Two Missense Mutations in the Primary Autosomal Recessive Microcephaly Gene MCPH1 Disrupt the Function of the Highly Conserved N-Terminal BRCT Domain of Microcephalin

Primary microcephaly MCPH1 is an extremely rare autosomal recessive disorder associated with congenital microcephaly, mental retardation and a distinctive cellular phenotype of misregulated chromosome condensation. The MCPH1 gene encodes an 835-amino acid protein, microcephalin, which contains 1 N-terminal and 2 C-terminal BRCT (BRCA1 C-terminus) domains. BRCT domains are predominantly found in proteins involved in cell cycle control and DNA repair. Here we describe 1 novel and 1 previously reported MCPH1 missense mutation, p.Trp75Arg and p.Ser72Leu, respectively, in the N-terminal BRCT domain of microcephalin associated with severe congenital microcephaly. Both residues are entirely conserved in the MCPH1 orthologs of all vertebrate species and Drosophila. Proliferating lymphocytes of the patients with p.Trp75Arg and p.Ser72Leu show the unique cellular MCPH1 phenotype of misregulated chromosome condensation, indicating that these missense alterations disrupt the function of the N-terminal BRCT domain of the protein. Interestingly, both residues are strictly conserved in BRCT domains of BRCA1. ClustalW alignments show that the residue p.Ser72 of microcephalin corresponds to p.Ser1715 of the N-terminal BRCT domain of BRCA1, while the microcephalin residue p.Trp75 is analogous to p.Trp1718 in the N-terminal BRCT and to p.Trp1837 in C-terminal BRCT domains of BRCA1. Missense alterations for all 3 corresponding BRCA1 residues were described and are predicted to be deleterious resulting in the destabilization of the BRCA1 protein. Our data on the 2 MCPH1 missense alterations provide further evidence for the functional significance of these residues in BRCT domains.

Computerized evaluation of mean residence times in multicompartmental linear system and pharmacokinetics

Deriving mean residence times (MRTs) is an important task both in pharmacokinetics and in multicompartmental linear systems. Taking as starting point the analysis of MRTs in open or closed (Garcia-Meseguer et al., Bull Math Biol 2003, 65, 279) multicompartmental linear systems, we implement a versatile software, using the Visual Basic 6.0 language for MS-Windows, that is easy to use and with a user-friendly format for the input of data and the output of results. For any multicompartmental linear system of up to 512 compartments, whether closed or open, with traps or without traps and with zero input in one or more of the compartments, this software allows the user to obtain the symbolic expressions, in the most simplified form, and/or the numerical values of the MRTs in any of its compartments, in the entire system or in a part of the system. As far as we known from the literature, such a software has not been implemented before. The advantage of the present software is that it reduces on the work time needed and minimizes the human errors that are frequent in compartmental systems even those that are relatively staightforward. The software bioCelTer, along with instructions, can be downloaded from http://oretano.iele-ab.uclm.es/~fgarcia/bioCelTer/.

High prevalence of the NBN gene mutation c.657-661del5 in Southeast Germany

Nijmegen breakage syndrome (NBS), a rare autosomal recessive chromosomal instability disorder, is caused by mutations in the NBN gene. Most patients known so far are of Slavic origin and carry the major founder mutation c.657-661del5. Due to an unexpectedly high incidence of NBS patients (homozygous for the c.657-661del5 mutation) in a Northeast Bavarian region in Southeast Germany, we estimated the prevalence of this mutation in this area and compared it to another German region. We found a high carrier frequency of 1/176 for the c.657-661del5 mutation among newborns in Northeast Bavaria, while the frequency of the mutation in Berlin was 1/990. We further studied families from a Slavic population isolate, the Sorbs, in the Lusatian region in Northeast Saxony, and revealed a prevalence of the c.657-661del5 mutation of 1/34. Whereas the Slavic origin of the Sorbs has been known, we attribute the surprisingly high frequencies of c.657-661del5 mutation in Bavaria (similar to frequencies of this mutation in various Eastern European countries) to a high percentage of people of Slavic origin in Northeast Bavaria.

