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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Genetic basis of clinical catecholamine disorders

Norepinephrine and epinephrine are critical determinants of minute-to-minute regulation of blood pressure. Here we review the characterization of two syndromes associated with a genetic abnormality in the noradrenergic pathway. In 1986, we reported a congenital syndrome of undetectable tissue and circulating levels of norepinephrine and epinephrine, elevated levels of dopamine, and absence of dopamine-beta-hydroxylase (DBH). These patients appeared with ptosis and severe orthostatic hypotension and lacked sympathetic noradrenergic function. In two persons with DBH deficiency, we identified seven novel polymorphisms. Both patients are compound heterozygotes for a variant that affects expression of DBH protein via impairment of splicing. Patient 1 also has a missense mutation in DBH exon 2, and patient 2 carries missense mutations in exons 1 and 6. Orthostatic intolerance is a common syndrome affecting young women, presenting with orthostatic tachycardia and symptoms of cerebral hypoperfusion on standing. We tested the hypothesis that abnormal norepinephrine transporter (NET) function might contribute to its etiology. In our proband, we found an elevated plasma norepinephrine with standing that was disproportionate to the increase in levels of dihydroxphenylglycol, as well as impaired norepinephrine clearance and tyramine resistance. Studies of NET gene structure revealed a coding mutation converting a conserved alanine residue in transmembrane domain 9 to proline. Analysis of the protein produced by the mutant cDNA demonstrated greater than 98% reduction in activity relative to normal. The finding of genetic mutations responsible for DBH deficiency and orthostatic intolerance leads us to believe that genetic causes of other autonomic disorders will be found, enabling us to design more effective therapeutic interventions.

Non-NASA Center↗
DOE OSTI2013

Determination of the physiological 2:2 TLR5:flagellin activation stoichiometry revealed by the activity of a fusion receptor

Highlights: •The chimeric protein fusing flagellin to the TLR5 ectodomain is constitutively active. •Mutation P736H within the BB-loop of TLR5 TIR domain renders the receptor inactive. •The R90D mutation in flagellin inactivated autoactivation of the chimeric protein. •The 2:2 stoichiometry of the TLR5:flagellin complex is physiologically relevant. -- Abstract: Toll-like receptor 5 (TLR5) recognizes flagellin of most flagellated bacteria, enabling activation of the MyD88-dependent signaling pathway. The recently published crystal structure of a truncated zebrafish TLR5 ectodomain in complex with an inactive flagellin fragment indicated binding of two flagellin molecules to a TLR5 homodimer, however this complex did not dimerize in solution. In the present study, we aimed to determine the physiological stoichiometry of TLR5:flagellin activation by the use of a chimeric protein composed of an active flagellin fragment linked to the N-terminus of human TLR5 (SF-TLR5). This construct was constitutively active. Inactivation by the R90D mutation within flagellin demonstrated that autoactivation of the chimeric protein depended solely on the specific interaction between TLR5 and flagellin. Addition of wild-type hTLR5 substantially lowered autoactivation of SF-TLR5 in a concentration dependent manner, an effect which was reversible by the addition of exogenous Salmonella typhimurium flagellin, indicating the biological activity of a TLR5:flagellin complex with a 2:2 stoichiometry. These results, in addition to the combinations of inactive P736H mutation within the BB-loop of the TIR domain of TLR5 and SF-TLR5, further confirm the mechanism of TLR5 activation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

The cataract-associated V41M mutant of human γS-crystallin shows specific structural changes that directly enhance local surface hydrophobicity

