Genomic Runs of Homozygosity Record Population History and Consanguinity

The human genome is characterised by many runs of homozygous genotypes, where identical haplotypes were inherited from each parent. The length of each run is determined partly by the number of generations since the common ancestor: offspring of cousin marriages have long runs of homozygosity (ROH), while the numerous shorter tracts relate to shared ancestry tens and hundreds of generations ago. Human populations have experienced a wide range of demographic histories and hold diverse cultural attitudes to consanguinity. In a global population dataset, genome-wide analysis of long and shorter ROH allows categorisation of the mainly indigenous populations sampled here into four major groups in which the majority of the population are inferred to have: (a) recent parental relatedness (south and west Asians); (b) shared parental ancestry arising hundreds to thousands of years ago through long term isolation and restricted effective population size (N(e)), but little recent inbreeding (Oceanians); (c) both ancient and recent parental relatedness (Native Americans); and (d) only the background level of shared ancestry relating to continental N(e) (predominantly urban Europeans and East Asians; lowest of all in sub-Saharan African agriculturalists), and the occasional cryptically inbred individual. Moreover, individuals can be positioned along axes representing this demographic historic space. Long runs of homozygosity are therefore a globally widespread and under-appreciated characteristic of our genomes, which record past consanguinity and population isolation and provide a distinctive record of the demographic history of an individual's ancestors. Individual ROH measures will also allow quantification of the disease risk arising from polygenic recessive effects

Argininosuccinate synthetase activity in cultured human lymphocytes

The activity of argininosuccinate synthetase (E.C. 6.3.4.5), a urea cycle enzyme, was measured in cultured human lymphocytes using a new radioactive assay. Control cells had a maximum specific activity of 15.7±8.7 nmoles per hour per milligram of protein and an apparent K m for citrulline of 2 × 10 −4 m , whereas cells derived from a patient with citrullinemia had no detectable activity. A nutritional variant, selected out of the citrullinemic lymphocyte population by ability to grow in citrulline, had a maximum specific activity of 10.7±3.8 nmoles/hr/mg and an apparent K m for citrulline of 2 × 10 −2 m . These measurements confirm the observation that citrullinemia is associated with a defect in argininosuccinate synthetase activity and provide further evidence that citrullinemia is expressed in cultured lymphocytes. The emergence of a nutritional variant with a partial defect in argininosuccinate synthetase enzyme suggests that this citrullinemic patient has a heterogeneous population of cells, some totally defective and others only partially defective in argininosuccinate synthetase. The new activity assay is described in detail.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44125/1/10528_2004_Article_BF00484469.pd

Citrulline metabolism in normal and citrullinemic human lymphocyte lines

Citrullinemia is one of the five aminoacidurias associated with the Krebs-Henseleit urea cycle. A long-term lymphocyte line (UM-21) derived from a patient with this disease and nine of ten clones of this line were found to have no activity for the enzyme argininosuccinate synthetase (AS), as demonstrated by their inability to grow in medium in which citrulline had been substituted for arginine, by their inability to incorporate arginine-C 14 derived from citrulline-C 14 into cellular protein, and by direct enzyme assay. One clone had normal or nearly normal argininosuccinate synthetase activity, as demonstrated by the same criteria. Nutritional “variants” able to grow logarithmically in medium containing citrulline were isolated from UM-21 and three clones. The apparent K m s of AS for citrulline in UM-21, the ten clones, the variant lines, and a normal line were measured and fell into three groups: AS in UM-21 and nine clones had no measurable apparent K m for citrulline; AS in the variant cells had apparent K m s for citrulline of approximately 20 m m ; and AS in the normal cell line and one clone had apparent K m s for citrulline of 0.2 m m . The data suggest that the defect in the citrullinemic cell lines is due to a mutation in the structural gene coding for argininosuccinate synthetase.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44122/1/10528_2004_Article_BF00485789.pd

Localization of the membrane defect in transepithelial transport of taurine by parallel studies in vivo and in vitro in hypertaurinuric mice.

