Autosomal dominant ApoA4 mutations present as tubulointerstitial kidney disease with medullary amyloidosis.

Sporadic cases of apolipoprotein A-IV medullary amyloidosis have been reported. Here we describe five families found to have autosomal dominant medullary amyloidosis due to two different pathogenic APOA4 variants. A large family with autosomal dominant chronic kidney disease (CKD) and bland urinary sediment underwent whole genome sequencing with identification of a chr11:116692578 G>C (hg19) variant encoding the missense mutation p.L66V of the ApoA4 protein. We identified two other distantly related families from our registry with the same variant and two other distantly related families with a chr11:116693454 C>T (hg19) variant encoding the missense mutation p.D33N. Both mutations are unique to affected families, evolutionarily conserved and predicted to expand the amyloidogenic hotspot in the ApoA4 structure. Clinically affected individuals suffered from CKD with a bland urinary sediment and a mean age for kidney failure of 64.5 years. Genotyping identified 48 genetically affected individuals; 44 individuals had an estimated glomerular filtration rate (eGFR) under 60 ml/min/1.73 m2, including all 25 individuals with kidney failure. Significantly, 11 of 14 genetically unaffected individuals had an eGFR over 60 ml/min/1.73 m2. Fifteen genetically affected individuals presented with higher plasma ApoA4 concentrations. Kidney pathologic specimens from four individuals revealed amyloid deposits limited to the medulla, with the mutated ApoA4 identified by mass-spectrometry as the predominant amyloid constituent in all three available biopsies. Thus, ApoA4 mutations can cause autosomal dominant medullary amyloidosis, with marked amyloid deposition limited to the kidney medulla and presenting with autosomal dominant CKD with a bland urinary sediment. Diagnosis relies on a careful family history, APOA4 sequencing and pathologic studies.

Mechanism of efficient double-strand break repair by a long non-coding RNA.

Mechanistic studies in DNA repair have focused on roles of multi-protein DNA complexes, so how long non-coding RNAs (lncRNAs) regulate DNA repair is less well understood. Yet, lncRNA LINP1 is over-expressed in multiple cancers and confers resistance to ionizing radiation and chemotherapeutic drugs. Here, we unveil structural and mechanistic insights into LINP1's ability to facilitate non-homologous end joining (NHEJ). We characterized LINP1 structure and flexibility and analyzed interactions with the NHEJ factor Ku70/Ku80 (Ku) and Ku complexes that direct NHEJ. LINP1 self-assembles into phase-separated condensates via RNA-RNA interactions that reorganize to form filamentous Ku-containing aggregates. Structured motifs in LINP1 bind Ku, promoting Ku multimerization and stabilization of the initial synaptic event for NHEJ. Significantly, LINP1 acts as an effective proxy for PAXX. Collective results reveal how lncRNA effectively replaces a DNA repair protein for efficient NHEJ with implications for development of resistance to cancer therapy.

PAICS deficiency, a new defect of de novo purine synthesis resulting in multiple congenital anomalies and fatal outcome.

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.

An international cohort study of autosomal dominant tubulointerstitial kidney disease due to REN mutations identifies distinct clinical subtypes.

There have been few clinical or scientific reports of autosomal dominant tubulointerstitial kidney disease due to REN mutations (ADTKD-REN), limiting characterization. To further study this, we formed an international cohort characterizing 111 individuals from 30 families with both clinical and laboratory findings. Sixty-nine individuals had a REN mutation in the signal peptide region (signal group), 27 in the prosegment (prosegment group), and 15 in the mature renin peptide (mature group). Signal group patients were most severely affected, presenting at a mean age of 19.7 years, with the prosegment group presenting at 22.4 years, and the mature group at 37 years. Anemia was present in childhood in 91% in the signal group, 69% prosegment, and none of the mature group. REN signal peptide mutations reduced hydrophobicity of the signal peptide, which is necessary for recognition and translocation across the endoplasmic reticulum, leading to aberrant delivery of preprorenin into the cytoplasm. REN mutations in the prosegment led to deposition of prorenin and renin in the endoplasmic reticulum-Golgi intermediate compartment and decreased prorenin secretion. Mutations in mature renin led to deposition of the mutant prorenin in the endoplasmic reticulum, similar to patients with ADTKD-UMOD, with a rate of progression to end stage kidney disease (63.6 years) that was significantly slower vs. the signal (53.1 years) and prosegment groups (50.8 years) (significant hazard ratio 0.367). Thus, clinical and laboratory studies revealed subtypes of ADTKD-REN that are pathophysiologically, diagnostically, and clinically distinct.

