Longitudinal profiling identifies co-occurring BRCA1/2 reversions, TP53BP1, RIF1 and PAXIP1 mutations in PARP inhibitor-resistant advanced breast cancer

BACKGROUND

Resistance to therapies that target homologous recombination deficiency (HRD) in breast cancer limits their overall effectiveness. Multiple, preclinically validated, mechanisms of resistance have been proposed, but their existence and relative frequency in clinical disease are unclear, as is how to target resistance.

PATIENTS AND METHODS

Longitudinal mutation and methylation profiling of circulating tumour (ct)DNA was carried out in 47 patients with metastatic BRCA1-, BRCA2- or PALB2-mutant breast cancer treated with HRD-targeted therapy who developed progressive disease-18 patients had primary resistance and 29 exhibited response followed by resistance. ctDNA isolated at multiple time points in the patient treatment course (before, on-treatment and at progression) was sequenced using a novel >750-gene intron/exon targeted sequencing panel. Where available, matched tumour biopsies were whole exome and RNA sequenced and also used to assess nuclear RAD51.

RESULTS

BRCA1/2 reversion mutations were present in 60% of patients and were the most prevalent form of resistance. In 10 cases, reversions were detected in ctDNA before clinical progression. Two new reversion-based mechanisms were identified: (i) intragenic BRCA1/2 deletions with intronic breakpoints; and (ii) intragenic BRCA1/2 secondary mutations that formed novel splice acceptor sites, the latter being confirmed by in vitro minigene reporter assays. When seen before commencing subsequent treatment, reversions were associated with significantly shorter time to progression. Tumours with reversions retained HRD mutational signatures but had functional homologous recombination based on RAD51 status. Although less frequent than reversions, nonreversion mechanisms [loss-of-function (LoF) mutations in TP53BP1, RIF1 or PAXIP1] were evident in patients with acquired resistance and occasionally coexisted with reversions, challenging the notion that singular resistance mechanisms emerge in each patient.

CONCLUSIONS

These observations map the prevalence of candidate drivers of resistance across time in a clinical setting, information with implications for clinical management and trial design in HRD breast cancers.

Familial rare EGFR-mutant lung cancer syndrome: Review of literature and description of R776H family

BACKGROUND

Interest in hereditary lung cancer is increasing, in particular germline mutations in the Epidermal Growth Factor Receptor (EGFR) gene. We review the current literature on this topic, discuss risk of developing lung cancer, treatment and screening options and describe a family of 3 sisters with lung cancer and their unaffected mother all with a rare EGFR germline mutation (EGFR p.R776H).

METHODS

We searched PubMed, Medline, Embase, the Cochrane Library, Google Scholar and scanned reference lists of articles. Search terms included "EGFR germline" and "familial lung cancer" or "EGFR familial lung cancer". We also describe our experience of managing a family with rare germline EGFR mutant lung cancer.

RESULTS

Although the numbers are small, the described cases in the literature show several similarities. The patients are younger and usually have no or light smoking history. 50% of the patients were treated with a tyrosine kinase inhibitor (TKIs) with OS over six months.

CONCLUSION

Although rare, germline p.R776H EGFR lung cancer mutations are over-represented in light or never smoking female patients who often also possess an additional somatic EGFR mutation. Treatment with TKIs appears suitable but further research is needed into the appropriate screening regime for unaffected carriers or light/never smokers.

Results of the c-TRAK TN trial: a clinical trial utilising ctDNA mutation tracking to detect molecular residual disease and trigger intervention in patients with moderate- and high-risk early-stage triple-negative breast cancer

BACKGROUND

Post-treatment detection of circulating tumour DNA (ctDNA) in early-stage triple-negative breast cancer (TNBC) patients predicts high risk of relapse. c-TRAK TN assessed the utility of prospective ctDNA surveillance in TNBC and the activity of pembrolizumab in patients with ctDNA detected [ctDNA positive (ctDNA+)].