Tyrosinase inactivation in its action on dopa

Under aerobic or anaerobic conditions, tyrosinase undergoes a process of irreversible inactivation induced by its physiological substrate L-dopa. Under aerobic conditions, this inactivation occurs through a process of suicide inactivation involving the form oxy-tyrosinase. Under anaerobic conditions, both the met- and deoxy-tyrosinase forms undergo irreversible inactivation. Suicide inactivation in aerobic conditions is slower than the irreversible inactivation under anaerobic conditions. The enzyme has less affinity for the isomer D-dopa than for L-dopa but the velocity of inactivation is the same. We propose mechanisms to explain these processes.

[Distal spinal-muscular atrophy 1 (DSMA1 or SMARD1)]

In this article, we review the clinical, neuropathological and genetic aspects of distal spinal-muscular atrophy 1 (DSMA1; MIM#604320), formerly designated as autosomal recessive spinal muscular atrophy with respiratory distress type 1 (SMARD1) and also known as distal hereditary-motor neuropathy type 6 (dHMN6 or HMN6).

Mean lifetime and first-passage time of the enzyme species involved in an enzyme reaction. Application to unstable enzyme systems

Taking as starting point the complete analysis of mean residence times in linear compartmental systems performed by Garcia-Meseguer et al. (Bull. Math. Biol. 65:279-308, 2003) as well as the fact that enzyme systems, in which the interconversions between the different enzyme species involved are of first or pseudofirst order, act as linear compartmental systems, we hereby carry out a complete analysis of the mean lifetime that the enzyme molecules spend as part of the enzyme species, forms, or groups involved in an enzyme reaction mechanism. The formulas to evaluate these times are given as a function of the individual rate constants and the initial concentrations of the involved species at the onset of the reaction. We apply the results to unstable enzyme systems and support the results by using a concrete example of such systems. The practicality of obtaining the mean times and their possible application in a kinetic data analysis is discussed.

Kinetic analysis of a general model of activation of aspartic proteinase zymogens involving a reversible inhibitor. I. Kinetic analysis

Starting from a simple general reaction mechanism of activation of aspartic proteinases zymogens involving a uni- and a bimolecular simultaneous activation route and a reversible inhibition step, the time course equation of the zymogen, inhibitor and activated enzyme concentrations have been derived. Likewise, expressions for the time required for any reaction progress and the corresponding mean activation rates as well as the half-life of the global zymogen activation have been derived. An experimental design and kinetic data analysis is suggested to estimate the kinetic parameters involved in the reaction mechanism proposed.

Kinetic analysis of a general model of activation of aspartic proteinase zymogens involving a reversible inhibitor. II. Contribution of the uni- and bimolecular activation routes

From the kinetic study carried out in part I of this series (preceding article) an analysis quantifying the relative contribution to the global process of the uni- and bimolecular routes has been carried out. This analysis suggests a way to predict the time course of the relative contribution as well as the effect on this relative weight of the initial zymogen, inhibitor and activating enzyme concentrations.

Kinetic characterization of the oxidation of esculetin by polyphenol oxidase and peroxidase

Esculetin has been described as an inhibitor of tyrosinase and polyphenol oxidase and, therefore, of melanogenesis. In this work, we demonstrate that esculetin is not an inhibitor but a substrate of mushroom polyphenol oxidase (PPO) and horseradish peroxidase (POD), enzymes which oxidize esculetin, generating its o-quinone. Since o-quinones are very unstable, the usual way of determining the enzymatic activity (slope of recordings) is difficult. For this reason, we developed a chronometric method to characterize the kinetics of this substrate, based on measurements of the lag period in the presence of micromolar concentrations of ascorbic acid. The catalytic constant determined was of the same order for both enzymes. However, polyphenol oxidase showed greater affinity (a lower Michaelis constant) than peroxidase for esculetin. The affinity of PPO and POD towards oxygen and hydrogen peroxide was very high, suggesting the possible catalysis of both enzymes in the presence of low physiological concentrations of these oxidizing substrates. Taking into consideration optimum pHs of 4.5 and 7 for POD and PPO respectively, and the acidic pHs of melanosomes, the studies were carried out at pH 4.5 and 7. The in vivo pH might be responsible for the stronger effect of these enzymes on L-tyrosine and L-3,4-dihydroxyphenylanaline (L-DOPA) (towards melanogenesis) and on cumarins such as esculetin towards an alternative oxidative pathway.