PDGFRB mutation and tyrosine kinase inhibitor resistance in Ph-like acute lymphoblastic leukemia.

Philadelphia chromosome (Ph)-like acute lymphoblastic leukemia (ALL) comprises ∼10% to 15% of childhood ALL cases, many of which respond exquisitely to tyrosine kinase inhibitors (TKIs), for example, imatinib in PDGFRB-rearranged ALL. However, some cases developed drug resistance to TKIs and the mechanisms are poorly understood. In this study, we identified a novel PDGFRB fusion gene, namely AGGF1-PDGFRB, and functionally characterized its oncogenic potential in vitro. Further genomic profiling of longitudinally collected samples during treatment revealed the emergence of a mutation, PDGFRBC843G , which directly conferred resistance to all generations of ABL TKIs, including imatinib, dasatinib, nilotinib, and ponatinib. PDGFRB-mutant leukemia cells are highly sensitive to multitarget kinase inhibitor CHZ868, suggesting potential therapeutic options for some patients resistant to ABL TKIs. In summary, we describe a complex clonal evolution pattern in Ph-like ALL and identified a novel PDGFRB point mutation that drives leukemia relapse after ABL TKI treatment.

Heterozygous Loss-of-Function SEC61A1 Mutations Cause Autosomal-Dominant Tubulo-Interstitial and Glomerulocystic Kidney Disease with Anemia.

Autosomal-dominant tubulo-interstitial kidney disease (ADTKD) encompasses a group of disorders characterized by renal tubular and interstitial abnormalities, leading to slow progressive loss of kidney function requiring dialysis and kidney transplantation. Mutations in UMOD, MUC1, and REN are responsible for many, but not all, cases of ADTKD. We report on two families with ADTKD and congenital anemia accompanied by either intrauterine growth retardation or neutropenia. Ultrasound and kidney biopsy revealed small dysplastic kidneys with cysts and tubular atrophy with secondary glomerular sclerosis, respectively. Exclusion of known ADTKD genes coupled with linkage analysis, whole-exome sequencing, and targeted re-sequencing identified heterozygous missense variants in SEC61A1-c.553A>G (p.Thr185Ala) and c.200T>G (p.Val67Gly)-both affecting functionally important and conserved residues in SEC61. Both transiently expressed SEC6A1A variants are delocalized to the Golgi, a finding confirmed in a renal biopsy from an affected individual. Suppression or CRISPR-mediated deletions of sec61al2 in zebrafish embryos induced convolution defects of the pronephric tubules but not the pronephric ducts, consistent with the tubular atrophy observed in the affected individuals. Human mRNA encoding either of the two pathogenic alleles failed to rescue this phenotype as opposed to a complete rescue by human wild-type mRNA. Taken together, these findings provide a mechanism by which mutations in SEC61A1 lead to an autosomal-dominant syndromic form of progressive chronic kidney disease. We highlight protein translocation defects across the endoplasmic reticulum membrane, the principal role of the SEC61 complex, as a contributory pathogenic mechanism for ADTKD.

Clinical manifestations and molecular aspects of phosphoribosylpyrophosphate synthetase superactivity in females.

Objectives: Phosphoribosylpyrophosphate synthetase (PRPS1) superactivity is an X-linked disorder characterized by urate overproduction Online Mendelian Inheritance in Man (OMIM) gene reference 300661. This condition is thought to rarely affect women, and when it does, the clinical presentation is mild. We describe a 16-year-old African American female who developed progressive tophi, nephrolithiasis and acute kidney failure due to urate overproduction. Family history included a mother with tophaceous gout who developed end-stage kidney disease due to nephrolithiasis and an affected sister with polyarticular gout. The main aim of this study was to describe the clinical manifestations of PRPS1 superactivity in women. Methods: Whole exome sequencing was performed in affected females and their fathers. Results: Mutational analysis revealed a new c.520 G > A (p.G174R) mutation in the PRPS1 gene. The mutation resulted in decreased PRPS1 inhibition by ADP. Conclusion: Clinical findings in previously reported females with PRPS1 superactivity showed a high clinical penetrance of this disorder with a mean serum urate level of 8.5 (4.1) mg/dl [506 (247) µmol/l] and a high prevalence of gout. These findings indicate that all women in families with PRPS1 superactivity should be genetically screened for a mutation (for clinical management and genetic counselling). In addition, women with tophaceous gout, gout presenting in childhood, or a strong family history of severe gout should be considered for PRPS1 mutational analysis.

