Discussion
The molybdenum cofactor deficiency is an ultra-rare autosomal recessive disease, characterized by rapidly progressive and severe neurological damage, sometimes misdiagnosed as hypoxic-ischemic encephalopathy. The clinical and molecular characteristics of molybdenum cofactor deficiency due to the MOCS2 variant have been recently extensively reviewed by Arican et al. (2019). The presented group encompassed 35 patients with proven molecular etiology and 29 different pathogenic variants identified in the MOCS2 gene.
Our proband’s history and clinical features are consistent with the previous descriptions. That prompted us to test toward a deficiency of molybdenum cofactor, while neonatal-onset, refractory epilepsy, feeding difficulties, facial dysmorphism, and brain images were the most suggestive phenotypic feature. This initial clinical diagnosis, supported by undetectable serum uric acid, very low serum homocysteine, and positive Sulfite dipstick results, was finally verified in whole-exome sequencing analysis. It revealed a variant of unknown significance in the MOSC2 gene. The homozygous variant c.472_477del on MOCS2 deletes nucleotides ’CTTTTA’ from chromosome5 52396264, which does not change frameshift in the protein-coding sequence. It has been reported with an extremely low frequency in the large population cohorts (GenomAD). This inframe deletion in the non-repeat region can change the length of proteins and disrupt protein function. This variant is classified as uncertain significance according to the recommendation of the American College of Medical Genetics (ACMG) and Association for Molecular Pathology (AMP) guideline. The child’s clinical features were, however, highly consistent with the disease. Additionally, taking into account the homozygous state of this variant, we suggest as causative.
We have performed an analysis of the crystal structure of the human molybdopterin synthase complex to support this statement. The Leu158, Lys159 are localized at the end of the last helix of the catalytic subunit, just before the 13 aa of the C-terminus. This region of the molybdopterin synthase has dual functionality. It binds the sulfur carrier subunit and the precursor Z. The alignment of the crystal structure of human molybdopterin synthase with the one ofStaphylococcus aureus complexed with precursor Z reflects that Lys159 corresponds to Leu116 of (S. aureus ) is essential residue for proper binding of the precursor Z.
In consequence of the mutation, the Leu158 will be replaced by Ala160, and Lys159 by Lys161. Luckily, the deleted Lys159 should be replaced by Lys161, and thus it’s role can be preserved (similar replacement of the key amino acid was observed for example in D-amino acid oxidase, where Tyr314 repossess the function of the Tyr55, eliminated by the Tyr55Ala mutation and modifies the enzyme-substrate specificity (Subramanian, 2014). Since the second one replaces the crucial lysine, one could expect, that the precursor Z binding can still occur, however, the protein-ligand affinity can be already modified. However, in this case, the situation is more complicated. The second lysine residue (123 inS. aureus and 166 in human) is essential for precursor Z binding, and this crucial residue is located at the C-terminus. It is highly probable that the deletion of two residues can modify the position of the second lysine.
Moreover, it will have consequences on the folding of the C-terminus and can result in changes in binding affinity of the sulfur carrier subunit to the catalytic one. Thus, the described deletion could have significant consequences for both, the heterodimer and the active complex formation, which may suggest another mechanism of pathogenicity of c.472_477del variant in the MOCS2 gene. As results from predictSNP have shown (Suppl. 2), no deletion analysis is possible, but the report clearly indicates that any mutation in this area has consequences. However, to verify, which functionality is mostly disturbed, the precursor binding or carrier subunit binding, further study has to be considered.