likely to have BDNF levels in the lower 10th percentile (crude OR: 1.53 [95% CI: 1.04 – 2.24], P value=0.03), and a similar pattern was seen for TGF-β in females with ASD (crude OR: 2.36 [95% CI: 1.05 – 5.33], P value=0.04). In ASD cases diagnosed with ICD-10, however, NT-4 levels were less likely to be in upper 10th percentile compared to controls (crude OR: 0.22 [95% CI: 0.05 – 0.98], P value=0.05).

Conclusions:

Findings from this study suggest a differential pattern of MMP-9 during pregnancy and of NTFs after birth. Elevated levels MMP-9 during pregnancy may be an indicator of a hyper-plasticity state within the central nervous system. On the other hand, decreased NTFs levels during neonatal period may contribute to the pathophysiology of ASD through impairments of neuroplasticity. Further research is required to confirm our results and to examine the specificity of the findings to ASD.