Highlights: •We present NMR analysis of V41M, a cataract-causing mutant of human γS-crystallin. •Mutation alters strand–strand interactions throughout the N-terminal domain. •Mutation directly affects Trp46 due to key Met41-S–Trp46-pi interactions. •We identify the basis of the surface hydrophobicity increase and residues involved. -- Abstract: The major crystallins expressed in the human lens are γS-, γC- and γD-crystallins. Several mutations in γS-crystallin are associated with hereditary cataracts, one of which involves the substitution of a highly conserved Valine at position 41 to Methionine. According to a recent report, the mutant protein, V41M, shows lower stability and increased surface hydrophobicity compared to the wild-type, and a propensity for self-aggregation. Here we address the structural differences between the two proteins, with residue-level specificity using NMR spectroscopy. Based on the structural model of the mutant protein, our results clearly show that the mutation creates a major local perturbation almost at the junction of the first and second “Greek-key” motifs in the N-terminal domain. A larger section of the second motif (residues 44–86) appears to be mainly affected. Based on the sizeable chemical shift of the imino proton of the indole side-chain of Trp46 in V41M, we suggest that the sulphur atom of Met41 is involved in an S–π interaction with Trp46. This interaction would bring the last β-strand of the first “Greek-key” motif closer to the first β-strand of the second motif. This appears to lead to a domino effect, towards both the N- and C-terminal ends, even as it decays off substantially beyond the domain interface. During this process discreet hydrophobic surface patches are created, as revealed by ANS-binding. Such changes would not affect the secondary structure or cause a major change in the tertiary structure, but can lead to self-aggregation or aberrant binding interactions of the mutant protein in vivo, and lead to lens opacity or cataract.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Comparative genomic and proteomic analyses of Clostridium acetobutylicum Rh8 and its parent strain DSM 1731 revealed new understandings on butanol tolerance

Highlights: • Genomes of a butanol tolerant strain and its parent strain were deciphered. • Comparative genomic and proteomic was applied to understand butanol tolerance. • None differentially expressed proteins have mutations in its corresponding genes. • Mutations in ribosome might be responsible for the global difference of proteomics. - Abstract: Clostridium acetobutylicum strain Rh8 is a butanol-tolerant mutant which can tolerate up to 19 g/L butanol, 46% higher than that of its parent strain DSM 1731. We previously performed comparative cytoplasm- and membrane-proteomic analyses to understand the mechanism underlying the improved butanol tolerance of strain Rh8. In this work, we further extended this comparison to the genomic level. Compared with the genome of the parent strain DSM 1731, two insertion sites, four deletion sites, and 67 single nucleotide variations (SNVs) are distributed throughout the genome of strain Rh8. Among the 67 SNVs, 16 SNVs are located in the predicted promoters and intergenic regions; while 29 SNVs are located in the coding sequence, affecting a total of 21 proteins involved in transport, cell structure, DNA replication, and protein translation. The remaining 22 SNVs are located in the ribosomal genes, affecting a total of 12 rRNA genes in different operons. Analysis of previous comparative proteomic data indicated that none of the differentially expressed proteins have mutations in its corresponding genes. Rchange Algorithms analysis indicated that the mutations occurred in the ribosomal genes might change the ribosome RNA thermodynamic characteristics, thus affect the translation strength of these proteins. Take together, the improved butanol tolerance of C. acetobutylicum strain Rh8 might be acquired through regulating the translational process to achieve different expression strength of genes involved in butanol tolerance.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Emerging (and converging) pathways in Parkinson's disease: keeping mitochondrial wellness

The selective cell loss in the ventral component of the substantia nigra pars compacta and the presence of alpha-synuclein (α-syn)-rich intraneuronal inclusions called Lewy bodies are the pathological hallmarks of Parkinson's disease (PD), the most common motor system disorder whose aetiology remains largely elusive. Although most cases of PD are idiopathic, there are rare familial forms of the disease that can be traced to single gene mutations that follow Mendelian inheritance pattern. The study of several nuclear encoded proteins whose mutations are linked to the development of autosomal recessive and dominant forms of familial PD enhanced our understanding of biochemical and cellular mechanisms contributing to the disease and suggested that many signs of neurodegeneration result from compromised mitochondrial function. Here we present an overview of the current understanding of PD-related mitochondrial dysfunction including defects in bioenergetics and Ca{sup 2+} homeostasis, mitochondrial DNA mutations, altered mitochondrial dynamics and autophagy. We emphasize, in particular, the convergence of many “apparently” different pathways towards a common route involving mitochondria. Understanding whether mitochondrial dysfunction in PD represents the cause or the consequence of the disease is challenging and will help to define the pathogenic processes at the basis of the PD onset and progression. - Highlights: • Mitochondrial dysfunctions are a common feature of neurodegenerative diseases. • Many familial PD related proteins ensure mitochondrial function. • Mutations in PD genes differently affect mitochondria related activities.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Molecular interaction between K-Ras and H-REV107 in the Ras signaling pathway

