Identifying mechanisms of diseases with complex inheritance patterns, such as macular telangiectasia type 2, is challenging. A link between macular telangiectasia type 2 and altered serine metabolism has been established previously. METHODS Through exome sequence analysis of a patient with macular telangiectasia type 2 and his family members, we identified a variant in SPTLC1 encoding a subunit of serine palmitoyltransferase (SPT). Because mutations affecting SPT are known to cause hereditary sensory and autonomic neuropathy type 1 (HSAN1), we examined 10 additional persons with HSAN1 for ophthalmologic disease. We assayed serum amino acid and sphingoid base levels, including levels of deoxysphingolipids, in patients who had macular telangiectasia type 2 but did not have HSAN1 or pathogenic variants affecting SPT. We characterized mice with low serine levels and tested the effects of deoxysphingolipids on human retinal organoids. RESULTS Two variants known to cause HSAN1 were identified as causal for macular telangiectasia type 2: of 11 patients with HSAN1, 9 also had macular telangiectasia type 2. Circulating deoxysphingolipid levels were 84.2% higher among 125 patients with macular telangiectasia type 2 who did not have pathogenic variants affecting SPT than among 94 unaffected controls. Deoxysphingolipid levels were negatively correlated with serine levels, which were 20.6% lower than among controls. Reduction of serine levels in mice led to increases in levels of retinal deoxysphingolipids and compromised visual function. Deoxysphingolipids caused photoreceptor-cell death in retinal organoids, but not in the presence of regulators of lipid metabolism. CONCLUSIONS Elevated levels of atypical deoxysphingolipids, caused by variant SPTLC1 or SPTLC2 or by low serine levels, were risk factors for macular telangiectasia type 2, as well as for peripheral neuropathy.

Summary Serine, glycine, and other non-essential amino acids are critical for tumor progression, and strategies to limit their availability are emerging as potential cancer therapies 1 – 3 . However, the molecular mechanisms driving this response remain unclear, and the impact on lipid metabolism is relatively unexplored. Serine palmitoyltransferase (SPT) catalyzes the de novo biosynthesis of sphingolipids but also produces non-canonical 1-deoxysphingolipids (doxSLs) when using alanine as a substrate 4 , 5 . DoxSLs accumulate in the context of SPTLC1 or SPTLC2 mutations 6 , 7 or low serine availability 8 , 9 to drive neuropathy, and deoxysphinganine (doxSA) has been investigated as an anti-cancer agent 10 . Here we exploit amino acid metabolism and SPT promiscuity to modulate the endogenous synthesis of toxic doxSLs and slow tumor progression. Anchorage-independent growth reprograms a metabolic network involving serine, alanine, and pyruvate resulting in increased susceptibility to endogenous doxSL synthesis. Targeting the mitochondrial pyruvate carrier (MPC) promotes alanine oxidation to mitigate doxSL synthesis and improves spheroid growth, while direct inhibition of doxSL synthesis drives similar phenotypes. Restriction of dietary serine/glycine potently induces accumulation of doxSLs in xenografts while decreasing tumor growth. Pharmacological modulation of SPT rescues xenograft growth on serine/glycine-restricted diets, while reduction of circulating serine by inhibition of phosphoglycerate dehydrogenase (PHGDH) leads to doxSL accumulation and mitigates tumor growth. SPT promiscuity therefore links serine and mitochondrial alanine metabolism to membrane lipid diversity, which sensitizes tumors to metabolic stress.

SUMMARY The AMP-activated protein kinase (AMPK) is a highly conserved master regulator of metabolism, whose activation has been proposed to be therapeutically beneficial for the treatment of several metabolic diseases, including nonalcoholic fatty liver disease (NAFLD). NAFLD, characterized by excessive accumulation of hepatic lipids, is the most common chronic liver disease and a major risk factor for development of nonalcoholic steatohepatitis, type 2 diabetes, and other metabolic conditions. To assess the therapeutic potential of AMPK activation, we have generated a genetically engineered mouse model, termed iAMPKCA, where AMPK can be inducibly activated in vivo in mice in a spatially and temporally restricted manner. Using this model, we show that liver-specific AMPK activation reprograms lipid metabolism, reduces liver steatosis, decreases expression of inflammation and fibrosis genes, and leads to significant therapeutic benefits in the context of diet-induced obesity. These findings further support AMPK as a target for the prevention and treatment of NAFLD.