Kinetic characterization of the oxidation of chlorogenic acid by polyphenol oxidase and peroxidase. Characteristics of the o-quinone

Chlorogenic acid is the major diphenol of many fruits, where it is oxidized enzymatically by polyphenol oxidase (PPO) or peroxidase (POD) to its o-quinone. In spectrophotometric studies of chlorogenic acid oxidation with a periodate ratio of [CGA]0/[IO4-]0 < 1 and [CGA]0/[IO4-]0 > 1, the o-quinone was characterized as follows: lambda(max) at 400 nm and epsilon = 2000 and 2200 M-1 cm-1 at pH 4.5 and 7.0, respectively. In studies of o-quinone generated by the oxidation of chlorogenic acid using a periodate at ratio of [CGA]0/[IO4-]0 > 1, a reaction with the remaining substrate was detected, showing rate constants of k = 2.73 +/- 0.17 M-1 s-1 and k' = 0.05 +/- 0.01 M-1 s-1 at the above pH values. A chronometric spectrophotometric method is proposed to kinetically characterize the action of the PPO or POD on the basis of measuring the time it takes for a given amount of ascorbic acid to be consumed in the reaction with the o-quinone. The kinetic constants of mushroom PPO and horseradish POD are determined.

Increased prevalence of imprinting defects in patients with Angelman syndrome born to subfertile couples

Recent case reports have suggested that infertility treatment with intracytoplasmic sperm injection (ICSI) may increase the risk of imprinting defects leading to Angelman syndrome (AS). Although imprinting defects account for only 4% of patients with AS, we have found four cases among 16 AS patients born to subfertile couples, who conceived with or without infertility treatment (25%; relative risk (RR) 6.25; 95% confidence interval (CI) 1.68 to 16.00). The risk in untreated couples with time to pregnancy (TTP) exceeding 2 years was identical to that of those treated by ICSI or by hormonal stimulation alone (RR 6.25; 95% CI 0.70 to 22.57). It was twice as high in couples who had received treatment and also had TTP >2 years (RR 12.5; 95% CI 1.40 to 45.13). Our findings suggest that imprinting defects and subfertility may have a common cause, and that superovulation rather than ICSI may further increase the risk of conceiving a child with an imprinting defect.

[Mutations in tumor suppressor gene NBS1 in adult patients with malignancies]

BACKGROUND

Mutations 657del5 and R215W in exon 6 of tumor suppressor gene NBS I are found in 1% Slavic populations. Increased occurrence of cancer was repeatedly reported in adult relatives of patients with Nijmegen breakage syndrome. Among children with oncological problematic, nonsignificantly increased frequency of NBS1 heterozygotes was found, which seems not to play any important role in cancerogenesis in childhood. However, the proportion of NBS heterozygotes among adult patients with malignancies could be significant and their therapy and follow up should respect their hyperradiosensitivity.

METHODS AND RESULTS

Mutations in exon were studied in 706 adult patients with malignancies. We found 5 NBS heterozygotes, which not more than the population prevalence (1:129-165). Increased frequency of NBS heterozygotes was found among patients with colon and rectal cancer (2/101), breast cancer (1/60), skin malignancies (1/98).

CONCLUSIONS

Surprisingly only one NBS heterozygote was found among 228 patients with nonHodgkin lymphoma, the malignancy which is a common complication in NBS homozygotes. Other types of malignancies were uncommon and only one R215W heterozygote was found. Comparison frequency of NBS heterozygotes with incidence NBS among person older than 70 years shows significant difference. Prevention of malignancies by avoidance from ionisation could be realized also in relatives of patients after identification of their genotype.

[Increased risk of malignancies in heterozygotes in families of patients with Nijmegen breakage syndrome]

BACKGROUND

The autosomal recessive chromosomal instability and hyperradiosensitivity Nijmegen breakage syndrome (NBS) in consequence of a mutation in the NBSI gene at 8q21 is associated with high occurrence of lymphoreticular malignancies due to deficient DNA reparation (double strand breaks). In the Slavic population the majority of patients are homozygotes of the so-called "Slavic mutation" 657de15 in exon 6. Increased occurrence of malignant solid tumors (1) in families of NBS patients has been described already prior to the identification of the responsible gene, and the increased risk of malignancies in heterozygotes was thus hypothetical.