PATIENTS AND METHODS

c-TRAK TN, a multicentre phase II trial, with integrated prospective ctDNA surveillance by digital PCR, enrolled patients with early-stage TNBC and residual disease following neoadjuvant chemotherapy, or stage II/III with adjuvant chemotherapy. ctDNA surveillance comprised three-monthly blood sampling to 12 months (18 months if samples were missed due to coronavirus disease), and ctDNA+ patients were randomised 2 : 1 to intervention : observation. ctDNA results were blinded unless patients were allocated to intervention, when staging scans were done and those free of recurrence were offered pembrolizumab. A protocol amendment (16 September 2020) closed the observation group; all subsequent ctDNA+ patients were allocated to intervention. Co-primary endpoints were (i) ctDNA detection rate and (ii) sustained ctDNA clearance rate on pembrolizumab (NCT03145961).

RESULTS

Two hundred and eight patients registered between 30 January 2018 and 06 December 2019, 185 had tumour sequenced, 171 (92.4%) had trackable mutations, and 161 entered ctDNA surveillance. Rate of ctDNA detection by 12 months was 27.3% (44/161, 95% confidence interval 20.6% to 34.9%). Seven patients relapsed without prior ctDNA detection. Forty-five patients entered the therapeutic component (intervention n = 31; observation n = 14; one observation patient was re-allocated to intervention following protocol amendment). Of patients allocated to intervention, 72% (23/32) had metastases on staging at the time of ctDNA+, and 4 patients declined pembrolizumab. Of the five patients who commenced pembrolizumab, none achieved sustained ctDNA clearance.

CONCLUSIONS

c-TRAK TN is the first prospective study to assess whether ctDNA assays have clinical utility in guiding therapy in TNBC. Patients had a high rate of metastatic disease on ctDNA detection. Findings have implications for future trial design, emphasising the importance of commencing ctDNA testing early, with more sensitive and/or frequent ctDNA testing regimes.

Circulating tumour DNA is a potential biomarker for disease progression and response to targeted therapy in advanced thyroid cancer

BACKGROUND

Conventional biomarkers in thyroid cancer are not disease specific and fluctuate in advanced disease, making interpretation difficult. Circulating tumour DNA (ctDNA) has been shown to be a useful biomarker in other solid tumours. This is a multimutational study of ctDNA over multiple timepoints, designed to test the hypothesis that ctDNA is a potential biomarker in patients with advanced thyroid cancer.

METHODS

Mutational analysis of archival tumour tissue was performed using NGS with a targeted gene panel. Custom TaqMan assays were designed for plasma ctDNA testing using digital droplet polymerase chain reaction. Concentrations of detected ctDNA were correlated with the conventional biomarker concentration and axial imaging status defined by the Response Evaluation Criteria in Solid Tumours criteria.

RESULTS

Tumour tissue from 51 patients was obtained, with the following histologies: 32 differentiated (differentiated thyroid cancer [DTC]), 15 medullary (medullary thyroid cancer [MTC]), three poorly differentiated and one anaplastic. NGS analysis detected variants in 42 (82%) of cases. Plasma was assayed for these patients in 190 samples, and ctDNA was detected in 67% of patients. Earlier detection of disease progression was noted in three patients with MTC. In two cases (PTC and ATC), where conventional biomarkers were not detectable, ctDNA was detected before disease progression. Changes in ctDNA concentration occurred earlier than conventional markers in response to disease progression in multiple patients with DTC receiving targeted therapies.

CONCLUSION

The majority of patients with advanced thyroid cancer had detectable ctDNA. ctDNA measurement may offer superiority over conventional markers in several scenarios: earlier detection of progression in MTC; as an alternative biomarker when conventional markers are not available; more rapid assessment of the disease status in response to targeted therapies, thereby potentially allowing prompter discontinuation of futile therapies. These early results support the hypothesis that ctDNA may be a clinically useful biomarker in thyroid cancer.

Abstract P2-02-17: Circulating tumor DNA analysis with ultra-high sensitivity sequencing in metastatic breast cancer

Introduction.

Circulating tumor DNA analysis has the potential to transform the clinical management of patients with breast cancer. We assessed the accuracy of ultra-high sensitivity ctDNA testing in patients with advanced breast cancer.

Methods.

