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Molecular heterogeneity of PK deficiency
accompany other red cell diseases, confounding the hematologic pattern and sometimes making the diagnosis challenging; likewise, concomitant causes of anemia may explain some patients with decreased PK activity and only one mutation detected upon molecular analysis. In a series of 56 French patients diagnosed with PK deficiency by enzymatic assay and submitted to a molecular diagno- sis, 17 cases were reported to carry a heterozygote PK mutation; in three of them, an association with other defects was found, namely a membrane defect, a hexoki- nase deficiency and a glucose-6-phosphate dehydroge- nase deficiency.48 In these cases, complete hematologic investigation and molecular characterization of the involved genes are needed to clarify the correct diagnosis, also in view of therapeutic approaches and genetic coun- seling.85,86
Co-inheritance of heterozygous HbS and PK deficiency (either in the homozygous or heterozygous state)87-89 may induce sickling and worsening of phenotype. In the reported cases, the increase of intraerythrocytic 2,3- diphosphogylcerate concentration induced by the PK deficiency resulted in a decreased oxygen affinity which favored sickling.
The possible contribution of a heterozygous PK defi- ciency to modifying the clinical expression of a mem- brane defect or other congenital anemias is still debated. Some authors excluded a synergetic effect between car- riership for PK deficiency in patients with concomitant hereditary spherocytosis,85,90 while others reported that the co-inheritance of heterozygous PK deficiency was associated with an aggravation of the phenotype in two families, one affected by hereditary spherocytosis91 and the other by congenital dyserythropoietic anemia associ- ated with a GATA1 mutation.92
From genotype to new therapies
The treatment of PK deficiency is based on supportive measures, including blood transfusions, splenectomy, and managing complications. The only curative treat- ment is hematopoietic stem cell transplantation (HSCT); however, due the risk of graft-versus-host disease, this should be considered only in severe cases or when it rep- resents the only realistic therapeutic option.
New therapeutic options that range from a small mol- ecule PK activator to gene therapy are being developed, and may change the way of treating PK deficiency in the future. In this context, a confirmed diagnosis is crucial to have access to these new therapies, and consequently genotyping is becoming a need for most patients; more- over, it may influence the outcome of the treatment and must therefore be taken into account when directing the patient to possible therapies.
Hematopoietic stem cell transplantation
van Straaten et al. recently evaluated the indications, procedures employed and outcomes of HSCT in the series of all the patients with PK deficiency treated between 1996 and 2015 (16 patients from Europe and Asia, no patients resulted as being treated in the USA in that peri- od).93 Two additional cases were recently reported.94,95
The analysis of the genotypes of the treated patients showed a great heterogeneity, and surprisingly, no preva- lence of nonsense pathogenic variants. Despite this, as reported in Table 3, most of the missense variants in this series affected amino acid residues that participate directly in the allosteric and catalytic binding site of the enzyme, supporting the observation of genotype-pheno- type analysis. As expected, no correlation was observed
Table 3. Patients who have undergone hematopoietic stem cell transplantation and their genotype.
Sex Country
Pt1 M Asia
Pt2 F EU Pt3 F Asia Pt4 F EU Pt5 M Asia Pt6 F EU Pt7 F EU Pt8 F EU Pt9 M EU Pt 10 M Asia Pt 11 M EU Pt 12 M Asia Pt 13 M Asia Pt 14 M EU Pt 15 M Asia Pt 16 F Asia Ref 94 F China
Ref 95 M Japan
Genotype
Unknown
p. [E241*; R532W] p.[K348N; R359H] p.[E241*; R488Q]
p. [R40Q; D339N]
p. [M377fs; M377fs] p.[G165V; R510Q] p.[G511E; E538*] p.[I494T; R559*] p.[V283A; I314T] p.[K541fs; K541fs] p.[D221Y; I314T] p.[V283A;V283A] p.[D331Q;D339H] c.[1270-3C>A];p.[G540*] c.[1270-3C>A];p.[G540*] p.[I314T; I314T]
p.[Pro145Hisfs;Pro145Hisfs]
Mutation effect
Splenectomy Age at HCST
No 5 y
Yes 15 No 1y7mo No 3y No 2y6mo
Yes 17y Yes 39y Yes 7y No 6y
No 1y6mo Yes 10y No 9y
No 1y6mo Yes 41y Yes 11 y
Year
1996
2002 2009 2009 2009 2010 2011 2013 2013 2013 2014 2014 2015 2015 Unknown Unknown
Outcome Structure ref
Alive
Deceased 10,11 Alive 47 Alive
Alive 10
Deceased Deceased 16
Unknown
Nonsense Missense 2 Nonsense Missense Nonsense Missense Missense Missense Missense Nonsense Missense 3 Missense Missense 5 Nonsense Nonsense Missense 3
Nonsense
Unknown
Missense 1
Missense 2
Missense
Missense 3
Nonsense
Missense 4
Nonsense
Nonsense
Missense 3
Nonsense
Missense 3
Missense
Missense 3
Nonsense
Nonsense
Missense 3
Nonsense
Alive
Deceased
Alive
Deceased
Alive
Alive
Alive
Alive
Alive
Alive
Alive
10 10,17 10,14
No 8 y
No Unknown Unknown
Yes 32 y Unknown
Missense variants falling in “strategic” functional amino acid residues or associated with documented thermo-unstable variants are reported in bold. 1Directly involved in the fructose 1,6 bisphosphate activator.2Directly involved in the substrate and cation binding sites.3Residues directly involved in the allosteric site and catalytic center.4Highly unsta- ble. 5Proximity of the substrate-binding site.
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