Ras proteins are small GTPases that serve as master moderators of a large number of signaling pathways involved in various cellular processes. Activating mutations in Ras are found in about one-third of cancers. H-REV107, a K-Ras binding protein, plays an important role in determining K-Ras function. H-REV107 is a member of the HREV107 family of class II tumor suppressor genes and a growth inhibitory Ras target gene that suppresses cellular growth, differentiation, and apoptosis. Expression of H-REV107 was strongly reduced in about 50% of human carcinoma cell lines. However, the specific molecular mechanism by which H-REV107 inhibits Ras is still unknown. In the present study, we suggest that H-REV107 forms a strong complex with activating oncogenic mutation Q61H K-Ras from various biochemical binding assays and modeled structures. In addition, the interaction sites between K-Ras and H-REV107 were predicted based on homology modeling. Here, we found that some structure-based mutants of the K-Ras disrupted the complex formation with H-REV107. Finally, a novel molecular mechanism describing K-Ras and H-REV107 binding is suggested and insights into new K-Ras effector target drugs are provided. - Highlights: • H-REV107 interacts strongly with active GNP-bound K-Ras mutation. • H-REV107 rotated dramatically to interact with K-Ras Q61H (GNP) from the binding position with K-Ras WT (GDP). • In the K-Ras and H-REV107 interaction, E31 of K-Ras is an important binding residue. • H-REV107 can be a stable drug target for oncogenic K-Ras mutation.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Genomic Evolution of Breast Cancer Metastasis and Relapse

Patterns of genomic evolution between primary and metastatic breast cancer have not been studied in large numbers, despite patients with metastatic breast cancer having dismal survival. We sequenced whole genomes or a panel of 365 genes on 299 samples from 170 patients with locally relapsed or metastatic breast cancer. Several lines of analysis indicate that clones seeding metastasis or relapse disseminate late from primary tumors, but continue to acquire mutations, mostly accessing the same mutational processes active in the primary tumor. Most distant metastases acquired driver mutations not seen in the primary tumor, drawing from a wider repertoire of cancer genes than early drivers. Lastly, these include a number of clinically actionable alterations and mutations inactivating SWI-SNF and JAK2-STAT3 pathways.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Circulating SARS-CoV-2 spike N439K variants maintain fitness while evading antibody-mediated immunity

SARS-CoV-2 can mutate and evade immunity, with consequences for efficacy of emerging vaccines and antibody therapeutics. Here, we demonstrate that the immunodominant SARS-CoV-2 spike (S) receptor binding motif (RBM) is a highly variable region of S and provide epidemiological, clinical, and molecular characterization of a prevalent, sentinel RBM mutation, N439K. We demonstrate N439K S protein has enhanced binding affinity to the hACE2 receptor, and N439K viruses have similar in vitro replication fitness and cause infections with similar clinical outcomes as compared to wild type. We show the N439K mutation confers resistance against several neutralizing monoclonal antibodies, including one authorized for emergency use by the US Food and Drug Administration (FDA), and reduces the activity of some polyclonal sera from persons recovered from infection. Immune evasion mutations that maintain virulence and fitness such as N439K can emerge within SARS-CoV-2 S, highlighting the need for ongoing molecular surveillance to guide development and usage of vaccines and therapeutics.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2022

Highly efficient and simple SSPER and rrPCR approaches for the accurate site-directed mutagenesis of large and small plasmids

