Phosphoribosylaminoimidazole carboxylase (PAICS) deficiency, caused by biallelic variants in PAICS gene, is an inborn error of de novo purine synthesis. Only two patients from a consanguineous family have been reported, with multiple congenital malformations, resulting in early neonatal death. Molecular analysis identified a homozygous p.(Lys53Arg) missense variant. We report the third case of PAICS deficiency in a 7 years old boy, presenting with polymalformative syndrome, but normal neurodevelopment. We report malformations not previously described in PAICS deficiency, notably congenital cardiopathy, and support the consistency of skeletal and oesophageal defects. Genome Sequencing identified the homozygous pathogenic variant p.(Lys53Arg), suggesting a recurrent variant in PAICS. A probable recurrence of PAICS deficiency occurred in a sibling, with a similar polymalformative syndrome antenatally diagnosed, but could not be confirmed molecularly. We further delineate the phenotype of PAICS deficiency and provide new insights concerning prognosis, notably in terms of neurodevelopment.

De novo synthesis of purines (DNPS) is a biochemical pathway that provides the purine bases for synthesis of essential biomolecules such as nucleic acids, energy transfer molecules, signaling molecules and various cofactors. Inborn errors of DNPS enzymes present with a wide spectrum of neurodevelopmental and neuromuscular abnormalities and accumulation of characteristic metabolic intermediates of the DNPS in body fluids and tissues. In this study, we present the second case of PAICS deficiency due to bi-allelic variants of PAICS gene encoding for a missense p.Ser179Pro and truncated p.Arg403Ter forms of the PAICS proteins. Two affected individuals were born at term after an uncomplicated pregnancy and delivery and presented later in life with progressive cerebral atrophy, epileptic encephalopathy, psychomotor retardation, and retinopathy. Plasma and urinary concentrations of dephosphorylated substrates of PAICS, AIr and CAIr were elevated, though they remained undetectable in skin fibroblasts. Both variants affect structural domains in SAICARs catalytic site and the oligomerization interface. In silico modeling predicted negative effects on PAICS oligomerization, enzyme stability and enzymatic activity. Consistent with these findings, affected skin fibroblasts were devoid of PAICS protein and enzyme activity. This was accompanied by alterations in contents of other DNPS proteins, which had co-localized in granular structures that are characteristic of purinosome formation. Our observation expands the clinical spectrum of PAICS deficiency from recurrent abortions and fatal neonatal form to later onset neurodevelopmental disorders. The rarity of this condition may be based on poor clinical recognition and limited access to specialized laboratory tests diagnostic for PAICS deficiency.

Phosphoribosylaminoimidazole carboxylase, phosphoribosylaminoimidazole succinocarboxamide synthetase (PAICS), an essential enzyme involved in de novo purine biosynthesis, is connected with formation of various tumors. However, the specific biological roles and related mechanisms of PAICS in gastric cancer (GC) remain unclear. In the present study, we identified for the first time that PAICS was significantly upregulated in GC and high expression of PAICS was correlated with poor prognosis of patients with GC. In addition, knockdown of PAICS significantly induced cell apoptosis, and inhibited GC cell growth both in vitro and in vivo. Mechanistic studies first found that PAICS was engaged in DNA damage response, and knockdown of PAICS in GC cell lines induced DNA damage and impaired DNA damage repair efficiency. Further explorations revealed that PAICS interacted with histone deacetylase HDAC1 and HDAC2, and PAICS deficiency decreased the expression of DAD51 and inhibited its recruitment to DNA damage sites by impairing HDAC1/2 deacetylase activity, eventually preventing DNA damage repair. Consistently, PAICS deficiency enhanced the sensitivity of GC cells to DNA damage agent, cisplatin (CDDP), both in vitro and in vivo. Altogether, our findings demonstrate that PAICS plays an oncogenic role in GC, which act as a novel diagnosis and prognostic biomarker for patients with GC.

We report for the first time an autosomal recessive inborn error of de novo purine synthesis (DNPS)-PAICS deficiency. We investigated two siblings from the Faroe Islands born with multiple malformations resulting in early neonatal death. Genetic analysis of affected individuals revealed a homozygous missense mutation in PAICS (c.158A>G; p.Lys53Arg) that affects the structure of the catalytic site of the bifunctional enzyme phosphoribosylaminoimidazole carboxylase (AIRC, EC 4.1.1.21)/phosphoribosylaminoimidazole succinocarboxamide synthetase (SAICARS, EC 6.3.2.6) (PAICS). The mutation reduced the catalytic activity of PAICS in heterozygous carrier and patient skin fibroblasts to approximately 50 and 10% of control levels, respectively. The catalytic activity of the corresponding recombinant enzyme protein carrying the mutation p.Lys53Arg expressed and purified from E. coli was reduced to approximately 25% of the wild-type enzyme. Similar to other two known DNPS defects-adenylosuccinate lyase deficiency and AICA-ribosiduria-the PAICS mutation prevented purinosome formation in the patient's skin fibroblasts, and this phenotype was corrected by transfection with the wild-type but not the mutated PAICS. Although aminoimidazole ribotide (AIR) and aminoimidazole riboside (AIr), the enzyme substrates that are predicted to accumulate in PAICS deficiency, were not detected in patient's fibroblasts, the cytotoxic effect of AIr on various cell lines was demonstrated. PAICS deficiency is a newly described disease that enhances our understanding of the DNPS pathway and should be considered in the diagnosis of families with recurrent spontaneous abortion or early neonatal death.