Page 43 - 2020_09-Haematologica-web

Molecular heterogeneity of PK deficiency
mutations and it is therefore difficult to determine the severity of any one individual variant in critical regions of the gene.
Splice site variants
Splice site variants have been reported in all exon/intron boundary sequences. Most of these variants affect the ±1 or ±2 nucleotides of the donor/acceptor sequences, and are consequently considered to have a drastic effect on splicing, causing unstable and degraded mRNA.
Care should be taken in the interpretation of more internal variants that require functional analysis before
A
defining their pathogenicity; this is the case of rare PKLR variants such as c.1269+5G>A,48 c.507-20C>A14, c.100+10G>A and c.375+10G>T,48 considered by authors to affect the splicing only basing on in silico analysis.
It is worth noting that some missense mutations in the coding region may also affect splicing, in particular when located in the first/last nucleotides of the exons, e.g., c.507G>A,17 c.694G>T,48,51 c.1269G>C,52 or the c.1436G>A variant (p.R479H),38 located in the last nucleotide of exon 10, typically but not exclusively found in the Amish com- munity. The deleterious effect of these variants should always be considered in genetic counseling, or in evaluat-
B
C
Figure 2. Type of PKLR pathogenic variants. (A) The type of PKLR pathogenic variants (n=290) reported in the Human Genome Mutation Database (March 2020). (B) The type of PKLR mutations (n=127) reported in a series of 257 patients with pyruvate kinase deficiency.35 (C) Genotypes in a series of 177 unrelated patients with pyruvate kinase defi- ciency.35
haematologica | 2020; 105(9)
2221