Chronic tubulointerstitial kidney disease in untreated adenine phosphoribosyl transferase (APRT) deficiency: A case report
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Adenine phosphoribosyltransferase (APRT) deficiency (OMIM #614723) is a rare autosomal recessive defect in the purine salvage pathway that causes excessive production of 2,8-dihydroxyadenine, leading to nephrolithiasis and chronic kidney disease (CKD). This case report describes the natural history of CKD in untreated APRT deficiency. We describe a novel APRT mutation (chr16:88877985 G / C; c.195 C>/G; p.His54Asp) presenting with CKD without nephrolithiasis. The patient initially required dialysis, but kidney function improved with allopurinol. We reviewed APRT deficiency reported in the literature to determine the loss of kidney function in individuals with untreated APRT deficiency and its relationship to nephrolithiasis. We identified 95 individuals in whom kidney function was assessed prior to treatment. There was a bimodal distribution of kidney failure. AKI occurred frequently in childhood due to obstructing nephrolithiasis or crystalline nephropathy and was usually reversible. CKD developed after age 20 in all patients irrespective of nephrolithiasis history, with 36/42 patients > 40 years of age having at least stage 3 CKD, and 24/42 having an eGFR > 10 mL/min/1.73m2 or being on dialysis. There were 13 adults without nephrolithiasis and 50 adults with nephrolithiasis. The mean age of end-stage renal diesease (ESRD) was 50.52 ± 13.9 for those without nephrolithiasis and 43.4 ± 15.8 years for those with nephrolithiasis (p = 0.24). APRT deficiency is associated with slowly progressive CKD that occurs independently of nephrolithiasis. Diagnosis should be considered in all individuals with chronic tubulointerstitial kidney disease, with or without the presence of nephrolithiasis. In our patient, allopurinol 300 mg/day resulted in improvement of kidney function.
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Oligomeric interface modulation causes misregulation of purine 5´-nucleotidase in relapsed leukemia.

BACKGROUND: Relapsed acute lymphoblastic leukemia (ALL) is one of the main causes of mortality in childhood malignancies. Previous genetic studies demonstrated that chemoresistant ALL is driven by activating mutations in NT5C2, the gene encoding cytosolic 5´-nucleotidase (cN-II). However, molecular mechanisms underlying this hyperactivation are still unknown. Here, we present kinetic and structural properties of cN-II variants that represent 75 % of mutated alleles in patients who experience relapsed ALL (R367Q, R238W and L375F). RESULTS: Enzyme kinetics measurements revealed that the mutants are consitutively active without need for allosteric activators. This shows that hyperactivity is not caused by a direct catalytic effect but rather by misregulation of cN-II. X-ray crystallography combined with mass spectrometry-based techniques demonstrated that this misregulation is driven by structural modulation of the oligomeric interface within the cN-II homotetrameric assembly. These specific conformational changes are shared between the studied variants, despite the relatively random spatial distribution of the mutations. CONCLUSIONS: These findings define a common molecular mechanism for cN-II hyperactivity, which provides a solid basis for targeted therapy of leukemia. Our study highlights the cN-II oligomerization interface as an attractive pharmacological target.

Chaperone therapy for homocystinuria: the rescue of CBS mutations by heme arginate.

