Thread by @thatdnaguy

#ASHG20 MAC
Impairment of the mitochondrial one-carbon metabolism enzyme SHMT2 causes a novel brain and heart developmental syndrome.
Margot A. Cousin.

#ASHG20 MAC SHMT2 encodes the mitochrondrial serine hydroxymethyltransferase 2. Loss embryonic lethal in mice. Both mitochondrial and cytosolic functions.

#ASHG20 MAC [ primarily mitochondrial though. ] Individuals with biallelic SHMT2 variants - 5 individuals with similar phenotypes from 4 families.

#ASHG20 MAC Patient 3 had an indel in a canonical splice junction. RNA-Seq showed use of cryptic splice acceptor. No exon skipping.

#ASHG20 MAC All variants missense or single amino acid deletion. Extremely rare or absent in gnomAD.

#ASHG20 MAC Congenital microcephaly that improves with age. Arched eye brows. Long philtrum. Short 5th finger. Some syndactyly. Low-set thumbs. [ Deep phenotyping is important! ]

#ASHG20 MAC Most have microcephaly detected prenatally. Moderate to severe intellectual disability. Cardiomyopathy. All have some degree of motor dysfunction. Lower limb pyramidal signs common.

#ASHG20 MAC No biochemical or amino acid changes in urine or CSF when investigated. Brain MRIs see thin corpus callosum. Patient 4 had muscle and heart biopsy. Type 2 fiber atrophy. Ragged red fibers. Consistent with mitochondrial dysfunction.

#ASHG20 MAC Looked at SHMT2 dimers 3D structure. 3 variants near active site. 4 near the dimerization interface. Dynamic simulations: variants alter binding pockets and / or dimer interface over time. [ Don't know what software, maybe in paper? ]

#ASHG20 MAC Theme of 3D modeling - impaired oligomerization or altered active site shape / accessibility.

#ASHG20 MAC In patient fibroblasts, showed decrease in glycine / serine ratios. 5-methyltetrahydrofolate / total folate increased. Supports folate and glycine / serine metabolism affected in these individuals.

#ASHG20 MAC Look at oxidative capacity with Seahorse. Decreased basal and maximum respiration.

#ASHG20 MAC Shmt2 knockdown in Drosophila neurons using a gal4 driver. Two independent RNAi lines. Assayed motor functions and neuromuscular junction (NMJ) integrity. Reduced climbing in knockdowns.

#ASHG20 MAC The NMJs had significant increase in number of satellite Boutons. Abnormal synaptic growth. So knockdown has cell autonomous effects in motor neurons.