Results and discussion
The hematological profile and G6PD activity of both patients are de- scribed in Table 1.
The sequencing analysis of the exons and intronic flanking regions of the G6PD gene identified two novel DNA variants that have not been previously reported (Fig. 1). Neither of the nucleotide changes, searched in the databases Exome Aggregation Consortium (ExAC), 1000 Ge- nomes (1000G), Single Nucleotide Polymorphism Database (dbSNP), and Human Gene Mutation Database (HGMD), were found.
The NM_001042351.2:c.995C N T DNA change was detected in pro- band no. 1. This variant was found in exon 9 and determines the Serine332Phenylalanine (Ser332Phe) amino acid change. The NM_001042351.2:c.1226C N A change was detected in proband no. 2. The variant is located in exon 10 and is associated with the Proline409Glutamine (Pro409Gln) amino acid change.
PolyPhen-2 and SIFT predicted that both amino acid changes (Ser332Phe and Pro409Gln) are probably damaging and deleterious.
Both the mother and the sister of proband no. 1 were heterozygous for the DNA variants found and did not show signs or symptoms of the disease phenotype, a fact that is usually expected for X-linked conditions.
To analyze the impact of amino acid change, the results structures were modeled using the previous crystals of G6PD as a template. The model shows that both mutated amino acids are located far from the ac- tive site, meaning that the interpretation of the change in protein func- tion is not straightforward. The analysis of Ser332Phe using the X-ray structure shows that the main effect is a breakage of the hydrogen bond between the side chains of serine 332 and aspartic acid 251 2). We hypothesize that the amino acid replacement could generate a conformational change in the loop at the access of the active site, which may affect the enzyme activity disturbing the substrate uptake.
Additionally, the Pro409Gln change affects the domain which is rel- evant for dimerization and is located on an intersubunit contact surface. The affected proline is at the end of a β-sheet and, because it is a rigid amino acid, it usually provokes the rupture of secondary structure ele- ments. Consequently, the change of proline for another amino acid could alter the structure and β-sheet, affecting dimer formation. The ho- modimeric or tetrameric structure is essential for protein stability and enzymatic activity [4] and therefore, the Pro409Gln change could affect normal dimerization, decreasing the activity of the enzyme (Fig. 2).
A previously described different mutation (HGMD: CM044625) at the same position 409 (Pro409Arg) leads to changes in G6PD kinetics and determines an unstable variant in comparison to the wild type [14]. This finding allows us to hypothesize that proline plays an impor- tant role in enzyme stability and function.
Although functional assays would be important to demonstrate the pathogenicity of these new variants, unfortunately, it was not possible to perform them at our laboratory.