We investigated the mechanism of taurinuria in three inbred strains of mice: A/J, a normal taurine excretor (taut+); and two hypertaurinuric (taut-) strains, C57BL/6J and PRO/Re. Plasma taurine is comparable in the three strains (approximately 0.5 mM), but taurinuria is 10-fold greater in taut- animals. Fractional reabsorption of taurine is 0.967 +/- 0.013 (mean +/- SD) in A/J); and 0.839 +/- 0.08 and 0.787 +/- 0.05 in C57BL/6J and PRO/Re, respectively. Taurine concentration in renal cortex intracellular fluid (free of urine contamination) is similar in the three strains. Taurine reabsorption is inhibited by beta-alanine, in taut+ and taut- strains. These in vivo findings reveal residual taurine transport activity in the taut- phenotype and no evidence for impaired efflux at basilar membranes as the cause of impaired taurine reabsorption. Cortex slices provide information about uptake of amino acids at the antiluminal membrane. Taurine behaves as an inert metabolite in mouse kidney cortex slices. Taurine uptake by slices is active and, at less than 1 mM, is greater than normal in taut- slices. Concentration-dependent uptake studies reveal more than one taurine carrier in taut+ and taut- strains. The apparent Km values for uptake below 1 mM are different in taut- and taut+ slices (approximately 0.2 mM and approximately 0.7 mM, respectively); the apparent Km values above 1 mM taurine are similar in taut+ and taut- slices. Efflux from slices in all strains in the same (0.0105-0.0113 mumol-min-1-g-1 wet wt), but taut- tissue retains about 10% more radioactivity over the period of efflux. beta-Alanine is actively metabolized in mouse kidney. Its uptake in the presence of blocked transamination, is greater; its intracellular oxidation is attenuated; and its exchange with intracellular taurine is diminished in taut- slices. These findings indicate impaired beta-amino acid permeation on a low-Km uptake system at the luminal membrane in the taut- phenotype. beta-Amino acids are not reclaimed efficiently either from the innermost luminal pool in cortex slices or from the ultrafiltrate in the tubule lumen in vivo. The former leads to high uptake ratios in vitro, the latter to high clearance rates in vivo. In vitro and in vivo data are thus concordant. This is the first time that a hereditary defect in amino acid transport has been assigned to a specific membrane surface in mammalian kidney

Molecular genetic analysis of the NF2 gene in young patients with unilateral vestibular schwannomas

Neurofibromatosis type 2 (NF2) must be suspected in patients presenting with a unilateral vestibular schwannoma at a young age who are therefore at theoretical risk of developing bilateral disease. We identified 45 patients aged 30 years or less at the onset of symptoms of a unilateral vestibular schwannoma. Molecular genetic analysis of the NF2 gene was completed on peripheral blood samples in all 45 and on 28 tumour samples. No pathogenic NF2 mutations were identified in any of the blood samples. NF2 point mutations were identified in 21/28 (75%) tumour samples and loss of heterozygosity (LOH) in 21/28 (75%) tumour samples. Both mutational hits were identified in 18/28 (65%) tumour samples. In one multilobular tumour, one (presumably first hit) mutation was confirmed which was common to different foci of the tumour, while the second mutational event differed between foci. The molecular findings in this patient were consistent with somatic mosaicism for NF2 and the clinical diagnosis was confirmed with the presence of two meningiomas on a follow up MRI scan. A further patient developed a contralateral vestibular schwannoma on a follow up MRI scan in whom neither of the truncating mutations in the vestibular schwannoma were present in blood. It is important when counselling patients with unilateral vestibular schwannomas to identify (1) those at risk of bilateral disease, (2) those at risk of developing other tumours, and (3) other family members at risk of developing NF2. Comparing tumour and blood DNA cannot exclude mosaicism in the index case and cannot, therefore, be used to predict those at risk of developing further tumours. However, identification of both mutations or one mutation plus LOH in the tumour and exclusion of those mutations in the blood samples of the sibs or offspring of the affected case may be sufficient to render further screening unnecessary in these relatives