Classical homocystinuria is caused by mutations in the cystathionine ß-synthase (CBS) gene. Previous experiments in bacterial and yeast cells showed that many mutant CBS enzymes misfold and that chemical chaperones enable proper folding of a number of mutations. In the present study, we tested the extent of misfolding of 27 CBS mutations previously tested in E. coli under the more folding-permissive conditions of mammalian CHO-K1 cells and the ability of chaperones to rescue the conformation of these mutations. Expression of mutations in mammalian cells increased the median activity 16-fold and the amount of tetramers 3.2-fold compared with expression in bacteria. Subsequently, we tested the responses of seven selected mutations to three compounds with chaperone-like activity. Aminooxyacetic acid and 4-phenylbutyric acid exhibited only a weak effect. In contrast, heme arginate substantially increased the formation of mutant CBS protein tetramers (up to sixfold) and rescued catalytic activity (up to ninefold) of five out of seven mutations (p.A114V, p.K102N, p.R125Q, p.R266K, and p.R369C). The greatest effect of heme arginate was observed for the mutation p.R125Q, which is non-responsive to in vivo treatment with vitamin B(6). Moreover, the heme responsiveness of the p.R125Q mutation was confirmed in fibroblasts derived from a patient homozygous for this genetic variant. Based on these data, we propose that a distinct group of heme-responsive CBS mutations may exist and that the heme pocket of CBS may become an important target for designing novel therapies for homocystinuria.

Biochemical properties of nematode O-acetylserine(thiol)lyase paralogs imply their distinct roles in hydrogen sulfide homeostasis.

O-Acetylserine(thiol)lyases (OAS-TLs) play a pivotal role in a sulfur assimilation pathway incorporating sulfide into amino acids in microorganisms and plants, however, these enzymes have not been found in the animal kingdom. Interestingly, the genome of the roundworm Caenorhabditis elegans contains three expressed genes predicted to encode OAS-TL orthologs (cysl-1-cysl-3), and a related pseudogene (cysl-4); these genes play different roles in resistance to hypoxia, hydrogen sulfide and cyanide. To get an insight into the underlying molecular mechanisms we purified the three recombinant worm OAS-TL proteins, and we determined their enzymatic activities, substrate binding affinities, quaternary structures and the conformations of their active site shapes. We show that the nematode OAS-TL orthologs can bind O-acetylserine and catalyze the canonical reaction although this ligand may more likely serve as a competitive inhibitor to natural substrates instead of being a substrate for sulfur assimilation. In addition, we propose that S-sulfocysteine may be a novel endogenous substrate for these proteins. However, we observed that the three OAS-TL proteins are conformationally different and exhibit distinct substrate specificity. Based on the available evidences we propose the following model: CYSL-1 interacts with EGL-9 and activates HIF-1 that upregulates expression of genes detoxifying sulfide and cyanide, the CYSL-2 acts as a cyanoalanine synthase in the cyanide detoxification pathway and simultaneously produces hydrogen sulfide, while the role of CYSL-3 remains unclear although it exhibits sulfhydrylase activity in vitro. All these data indicate that C. elegans OAS-TL paralogs have distinct cellular functions and may play different roles in maintaining hydrogen sulfide homeostasis.

Conformationally constrained nucleoside phosphonic acids--potent inhibitors of human mitochondrial and cytosolic 5'(3')-nucleotidases.

This work describes novel in vitro inhibitors of human mitochondrial (mdN) and cytosolic (cdN) 5'(3')-deoxynucleotidases. We designed a series of derivatives of the lead compound (S)-1-[2-deoxy-3,5-O-(phosphonobenzylidene)-ß-d-threo-pentofuranosyl]thymine bearing various substituents in the para position of the benzylidene moiety. Detailed kinetic study revealed that certain para substituents increase the inhibitory potency (iodo derivative; K = 2.71 µM) and some induce a shift in selectivity toward cdN (carboxy derivative, K = 11.60 µM; iodoxy derivative, K = 6.60 µM). Crystal structures of mdN in complex with three of these compounds revealed that various para substituents lead to two alternative inhibitor binding modes within the enzyme active site. Two binding modes were also identified for cdN complexes by heteronuclear NMR spectroscopy.

Novel structural arrangement of nematode cystathionine ß-synthases: characterization of Caenorhabditis elegans CBS-1.