METHODS AND RESULTS

The possibility of discerning mutation carriers in families from normal homozygotes enables verification of that hypothesis. Through molecular genetics investigations of grandparents and immediate relatives, we have been successful in determining the genotype in 79 of 112 grandparents in 28 families of our 39 patients and 54 their parents and siblings. A single family had affected children in consequence of compound heterozygosity of the 657de15 and R215W mutations in the same exon of the NBSI gene. The proband's families were investigated genealogically and data on relatives were obtained over four generations. Obtained data were repeatedly supplemented and objectively verified in church books and in healthcare documentation. Seven families have been followed up for 20-30 years, six families for 10-20 years, and 15 families for 1-10 years. Out of 28 families we were successful in examining the genotype of both grandparents in 18 families, there having been revealed one non-paternity; in five families only one of the grandparents has been examined; in five families we were not successful in examining any grandparent. Among 40 grandparents - normal homozygotes, there has appeared a malignancy in three (7.4 %), while among 39 heterozygotes of mutation 657de15 in the NBSI gene malignancies were documented in 15 (38,2 %). Mean age of NBS heterozygotes at manifestation of malignancy was 59.3 year (range 47-72 years), in the group of homozygotes it was 52.6 years (range 44-62 years). Nine grandparents died of malignancy prior to the discovery of the NBSI gene and their genotype has been deduced genealogically in seven on the basis of the genotype in the sponse and children, in two from preserved DNA. Out of that number, from three grandparents that had died of malignancies we were successful in obtaining neoplastic tissue for molecular genetics investigation, aimed at LOH or amplification of the NBS1 gene. In another seven grandparents - heterozygotes, malignancies were manifested after determination of their genotype by DNA analysis, and consequently also from tumor tissue that has been obtained from three of them for molecular genetic investigation.

CONCLUSIONS

The age distribution and socio-economic status of both groups of grandparents did not differ, the sex ratio was slightly shifted towards females in the group of homozygotic grandparents (22 females and 18 males), and in the group of heterozygotes it was towards males (21 males and 18 females). The sex ratio between heterozygotic grandparents with malignancies was likewise shifted towards the male gender (11 males and 4 females), in the group of homozygotic grandparents malignancy affected one male and two females. As verified in healthcare and church books documentation, the occurrence of malignancies was significantly more frequent among grandparents heterozygotic for NBS1 mutation than in healthy homozygotes. Among sibs of grandparents and great-grandparents was found significant difference in frequency of malignancies in heterozygotes (5/18 = 27,7 %) and healthy homozygotes (2/36 = 5,5 %), too.

Nijmegen breakage syndrome (NBS) with neurological abnormalities and without chromosomal instability

BACKGROUND

Nijmegen breakage syndrome (NBS) is an autosomal recessive chromosomal instability disorder with hypersensitivity to ionising radiation. The clinical phenotype is characterised by congenital microcephaly, mild dysmorphic facial appearance, growth retardation, immunodeficiency, and greatly increased risk for lymphoreticular malignancy. Most NBS patients are of Slavic origin and homozygous for the founder mutation 657del5. The frequency of 657del5 heterozygotes in the Czech population is 1:150. Recently, another NBS1 mutation, 643C>T(R215W), with uncertain pathogenicity was found to have higher frequency among tumour patients of Slavic origin than in controls. This alteration results in the substitution of the basic amino acid arginine with the non-polar tryptophan and thus could potentially interfere with the function of the NBS1 protein, nibrin.

METHODS AND RESULTS

Children with congenital microcephaly are routinely tested for the 657del5 mutation in the Czech and Slovak Republics. Here, we describe for the first time a severe form of NBS without chromosomal instability in monozygotic twin brothers with profound congenital microcephaly and developmental delay who are compound heterozygotes for the 657del5 and 643C>T(R215W) NBS1 mutations. Both children showed reduced expression of full length nibrin when compared with a control and a heterozygote for the 657del5 mutation. Radiation response processes such as phosphorylation of ATM and phosphorylation/stabilisation of p53, which are promoted by NBS1, are strongly reduced in cells from these patients.

CONCLUSIONS

Interestingly, the patients are more severely affected than classical NBS patients. Consequently, we postulate that homozygosity for the 643C>T(R215W) mutation will also lead to a, possibly very, severe disease phenotype.