From a prospective tissue collection study, we identified 25 patients with a contemporaneous metastatic tissue biopsy and plasma for ctDNA testing. Tumour DNA from the metastatic tissue biopsy was sequenced with a validated clinical hybrid capture panel, while plasma cell free DNA was sequenced with AVENIO ctDNA technology – a molecular barcoded duplex sequencing based on CAPPseq technology. Sample collection is on-going and results from the full concordance series will be presented at the conference.

Results.

Circulating tumour DNA was detectable in 87% (20/23) of patients, with at least one variant from tissue sequencing identified in plasma. There was overall high agreement between tissue and plasma sequencing. The sensitivity of plasma testing for variants identified in tumour, positive percent agreement, was 75% (24/32). Plasma testing revealed a diversity of sub-clonal mutations including polyclonal ESR1, polyclonal FGFR2 and FGFR3 mutations, rare KRAS mutations, and TSC1 and MSH2 inactivating mutations.

Conclusions.

Circulating tumour DNA testing with molecular barcoded duplex sequencing offers high sensitivity for tumour variant detection. The extent of sub-clonal resistance mutations identified emphasises the genetic diversity of advanced breast cancer.

Citation Format: Garcia-Murillas I, Proszek P, Fribbens C, Yuan L, Bye H, Hubank M, Jiang J, Yuang S, Palma J, Johnston S, Ring A, Turner N. Circulating tumor DNA analysis with ultra-high sensitivity sequencing in metastatic breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-02-17.

Eye on the B-ALL: B-cell receptor repertoires reveal persistence of numerous B-lymphoblastic leukemia subclones from diagnosis to relapse

The strongest predictor of relapse in B-cell acute lymphoblastic leukemia (B-ALL) is the level of persistence of tumor cells after initial therapy. The high mutation rate of the B-cell receptor (BCR) locus allows high-resolution tracking of the architecture, evolution and clonal dynamics of B-ALL. Using longitudinal BCR repertoire sequencing, we find that the BCR undergoes an unexpectedly high level of clonal diversification in B-ALL cells through both somatic hypermutation and secondary rearrangements, which can be used for tracking the subclonal composition of the disease and detect minimal residual disease with unprecedented sensitivity. We go on to investigate clonal dynamics of B-ALL using BCR phylogenetic analyses of paired diagnosis-relapse samples and find that large numbers of small leukemic subclones present at diagnosis re-emerge at relapse alongside a dominant clone. Our findings suggest that in all informative relapsed patients, the survival of large numbers of clonogenic cells beyond initial chemotherapy is a surrogate for inherent partial chemoresistance or inadequate therapy, providing an increased opportunity for subsequent emergence of fully resistant clones. These results frame early cytoreduction as an important determinant of long-term outcome.

Effective transplantation of photoreceptor precursor cells selected via cell surface antigen expression

Retinal degenerative diseases are a major cause of untreatable blindness. Stem cell therapy to replace lost photoreceptors represents a feasible future treatment. We previously demonstrated that postmitotic photoreceptor precursors expressing an NrlGFP transgene integrate into the diseased retina and restore some light sensitivity. As genetic modification of precursor cells derived from stem cell cultures is not desirable for therapy, we have tested cell selection strategies using fluorochrome-conjugated antibodies recognizing cell surface antigens to sort photoreceptor precursors. Microarray analysis of postnatal NrlGFP-expressing precursors identified four candidate genes encoding cell surface antigens (Nt5e, Prom1, Podxl, and Cd24a). To test the feasibility of using donor cells isolated using cell surface markers for retinal therapy, cells selected from developing retinae by fluorescence-activated cell sorting based on Cd24a expression (using CD24 antibody) and/or Nt5e expression (using CD73 antibody) were transplanted into the wild-type or Crb1(rd8/rd8) or Prph2(rd2/rd2) mouse eye. The CD73/CD24-sorted cells migrated into the outer nuclear layer, acquired the morphology of mature photoreceptors and expressed outer segment markers. They showed an 18-fold higher integration efficiency than that of unsorted cells and 2.3-fold higher than cells sorted based on a single genetic marker, NrlGFP, expression. These proof-of-principle studies show that transplantation competent photoreceptor precursor cells can be efficiently isolated from a heterogeneous mix of cells using cell surface antigens without loss of viability for the purpose of retinal stem cell therapy. Refinement of the selection of donorphotoreceptor precursor cells can increase the number of integrated photoreceptor cells,which is a prerequisite for the restoration of sight.