CBSs (cystathionine ß-synthases) are eukaryotic PLP (pyridoxal 5 *-phosphate)-dependent proteins that maintain cellular homocysteine homoeostasis and produce cystathionine and hydrogen sulfide. In the present study, we describe a novel structural arrangement of the CBS enzyme encoded by the cbs-1 gene of the nematode Caenorhabditis elegans. The CBS-1 protein contains a unique tandem repeat of two evolutionarily conserved catalytic regions in a single polypeptide chain. These repeats include a catalytically active C-terminal module containing a PLP-binding site and a less conserved N-terminal module that is unable to bind the PLP cofactor and cannot catalyse CBS reactions, as demonstrated by analysis of truncated variants and active-site mutant proteins. In contrast with other metazoan enzymes, CBS-1 lacks the haem and regulatory Bateman domain essential for activation by AdoMet (S-adenosylmethionine) and only forms monomers. We determined the tissue and subcellular distribution of CBS-1 and showed that cbs-1 knockdown by RNA interference leads to delayed development and to an approximately 10-fold elevation of homocysteine concentrations in nematode extracts. The present study provides the first insight into the metabolism of sulfur amino acids and hydrogen sulfide in C. elegans and shows that nematode CBSs possess a structural feature that is unique among CBS proteins.

Conformational properties of nine purified cystathionine ß-synthase mutants.

Protein misfolding due to missense mutations is a common pathogenic mechanism in cystathionine ß-synthase (CBS) deficiency. In our previous studies, we successfully expressed, purified, and characterized nine CBS mutant enzymes containing the following patient mutations: P49L, P78R, A114V, R125Q, E176K, R266K, P422L, I435T, and S466L. These purified mutants exhibited full heme saturation, normal tetrameric assembly, and high catalytic activity. In this work, we used several spectroscopic and proteolytic techniques to provide a more thorough insight into the conformation of these mutant enzymes. Far-UV circular dichroism, fluorescence, and second-derivative UV spectroscopy revealed that the spatial arrangement of these CBS mutants is similar to that of the wild type, although the microenvironment of the chromophores may be slightly altered. Using proteolysis with thermolysin under native conditions, we found that the majority of the studied mutants is more susceptible to cleavage, suggesting their increased local flexibility or propensity for local unfolding. Interestingly, the presence of the CBS allosteric activator, S-adenosylmethionine (AdoMet), increased the rate of cleavage of the wild type and the AdoMet-responsive mutants, while the proteolytic rate of the AdoMet-unresponsive mutants was not significantly changed. Pulse proteolysis analysis suggested that the protein structure of the R125Q and E176K mutants is significantly less stable than that of the wild type and the other mutants. Taken together, the proteolytic data shows that the conformation of the pathogenic mutants is altered despite retained catalytic activity and normal tetrameric assembly. This study demonstrates that the proteolytic techniques are useful tools for the assessment of the biochemical penalty of missense mutations in CBS.

Cystathionine beta-synthase mutants exhibit changes in protein unfolding: conformational analysis of misfolded variants in crude cell extracts.

Protein misfolding has been proposed to be a common pathogenic mechanism in many inborn errors of metabolism including cystathionine ß-synthase (CBS) deficiency. In this work, we describe the structural properties of nine CBS mutants that represent a common molecular pathology in the CBS gene. Using thermolysin in two proteolytic techniques, we examined conformation of these mutants directly in crude cell extracts after expression in E. coli. Proteolysis with thermolysin under native conditions appeared to be a useful technique even for very unstable mutant proteins, whereas pulse proteolysis in a urea gradient had limited values for the study of the majority of CBS mutants due to their instability. Mutants in the active core had either slightly increased unfolding (p.A114V, p.E302K and p.G307S) or extensive unfolding with decreased stability (p.H65R, p.T191M, p.I278T and p.R369C). The extent of the unfolding inversely correlated with the previously determined degree of tetrameric assembly and with the catalytic activity. In contrast, mutants bearing aminoacid substitutions in the C-terminal regulatory domain (p.R439Q and p.D444N) had increased global stability with decreased flexibility. This study shows that proteolytic techniques can reveal conformational abnormalities even for CBS mutants that have activity and/or a degree of assembly similar to the wild-type enzyme. We present here a methodological strategy that may be used in cell lysates to evaluate properties of proteins that tend to misfold and aggregate and that may be important for conformational studies of disease-causing mutations in the field of inborn errors of metabolism.

Molecular characterization of the AdeI mutant of Chinese hamster ovary cells: a cellular model of adenylosuccinate lyase deficiency.