Prenatal diagnosis and molecular cytogenetic characterization of an unusual complex structural rearrangement in a pregnancy following intracytoplasmic sperm injection (ICSI)

We report on a balanced complex chromosomal aberration detected in a fetus after amniocentesis. The pregnancy was achieved after intracytoplasmic sperm injection. GTG-banding revealed a complex structurally rearranged karyotype with a translocation between chromosomes 5 and 15 and an additional paracentric inversion in the der(15) between bands 5q11.2 and 5q15. Ag-NOR staining showed an interstitial active nuclear organizer region in the der(15). Molecular cytogenetic analyses using whole-chromosome-painting probes, comparative genomic hybridization, and multicolor banding did not point to further structural aberrations or imbalances. Therefore, a complex rearrangement with three breakpoints has occurred, and the karyotype can be described as 46,XX,der(5)t(5;15) (q11.2;p12),der(15)t(5;15)(q11.2;p12)inv(5)(q11.2q15).

Long-term observations of patients with infantile spinal muscular atrophy with respiratory distress type 1 (SMARD1)

We describe 6 unrelated patients affected by infantile spinal muscular atrophy with respiratory distress type 1 (SMARD1) with prolonged survival upon mechanical ventilation (4.5-11 years), which has not been reported before. Biallelic mutations in the IGHMBP2 gene proved the diagnosis of SMARD1 in all patients. Disease onset was in the first 2 months in the described patients, starting with generalised hypotonia, failure to thrive, and early breathing difficulties. Diaphragmatic palsy was diagnosed and permanent ventilation was initiated 2-8 months after onset. Within months a more distal muscular atrophy became evident associated with joint contractures (talipes), hand drops, and fatty finger pads. Motor development remained minimal, loss of function was observed within the first year after which no further progression was recorded. Voiding dysfunction with reflux nephropathy was observed in 3 patients and has not been reported before. Further evidence of autonomic nerve dysfunction resulting in cardiac arrhythmia, hypertension, and excessive sweating was given in 2 patients. Investigative results were largely compatible with those obtained in classic SMA. However, neurogenic atrophy muscle was more pronounced in distal muscles, if examined, and there was evidence of peripheral nerve involvement at least in some patients.

[Nijmegen breakage syndrome in Slovakia]

BACKGROUND

The autosomal recessive Nijmegen breakage syndrome (NBS) is a DNA repair disorder due to a mutation in the NBS1 gene on 8q21. Hyperradiosensitivity and high risk for lymphoreticular malignancy are important reasons for early diagnosis and prevention by avoidance of ionisation. The frequency of NBS heterozygotes of the mutation 657de15, which is predominant in the Slavic population was estimated to be in the range of 1:90-1:314 in different parts of Poland, and 1:128-154 among Czech newborns, born 20 years ago.

METHODS AND RESULTS

Lower prevalence of affected homozygotes born in Czechoslovakia in the period 1969- 1992 (24 among 5.2 million newborns corresponds to 1:271000) than expected on the basis of carrier frequency is explained to be due to underdiagnosing because the rate of prenatal lethality in the NBS families is not increased or it is even lower than in the general population. The underdiagnosing of NBS is emphasized also by the mean age at diagnosis (7.5 years) although severe microcephaly is present at birth. The possibility to offer effective prevention of primary and secondary malignancies becomes the motivation for interdisciplinary collaboration with paediatricians, neurologists, immunologists and clinical geneticists. A decrease of the mean age down to 6 months at diagnosis among the 11 newly recognized patients has been achieved in the previous 4 years. The occurrence of homozygotes was relatively higher in Slovakia with 5 million inhabitants (14 patients in 11 families) than in the Czech Republic with a population of 10 million (21 patients in 14 families), and therefore the frequency of NBS heterozygotes was studied among 2996 newborns born in 2002-2003 in 12 maternity hospitals of west, middle and east Slovakia. Surprisingly, only 3 heterozygotes were found.

CONCLUSIONS

This discrepancy of heterozygote frequency and the number of homozygotes shows that due to traditional subisolates the population is not in the genetic equilibrium. It explains the high prevalence of alcaptonuria in Slovakia in the middle of last century, which is a rare disorder in other countries.