Advances are needed in the site-directed mutagenesis of large plasmids for protein structure-function studies, as current methods are often inefficient, complicated and time-consuming. Here two new methods are reported that overcome these difficulties, namely the single primer extension reaction (SSPER) strategy that reaches 100% efficiency and the reduce recycle PCR (rrPCR) method that is advantageous in generating single and pairwise combinations of mutations. Both methods are distinguished from current technologies by the addition of a step that easily removes the oligonucleotide primer(s) after the first reaction, thus allowing for the addition of a second reaction in chronological sequence to generate and isolate the appropriate DNA product with the site-directed mutation(s). High efficiency of the methods is demonstrated by generating single and paired combinations of the 11 site-directed mutations targeted on 5 different plasmid DNA templates ranging from 10 to 12 kb and 57–60% GC-content at a rate of 50–100%. Overall, the methods are demonstrated to be (i) highly accurate, allowing for screening of plasmids by DNA sequencing, (ii) streamlined to generate the mutations within a single day, (iii) cost-effective in requiring only two primers and two enzymes (DpnI and a proofreading DNA polymerase), (iv) straightforward in primer design, (v) applicable for both large and small plasmids, and (vi) easily implemented by entry level researchers.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2014

Mitochondrial DNA disease—molecular insights and potential routes to a cure

Mitochondrial DNA diseases are common neurological conditions caused by mutations in the mitochondrial genome or nuclear genes responsible for its maintenance. Current treatments for these disorders are focussed on the management of the symptoms, rather than the correction of biochemical defects caused by the mutation. This review focuses on the molecular effects of mutations, the symptoms they cause and current work focusing on the development of targeted treatments for mitochondrial DNA disease. - Highlights: • We discuss several common disease causing mtDNA mutations. • We highlight recent work linking pathogenicity to deletion size and heteroplasmy. • We discuss recent advances in the development of targeted mtDNA disease treatments.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Structure of the Newcastle disease virus hemagglutinin-neuraminidase (HN) ectodomain reveals a four-helix bundle stalk

The paramyxovirus hemagglutinin-neuraminidase (HN) protein plays multiple roles in viral entry and egress, including binding to sialic acid receptors, activating the fusion (F) protein to activate membrane fusion and viral entry, and cleaving sialic acid from carbohydrate chains. HN is an oligomeric integral membrane protein consisting of an N-terminal transmembrane domain, a stalk region, and an enzymatically active neuraminidase (NA) domain. Structures of the HN NA domains have been solved previously; however, the structure of the stalk region has remained elusive. The stalk region contains specificity determinants for F interactions and activation, underlying the requirement for homotypic F and HN interactions in viral entry. Mutations of the Newcastle disease virus HN stalk region have been shown to affect both F activation and NA activities, but a structural basis for understanding these dual affects on HN functions has been lacking. Here, we report the structure of the Newcastle disease virus HN ectodomain, revealing dimers of NA domain dimers flanking the N-terminal stalk domain. The stalk forms a parallel tetrameric coiled-coil bundle (4HB) that allows classification of extensive mutational data, providing insight into the functional roles of the stalk region. Mutations that affect both F activation and NA activities map predominantly to the 4HB hydrophobic core, whereas mutations that affect only F-protein activation map primarily to the 4HB surface. Two of four NA domains interact with the 4HB stalk, and residues at this interface in both the stalk and NA domain have been implicated in HN function.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Essential amino acids in the Plant-Conserved and Class-Specific Regions of cellulose synthases

The Plant-Conserved Region (P-CR) and the Class-Specific Region (CSR) are two plant-unique sequences in the catalytic core of cellulose synthases (CESAs) for which specific functions have not been established. Here, we used site-directed mutagenesis to replace amino acids and motifs within these sequences predicted to be essential for assembly and function of CESAs. We developed an in vivo method to determine the ability of mutated CesA1 transgenes to complement an Arabidopsis (Arabidopsis thaliana) temperature-sensitive root-swelling1 (rsw1) mutant. Replacement of a Cys residue in the CSR, which blocks dimerization in vitro, rendered the AtCesA1 transgene unable to complement the rsw1 mutation. Examination of the CSR sequences from 33 diverse angiosperm species showed domains of high-sequence conservation in a class-specific manner but with variation in the degrees of disorder, indicating a nonredundant role of the CSR structures in different CESA isoform classes. The Cys residue essential for dimerization was not always located in domains of intrinsic disorder. Expression of AtCesA1 transgene constructs, in which Pro417 and Arg453 were substituted for Ala or Lys in the coiled-coil of the P-CR, were also unable to complement the rsw1 mutation. Despite an expected role for Arg457 in trimerization of CESA proteins, AtCesA1 transgenes with Arg457Ala mutations were able to fully restore the wild-type phenotype in rsw1. Our data support that Cys662 within the CSR and Pro417 and Arg453 within the P-CR of Arabidopsis CESA1 are essential residues for functional synthase complex formation, but our data do not support a specific role for Arg457 in trimerization in native CESA complexes.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

