The paper by Remmelt Schür and colleagues  provides some (brief) blogging fodder today and the observation that following a "systematic literature review and meta-analysis of 1 H-MRS studies" brain GABA levels were found to be significantly lower in cases of autism spectrum disorder (ASD) than compared to control (not autism) populations.
GABA - gamma-Aminobutyric acid - has been something of interest for quite a few years in autism research circles (see here). It's particular role as an inhibitory neurotransmitter has perhaps been where the lion's share of research has been targeted, bearing in mind it's actions might extend quite a bit further . Indeed, whilst the over-representation of epilepsy in cases of autism (see here) hints at a possible dual role for GABA in relation to autism, I'd be minded to suggest that far more complicated processes might also be at work for some people (see here).
Schür and colleagues surveyed the peer-reviewed research literature for several developmental and psychiatric labels with measured brain levels of GABA in mind. They concluded that outside of autism, there was also some evidence for lower levels of brain GABA in those diagnosed with major depressive disorder (MDD) too (albeit those still presenting with symptoms). Further: "No significant differences in GABA levels were found in bipolar disorder, panic disorder, PTSD [post-traumatic stress disorder], and ADHD [attention-deficit hyperactivity disorder] compared with controls."
Minus any sweeping generalisations about how GABA levels could be a 'uniting' feature of autism and MDD (even though there may be overlap including at a clinical level) I do find the possibility of shared physiology to be an important one. Not least because of discussions about how interventions "aimed at either autism symptoms or symptoms of depression may improve the other"  could very much include GABA as one of several potential clinical parameters.
 Schür RR. et al. Brain GABA levels across psychiatric disorders: A systematic literature review and meta-analysis of 1 H-MRS studies. Hum Brain Mapp. 2016 May 4.
 Andersen PN. et al. Associations Among Symptoms of Autism, Symptoms of Depression and Executive Functions in Children with High-Functioning Autism: A 2 Year Follow-Up Study. J Autism Dev Disord. 2015 Aug;45(8):2497-507.
Schür RR, Draisma LW, Wijnen JP, Boks MP, Koevoets MG, Joëls M, Klomp DW, Kahn RS, & Vinkers CH (2016). Brain GABA levels across psychiatric disorders: A systematic literature review and meta-analysis of 1 H-MRS studies. Human brain mapping PMID: 27145016