Genetic heterogeneity for a Nijmegen breakage-like syndrome

Nijmegen breakage syndrome (NBS) is a rare, autosomal-recessive chromosome instability disorder characterized by growth and developmental defects, immunodeficiency, high susceptibility to lymphoid malignancies, hypersensitivity to ionizing radiation and aberrant cell-cycle checkpoint control. The disease is caused by mutations in the NBS1 gene, which encodes nibrin, a component of the hMre11-Rad50-p95 complex involved in cellular response to DNA double-strand breaks. Genetic heterogeneity has been suggested in at least two patients with the NBS phenotype, but no mutation in the NBS1 gene; recently, mutations in the gene encoding the enzyme ligase IV have been identified in patients with signs of NBS. We describe a boy with an NBS clinical phenotype but no mutation in either the NBS1 or the LIG4 genes. The analysis of his cellular phenotype reveals chromosome instability and radiosensitivity, but normal cell-cycle checkpoint control. In addition, a literature review was carried out to summarize and compare data of all NBS-like patients reported to date. This case confirms genetic heterogeneity for NBS. We believe that dissecting the clinical and cellular phenotypes of this and other NBS-like patients will provide useful information for the research of new genes involved in cellular response to DNA damage and the assessment of cancer risk in NBS-like syndrome.

DNA ligase IV mutations identified in patients exhibiting developmental delay and immunodeficiency

DNA ligase IV functions in DNA nonhomologous end-joining and V(D)J recombination. Four patients with features including immunodeficiency and developmental and growth delay were found to have mutations in the gene encoding DNA ligase IV (LIG4). Their clinical phenotype closely resembles the DNA damage response disorder, Nijmegen breakage syndrome (NBS). Some of the mutations identified in the patients directly disrupt the ligase domain while others impair the interaction between DNA ligase IV and Xrcc-4. Cell lines from the patients show pronounced radiosensitivity. Unlike NBS cell lines, they show normal cell cycle checkpoint responses but impaired DNA double-strand break rejoining. An unexpected V(D)J recombination phenotype is observed involving a small decrease in rejoining frequency coupled with elevated imprecision at signal junctions.

Mutations in the Nijmegen Breakage Syndrome gene (NBS1) in childhood acute lymphoblastic leukemia (ALL)

The Nijmegen Breakage Syndrome (NBS) is a rare autosomal recessive disorder associated with immune deficiency, chromosome fragility, and increased susceptibility to lymphoid malignancies. The aim of the present study was to elucidate the potential role of the gene mutated in NBS (NBS1) in the pathogenesis and disease progression of childhood acute lymphoblastic leukemia (ALL). Samples from 47 children with first relapse of ALL were analyzed for mutations in all 16 exons of the NBS1 gene, and in 7 of them (14.9%), four novel amino acid substitutions were identified. Mutations S93L, D95N, and I171V occur in the two known domains of nibrin that are probably involved in protein-protein interactions. Germ-line origin of the I171V mutation was confirmed in three patients, whereas the D95N exchange was present only in leukemic cells. The R215W mutation was observed in one ALL but also in a population-based study and probably represents a rare sequence variant. No additional mutations were found on the second allele in any of these seven patients. The observed NBS1 gene mutations in ALL patients points to its possible involvement in the pathogenesis of this disease.

No evidence for deletions of the NBS1 gene in lymphomas

Patients with Nijmegen breakage syndrome (NBS) have a high risk to develop malignant diseases, most frequently B-cell lymphomas. The NBS gene product, nibrin, is involved in DNA recombination repair, a function shared with known tumor suppressor genes like BRCA1 and BRCA2. This led us to investigate whether NBS acts as tumor suppressor gene in the development of non-Hodgkin lymphomas. Therefore, we performed fluorescence in situ hybridization analysis using a BAC clone containing the entire NBS1 region on eight B-cell and eight T-cell lymphomas, including one B-cell and two T-cell lymphomas with structural abnormalities of 8q. None of the tumors showed a deletion of the NBS1 gene, demonstrating that deletion of the NBS1 gene is not a major cause or a primary event in tumorigenesis of human B- and T-cell lymphomas.