Cofactor Specificity of the Bifunctional Alcohol and Aldehyde Dehydrogenase (AdhE) in Wild-Type and Mutant Clostridium thermocellum and Thermoanaerobacterium saccharolyticum

Clostridium thermocellum and Thermoanaerobacterium saccharolyticum are thermophilic bacteria that have been engineered to produce ethanol from the cellulose and hemicellulose fractions of biomass, respectively. Although engineered strains of T. saccharolyticum produce ethanol with a yield of 90% of the theoretical maximum, engineered strains of C. thermocellum produce ethanol at lower yields (~50% of the theoretical maximum). In the course of engineering these strains, a number of mutations have been discovered in their adhE genes, which encode both alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) enzymes. To understand the effects of these mutations, the adhE genes from six strains of C. thermocellum and T. saccharolyticum were cloned and expressed in Escherichia coli , the enzymes produced were purified by affinity chromatography, and enzyme activity was measured. In wild-type strains of both organisms, NADH was the preferred cofactor for both ALDH and ADH activities. In high-ethanol-producing (ethanologen) strains of T. saccharolyticum , both ALDH and ADH activities showed increased NADPH-linked activity. Interestingly, the AdhE protein of the ethanologenic strain of C. thermocellum has acquired high NADPH-linked ADH activity while maintaining NADH-linked ALDH and ADH activities at wild-type levels. When single amino acid mutations in AdhE that caused increased NADPH-linked ADH activity were introduced into C. thermocellum and T. saccharolyticum , ethanol production increased in both organisms. Structural analysis of the wild-type and mutant AdhE proteins was performed to provide explanations for the cofactor specificity change on a molecular level. This work describes the characterization of the AdhE enzyme from different strains of C. thermocellum and T. saccharolyticum . C. thermocellum and T. saccharolyticum are thermophilic anaerobes that have been engineered to make high yields of ethanol and can solubilize components of plant biomass and ferment the sugars to ethanol. In the course of engineering these strains, several mutations arose in the bifunctional ADH/ALDH protein AdhE, changing both enzyme activity and cofactor specificity. We show that changing AdhE cofactor specificity from mostly NADH linked to mostly NADPH linked resulted in higher ethanol production by C. thermocellum and T. saccharolyticum .

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Assessment of software methods for estimating protein-protein relative binding affinities

A growing number of computational tools have been developed to accurately and rapidly predict the impact of amino acid mutations on protein-protein relative binding affinities. Such tools have many applications, for example, designing new drugs and studying evolutionary mechanisms. In the search for accuracy, many of these methods employ expensive yet rigorous molecular dynamics simulations. By contrast, non-rigorous methods use less exhaustive statistical mechanics, allowing for more efficient calculations. However, it is unclear if such methods retain enough accuracy to replace rigorous methods in binding affinity calculations. This trade-off between accuracy and computational expense makes it difficult to determine the best method for a particular system or study. Here, eight non-rigorous computational methods were assessed using eight antibody-antigen and eight non-antibody-antigen complexes for their ability to accurately predict relative binding affinities (ΔΔG) for 654 single mutations. In addition to assessing accuracy, we analyzed the CPU cost and performance for each method using a variety of physico-chemical structural features. This allowed us to posit scenarios in which each method may be best utilized. Most methods performed worse when applied to antibody-antigen complexes compared to non-antibody-antigen complexes. Rosetta-based JayZ and EasyE methods classified mutations as destabilizing (ΔΔG < -0.5 kcal/mol) with high (83–98%) accuracy and a relatively low computational cost for non-antibody-antigen complexes. Some of the most accurate results for antibody-antigen systems came from combining molecular dynamics with FoldX with a correlation coefficient (r) of 0.46, but this was also the most computationally expensive method. Overall, our results suggest these methods can be used to quickly and accurately predict stabilizing versus destabilizing mutations but are less accurate at predicting actual binding affinities. This study highlights the need for continued development of reliable, accessible, and reproducible methods for predicting binding affinities in antibody-antigen proteins and provides a recipe for using current methods.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Origin and evolution of HIV-1 subtype A6