Adenylosuccinate lyase (ADSL, E. C. 4.3.2.2) carries out two non-sequential steps in de novo AMP synthesis, the conversion of succinylaminoimidazole carboxamide ribotide (SAICAR) to aminoimidazolecarboxamide ribotide (AICAR) and the conversion of succinyl AMP (AMPS) to AMP. In humans, mutations in ADSL lead to an inborn error of metabolism originally characterized by developmental delay, often with autistic features. There is no effective treatment for ADSL deficiency. Hypotheses regarding the pathogenesis include toxicity of high levels of SAICAR, AMPS, or their metabolites, deficiency of the de novo purine biosynthetic pathway, or lack of a completely functional purine cycle in muscle and brain. One important approach to understand ADSL deficiency is to develop cell culture models that allow investigation of the properties of ADSL mutants and the consequences of ADSL deficiency at the cellular level. We previously reported the isolation and initial characterization of mutants of Chinese hamster ovary (CHO-K1) cells (AdeI) that lack detectable ADSL activity, accumulate SAICAR and AMPS, and require adenine for growth. Here we report the cDNA sequences of ADSL from CHO-K1 and AdeI cells and describe a mutation resulting in an alanine to valine amino acid substitution at position 291 (A291V) in AdeI ADSL. This substitution lies in the "signature sequence" of ADSL, inactivates the enzyme, and validates AdeI as a cellular model of ADSL deficiency.

Determination of cystathionine beta-synthase activity in human plasma by LC-MS/MS: potential use in diagnosis of CBS deficiency.

Cystathionine ß-synthase (CBS) deficiency is usually confirmed by assaying the enzyme activity in cultured skin fibroblasts. We investigated whether CBS is present in human plasma and whether determination of its activity in plasma could be used for diagnostic purposes. We developed an assay to measure CBS activity in 20 µL of plasma using a stable isotope substrate - 2,3,3-(2)H serine. The activity was determined by measurement of the product of enzyme reaction, 3,3-(2)H-cystathionine, using LC-MS/MS. The median enzyme activity in control plasma samples was 404 nmol/h/L (range 66-1,066; n = 57). In pyridoxine nonresponsive CBS deficient patients, the median plasma activity was 0 nmol/ho/L (range 0-9; n = 26), while in pyridoxine responsive patients the median activity was 16 nmol/hour/L (range 0-358; n = 28); this overlapped with the enzyme activity from control subject. The presence of CBS in human plasma was confirmed by an in silico search of the proteome database, and was further evidenced by the activation of CBS by S-adenosyl-L-methionine and pyridoxal 5'-phosphate, and by configuration of the detected reaction product, 3,3-(2)H-cystathionine, which was in agreement with the previously observed CBS reaction mechanism. We hypothesize that the CBS enzyme in plasma originates from liver cells, as the plasma CBS activities in patients with elevated liver aminotransferase activities were more than 30-fold increased. In this study, we have demonstrated that CBS is present in human plasma and that its catalytic activity is detectable by LC-MS/MS. CBS assay in human plasma brings new possibilities in the diagnosis of pyridoxine nonresponsive CBS deficiency.

Stages, scaffolds and strings in the spatial organisation of non-homologous end joining: Insights from X-ray diffraction and Cryo-EM.

Non-homologous end joining (NHEJ) is the preferred pathway for the repair of DNA double-strand breaks in humans. Here we describe three structural aspects of the repair pathway: stages, scaffolds and strings. We discuss the orchestration of DNA repair to guarantee robust and efficient NHEJ. We focus on structural studies over the past two decades, not only using X-ray diffraction, but also increasingly exploiting cryo-EM to investigate the macromolecular assemblies.

Dimers of DNA-PK create a stage for DNA double-strand break repair.

DNA double-strand breaks are the most dangerous type of DNA damage and, if not repaired correctly, can lead to cancer. In humans, Ku70/80 recognizes DNA broken ends and recruits the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to form DNA-dependent protein kinase holoenzyme (DNA-PK) in the process of non-homologous end joining (NHEJ). We present a 2.8-Å-resolution cryo-EM structure of DNA-PKcs, allowing precise amino acid sequence registration in regions uninterpreted in previous 4.3-Å X-ray maps. We also report a cryo-EM structure of DNA-PK at 3.5-Å resolution and reveal a dimer mediated by the Ku80 C terminus. Central to dimer formation is a domain swap of the conserved C-terminal helix of Ku80. Our results suggest a new mechanism for NHEJ utilizing a DNA-PK dimer to bring broken DNA ends together. Furthermore, drug inhibition of NHEJ in combination with chemo- and radiotherapy has proved successful, making these models central to structure-based drug targeting efforts.