Clinical ascertainment of Nijmegen breakage syndrome (NBS) and prevalence of the major mutation, 657del5, in three Slav populations

Nijmegen breakage syndrome (NBS) is a chromosomal instability disorder, clinically characterised by microcephaly, immunodeficiency, radiosensitivity and a very high predisposition to lymphoid malignancy. Recently, it was demonstrated that mutations in the NBS1 gene are responsible for NBS. Most of the NBS patients known so far are of Slav origin and carry a major founder mutation 657del5 in exon 6 of the NBS1 gene. In this study we estimated the prevalence of the 657del5 mutation in the Czech Republic, Poland and the Ukraine. We found an unexpectedly high carrier frequency of the 657del5 mutation (1/177) in the three Slav populations, a factor that may contribute to cancer frequency in those countries. In addition, we show that NBS patients are often diagnosed late and therefore receive inappropriate therapy.

Clinical presentation and mutation identification in the NBS1 gene in a boy with Nijmegen breakage syndrome

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder which belongs to the group of inherited chromosomal instability syndromes. The clinical characteristics include severe microcephaly, a dysmorphic facies, and immunodeficiency with predisposition to malignancies. While the cellular characteristics of ataxia teleangiectasia (AT) and NBS are similar, the clinical findings are quite distinct. NBS patients show characteristic microcephaly, which is rare in association with AT and they do not develop ataxia and teleangiectasia. Recently, the gene mutated in NBS has been identified. Here we report a 5-year-old Bosnian boy with severe microcephaly. Because of multiple structural aberrations involving chromosomes 7 and 14 typical for AT (MIM 208900) and NBS (MIM 251260), AT was diagnosed. We suggested the diagnosis of NBS because of the boy's remarkable microcephaly, his facial appearance, and the absence of ataxia and teleangiectasia. DNA analysis was performed and revealed that the boy is homozygous for the major mutation (657de15) in the NBS1 gene. This finding confirms the diagnosis of NBS in our patient and offers the possibility to perform a most reliable prenatal diagnosis in a further pregnancy.

Nibrin, a novel DNA double-strand break repair protein, is mutated in Nijmegen breakage syndrome

Nijmegen breakage syndrome (NBS) is an autosomal recessive chromosomal instability syndrome characterized by microcephaly, growth retardation, immunodeficiency, and cancer predisposition. Cells from NBS patients are hypersensitive to ionizing radiation with cytogenetic features indistinguishable from ataxia telangiectasia. We describe the positional cloning of a gene encoding a novel protein, nibrin. It contains two modules found in cell cycle checkpoint proteins, a forkhead-associated domain adjacent to a breast cancer carboxy-terminal domain. A truncating 5 bp deletion was identified in the majority of NBS patients, carrying a conserved marker haplotype. Five further truncating mutations were identified in patients with other distinct haplotypes. The domains found in nibrin and the NBS phenotype suggest that this disorder is caused by defective responses to DNA double-strand breaks.

Computer program for the equations describing the steady state of enzyme reactions

MOTIVATION

The derivation of steady-state equations is frequently carried out in enzyme kinetic studies. Done manually, this becomes tedious and prone to human error. The computer programs now available which are able to accept reaction mechanisms of some complexity are focused only on the strict steady-state approach.

RESULTS

Here we present a computer program called REFERASS, with a short computation time and a user-friendly format for the input and output files, able to derive the strict steady-state equations and/or those corresponding to the usual assumption that one ore more of the reversible steps are in rapid equilibrium. This program handles enzyme-catalysed reactions with mechanisms involving up to 255 enzyme species connected by up to 255 reaction steps, subject to limits imposed by the memory and disk space available.

An analysis of the kinetics of enzymatic systems with unstable species

We present a general kinetic analysis of the Michaelis-Menten mechanism for the case in which the substrate, the enzyme-substrate complex, and the product are unstable. The equations for the rapid equilibrium conditions are obtained as a particular case of the general equations of the transient-phase. The kinetic data analysis which we suggest is based on the time progress curve of the product of the enzymatic reaction, or on the progress curve of the species into which the immediate product is transformed. This analysis allows the determination of the rate and equilibrium constants if adequate experimental results are available. It assumes, in contrast to most previous treatments of enzyme kinetics, that the concentration of the enzyme is much higher than that of the substrate. Since this condition often is satisfied inside cells, it can be more relevant to physiological problems than the classical assumption, [E] << [S], which sooner pertains to the situation in the laboratory.

