Purines are essential molecules that are components of vital biomolecules, such as nucleic acids, coenzymes, signaling molecules, as well as energy transfer molecules. The de novo biosynthesis pathway starts from phosphoribosylpyrophosphate (PRPP) and eventually leads to the synthesis of inosine monophosphate (IMP) by means of 10 sequential steps catalyzed by six different enzymes, three of which are bi-or tri-functional in nature. IMP is then converted into guanosine monophosphate (GMP) or adenosine monophosphate (AMP), which are further phosphorylated into nucleoside di- or tri-phosphates, such as GDP, GTP, ADP and ATP. This review provides an overview of inborn errors of metabolism pertaining to purine synthesis in humans, including either phosphoribosylpyrophosphate synthetase (PRS) overactivity or deficiency, as well as adenylosuccinate lyase (ADSL), 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (ATIC), phosphoribosylaminoimidazole succinocarboxamide synthetase (PAICS), and adenylosuccinate synthetase (ADSS) deficiencies. ITPase deficiency is being described as well. The clinical spectrum of these disorders is broad, including neurological impairment, such as psychomotor retardation, epilepsy, hypotonia, or microcephaly; sensory involvement, such as deafness and visual disturbances; multiple malformations, as well as muscle presentations or consequences of hyperuricemia, such as gouty arthritis or kidney stones. Clinical signs are often nonspecific and, thus, overlooked. It is to be hoped that this is likely to be gradually overcome by using sensitive biochemical investigations and next-generation sequencing technologies.

Adenylosuccinate synthase (ADSSL1) is a muscle specific enzyme involved in the purine nucleotide cycle and responsible for the conversion of inosine monophosphate to adenosine monophosphate. Since 2016, when mutations in the ADSSL1 gene were first described to be associated with an adult onset distal myopathy, nine patients with compound heterozygous variants in the ADSSL1 gene, all of Korean origin, have been identified. Here we report a novel ADSSL1 mutation and describe two sporadic cases of Turkish and Indian origin. Many of the clinical features of both patients and muscle histopathology and muscle MRI findings, were in accordance with previously reported findings in the adult onset distal myopathy individuals. However, one of our patients presented with progressive, proximally pronounced weakness, severe muscle atrophy and early contractures. Thus, mutations in ADSSL1 have to be considered in patients with both distal and proximal muscle weakness and across various ethnicities.

ADSSL1 myopathy was recently identified as the cause of muscular disorders in Korean patients with distal myopathy. We generated transcriptome profiles of muscles from control subjects and patients with ADSSL1 myopathy. In the present study, RNA sequencing was conducted with seven vastus lateralis muscle samples from four patients with ADSSL1 myopathy and three control subjects. The hierarchical clustering result revealed a separation between myopathy and control groups. A total of 1,260 transcripts were significantly differentially expressed (|fold change| ≥ 2, p < 0.05), with 740 upregulated transcripts and 520 downregulated transcripts in myopathy group. Eighteen transcripts that mapped to purine metabolism pathway were significantly differentially expressed between the two groups, with ten downregulated transcripts and eight upregulated transcripts in myopathy group. In particular, three genes involved in purine nucleotide cycle (ADSSL1, ADSL, and AMPD1) were significantly downregulated in myopathy group. Ten transcripts in glycolysis/gluconeogenesis pathway were also significantly differentially expressed. This is the first study on the altered expression of transcripts in muscle tissues from patients with ADSSL1 myopathy. Our results provide new insights into the pathogenesis of ADSSL1 myopathy.

To understand the characteristics of ADSSL1 myopathy, we investigated the clinical manifestation in Korean patients with ADSSL1 mutations. We developed a targeted panel of 16 distal-myopathy genes and recruited a total of 12 patients with genetically undetermined distal myopathy. We found four (33%) with ADSSL1 mutations and one (8%) with GNE mutations. ADSSL1 mutations consisted of c.910G>A, c.1048delA and c.1220T>C mutations. Patients with ADSSL1 mutations demonstrated distal muscle weakness in adolescence, followed by quadriceps muscle weakness in the early 30s. All patients had mild facial weakness and two patients complained of easy fatigue while eating and chewing. Vastus lateralis muscle biopsies revealed non-specific chronic myopathic features with a few nemaline rods. Whole body muscle MR imaging showed more fatty replacement in the distal limb and tongue muscles than in the proximal limb and axial muscles. This study showed that ADSSL1 myopathy was not rare among distal myopathy patients of Korean origin, and expanded the clinical and genetic spectrum. Therefore, we suggest that the screening test of ADSSL1 gene should be considered for the diagnosis of distal myopathy.

Late-onset Alzheimer\'s disease (LOAD) is a complex and multifactorial disease. So far ten loci have been identified for LOAD, including APOE, PICALM, CLU, BIN1, CD2AP, CR1, CD33, EPHA1, ABCA7, and MS4A4A/MS4A6E, but they explain about 50% of the genetic risk and thus additional risk genes need to be identified. Amyloid beta (Aβ) plaques develop in the brains of LOAD patients and are considered to be a pathological hallmark of this disease. Recently 12 new Aβ toxicity modifier genes (ADSSL1, PICALM, SH3KBP1, XRN1, SNX8, PPP2R5C, FBXL2, MAP2K4, SYNJ1, RABGEF1, POMT2, and XPO1) have been identified that potentially play a role in LOAD risk. In this study, we have examined the association of 222 SNPs in these 12 candidate genes with LOAD risk in 1291 LOAD cases and 958 cognitively normal controls. Single site and haplotype analyses were performed using PLINK. Following adjustment for APOE genotype, age, sex, and principal components, we found single nucleotide polymorphisms (SNPs) in PPP2R5C, PICALM, SH3KBP1, XRN1, and SNX8 that showed significant association with risk of LOAD. The top SNP was located in intron 3 of PPP2R5C (P=0.009017), followed by an intron 19 SNP in PICALM (P=0.0102). Haplotype analysis revealed significant associations in ADSSL1, PICALM, PPP2R5C, SNX8, and SH3KBP1 genes. Our data indicate that genetic variation in these new candidate genes affects the risk of LOAD. Further investigation of these genes, including additional replication in other case-control samples and functional studies to elucidate the pathways by which they affect Aβ, are necessary to determine the degree of involvement these genes have for LOAD risk.