Background: HIV outbreaks in the Former Soviet Union (FSU) countries were characterized by repeated transmission of the HIV variant AFSU, which is now classified as a distinct subtype A sub-subtype called A6. The current study used phylogenetic/phylodynamic and signature mutation analyses to determine likely evolutionary relationship between subtype A6 and other subtype A sub-subtypes. Methods: For this study, an initial Maximum Likelihood phylogenetic analysis was performed using a total of 553 full-length, publicly available, reverse transcriptase sequences, from A1, A2, A3, A4, A5, and A6 sub-subtypes of subtype A. For phylogenetic clustering and signature mutation analysis, a total of 5961 and 3959 pol and env sequences, respectively, were used. Results: Phylogenetic and signature mutation analysis showed that HIV-1 sub-subtype A6 likely originated from sub-subtype A1 of African origin. A6 and A1 pol and env genes shared several signature mutations that indicate genetic similarity between the two subtypes. For A6, tMRCA dated to 1975, 15 years later than that of A1. Conclusion: The current study provides insights into the evolution and diversification of A6 in the backdrop of FSU countries and indicates that A6 in FSU countries evolved from A1 of African origin and is getting bridged outside the FSU region.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2019

Effects of Copper Chelation on BRAF V600E Positive Colon Carcinoma Cells

High affinity copper binding to mitogen-activated protein kinase kinase 1 (MAP2K1, also known as MEK1) allosterically promotes the kinase activity of MEK1/2 on extracellular signal regulated kinases 1 and 2 (ERK1/2). Consequently, copper-dependent activation of the mitogen-activated (MAP) kinase pathway has a role in promoting tumor growth. Conversely, copper chelation may represent a possible therapeutic approach for a specific subset of tumors characterized by activating mutations in the serine/threonine protein kinase V-Raf Murine Sarcoma Viral Oncogene Homolog B1 (BRAF), such as the V600E, occurring within the kinase domain (BRAF V600E ). Tetrathiomolybdate (TM) is a specific copper chelating agent currently used for the treatment of Wilson’s disease and in preclinical studies for the management of metastatic cancers owing to its anti-angiogenic and anti-inflammatory properties. We evaluated in vitro and in vivo the effects of copper depletion achieved by pharmacological treatment with TM in human colorectal cells bearing the BRAF V600E mutation in comparison with BRAF wild type cells. We provide evidence that selective copper chelation differentially affects proliferation, survival and migration of colon cancer cells bearing the BRAF V600E mutation compared to BRAF wt acting via differential phosphorylation levels of ERK1/2. Moreover, tetrathiomolybdate treatment was also effective in reducing the clonogenic potential of colon cancer BRAF V600E cells resistant to BRAF pharmacological inhibition. In conclusion, these results support further assessment of copper chelation therapy as an adjuvant therapy for inhibiting the progression of colon cancers containing the BRAF V600E mutation.

60 APPLIED LIFE SCIENCES↗
NASA NTRS2001

Mechanisms of mutagenesis in human cells exposed to 55 MeV protons

Protons represent the major type of charged particle radiation in spaceflight environments. The purpose of this study was to assess mutations arising in human lymphoid cells exposed to protons. Mutations were quantitated at the thymidine kinase (TK1) locus in cell lines derived from the same donor: TK6 cells (wt TP53) and WTK1 cells (mutant TP53). WTK1 cells were much more susceptible to mutagenesis following proton exposure than TK6 cells. Intragenic deletions were observed among early-arising TK1 mutants in TK6 cells, but not in WTK1 cells where all of the mutants arose by LOH. Deletion was the predominant mode of LOH in TK6 cells, while allelic recombination was the major mode of LOH in WTK1 cells. Deletions were of variable lengths, from <1 cM to 64 cM, while mutations that arose by allelic recombination often extended to the telomere. In summary, proton exposures elicited many types of mutations at an autosomal locus in human cells. Most involved large scale loss of genetic information, either through deletion or by recombination.

NASA Discipline Radiation Health↗