Kinetic study of an enzyme-catalysed reaction in the presence of novel irreversible-type inhibitors that react with the product of enzymatic catalysis

In the present paper a kinetic study is made of the behaviour of a Michaelis-Menten enzyme-catalysed reaction in the presence of irreversible inhibitors rendered unstable in the medium by their reaction with the product of enzymatic catalysis. A general mechanism involving competitive, non-competitive, uncompetitive and mixed irreversible inhibition with one or two steps has been analysed. The differential equation that describes the kinetics of the reaction is non-linear and computer simulations of its dynamic behaviour are presented. The results obtained show that the systems studied here present kinetic co-operativity for a target enzyme that follows the simple Michaelis-Menten mechanism in its action on the substrate, except in the case of an uncompetitive-type inhibitor.

Pancreatic insufficiency and pulmonary disease in German and Slavic cystic fibrosis patients with the R347P mutation

Cystic fibrosis (CF) is caused by mutations in the gene for the cystic fibrosis transmembrane conductance regulator (CFTR) that codes for a cAMP-regulated chloride channel. The R347P is a missense mutation located within the first membrane spanning domain (MSD1) of the CFTR protein. This mutation occurs with an overall worldwide frequency of about 0.2%. The patients, originally described with this mutation were compound heterozygotes with the delta F508 mutation and had a very mild course of CF, suggesting that R347P, similar to other missense mutations affecting the MSD1 domain, causes a mild phenotype. We report here a group of 19 CF patients with the R347P mutation of German, Bulgarian, Czech, and Slovak origin, including two homozygotes. Most patients presented with early disease onset, pancreas insufficiency (PI), and early pulmonary involvement, suggesting that this mutation can lead to a severe course of CF. Most R347P alleles in the group studied share a common polymorphic haplotype. In addition, these analyses gave evidence for recurrence of the mutation in two CF patients of German and Czech origin.

Experimental methods for kinetic study of suicide substrates

The kinetic study of the enzymatic inactivation originated by suicide substrates can be carried out by means of two alternative approaches. One method considers the substrate concentration as practically constant during the assay time and provides explicit equations of product concentration vs. time. The other method involves the significant consumption of the substrate, yielding implicit equations of time vs. product concentration. The utility of both methods is discussed and adequate experimental conditions for their correct application are established.

Kinetic study of the activation process of frog epidermis pro-tyrosinase by trypsin

1. The rate of tyrosinase formation has been calculated by coupling the activatory process of frog epidermis pro-tyrosinase by trypsin to the oxidation of L-DOPA to dopachrome. Under certain conditions ([trypsin]/[pro-tyrosinase] greater than or equal to 300), the lag period of the coupled reactions, tau, is independent of trypsin concentration. 2. The specific rate constant of tyrosinase formation at different temperatures has been calculated, ranging from 0.025 sec-1, at 5 degrees C to 0.248 sec-1, at 30 degrees C. 3. Thermodynamic parameters of the activatory process (delta G not equal to = + 18.5 kcal/mol; delta H not equal to = + 14.8 kcal/mol; delta S not equal to = -12.4 e.u.; Ea = + 15.3 kcal/mol), have been determined by the study of the system at different temperatures. These values are characteristic for a normal chemical reaction. 4. From these kinetic data, the order of products formation in the proteolytic step, can be determined, active tyrosinase being the last product released.

Evidence for the deficiency of beta-glucosidase-activating factor in fibroblasts of patients with I-cell disease

Reduced activity of beta-glucosidase was shown in the cultured skin fibroblasts of four patients with I-cell disease when the enzyme was tested without the use of detergents. In the presence of taurocholate and triton X100 beta-glucosidase activity was normal. This suggested a deficiency of a beta-glucosidase activating factor in I-cell fibroblasts rather than of the enzyme itself. The deficiency of beta-glucosidase activity was corrected to some extent by mixing cell lysates, and more effectively by cocultivation and fusion of I-cell disease and Gaucher fibroblasts. These results present evidence for the presence of a beta-glucosidase-activating factor in normal and Gaucher fibroblasts. In fibroblasts of patients with I-cell disease this activator is probably deficient, as is the case for most lysosomal enzymes.