Tnfaip8 is an essential gene for the regulation of glucocorticoid-mediated apoptosis of thymocytes

Glucocorticoids have significant immunoregulatory actions on thymocytes and T cells and act by binding and activating cytosolic glucocorticoid receptors, which translocate to the nucleus and control gene expression through binding to specific response elements in target genes. Glucocorticoids promote cell death by activating an apoptotic program that requires transcriptional regulation. We set out to identify genes that are crucial to the process of glucocorticoid-mediated thymocyte apoptosis. Freshly isolated murine primary thymocytes were treated with dexamethasone, mRNA isolated and used to screen DNA microarrays. A set of candidate genes with upregulated expression was identified and selected members assayed in reconstituted fetal thymic organ culture (FTOC). Fetal liver-derived hematopoietic progenitor cells (HPCs) were infected with retroviruses expressing individual genes then used to repopulate depleted fetal thymic lobes. Reconstituted FTOCs expressing the gene Tnfaip8 were treated with dexamethasone and shown to be greatly sensitized to dexamethasone. Retrovirus-mediated RNA interference was applied to knock down Tnfaip8 expression in HPCs and these were used to reconstitute FTOCs. We observed that downregulating the expression of Tnfaip8 alone was sufficient to effectively protect thymocytes against glucocorticoid-induced apoptosis. We propose that Tnfaip8 is crucial in regulating glucocorticoid-mediated apoptosis of thymocytes.

rHVDM: an R package to predict the activity and targets of a transcription factor

SUMMARY

Highly parallel genomic platforms like microarrays often present researchers with long lists of differentially expressed genes but contain little or no information on how these genes are regulated. rHVDM is a novel R package which uses gene expression time course data to predict the activity and targets of a transcription factor. In the first step, rHVDM uses a small number of known targets to derive the activity profile of a given transcription factor. Then, in a subsequent step, this activity profile is used to predict other putative targets of that transcription factor. A dynamic and mechanistic model of gene expression is at the heart of the technique. Measurement error is taken into account during the process, which allows an objective assessment of the robustness of fit and, therefore, the quality of the predictions. The package relies on efficient algorithms and vectorization to accomplish potentially time consuming tasks including optimization and differential equation integration. We demonstrate the efficiency and accuracy of rHVDM by examining the activity of the tumour-suppressing transcription factor, p53.

AVAILABILITY

The version of the package presented here (1.8.1) is freely available from the Bioconductor Web site (http://bioconductor.org/packages/2.3/bioc/html/rHVDM.html).

Cardioprotection mediated by urocortin is dependent upon PKCε activation

Urocortin (Ucn) is an endogenous cardioprotective agent that protects against the damaging effects of ischemia and reperfusion injury in vitro and in vivo. We have found that the mechanism of action of Ucn involves both acute activation of specific target molecules, and using Affymetrix (Santa Clara, CA) gene chip technology, altered gene expression of different end effector molecules. Here, from our gene chip data, we show that after a 24 h exposure to Ucn, there was a specific increase in mRNA and protein levels of the protein kinase C epsilon (PKCepsilon) isozyme in primary rat cardiomyocytes compared with untreated cells and in the Langendorff perfused ex vivo heart. Furthermore, a short 10 min exposure of these cells to Ucn caused a specific translocation/activation of PKCepsilon in vitro and in the Langendorff perfused ex vivo heart. The importance of the PKCepsilon isozyme in cardioprotection and its relationship to cardioprotection produced by Ucn was assessed using PKCepsilon-specific inhibitor peptides. The inhibitor peptide, when introduced into cardiomyocytes, caused an increase in apoptotic cell death compared with control peptide after ischemia and reperfusion. When the inhibitor peptide was present with Ucn, the cardioprotective effect of Ucn was lost. This loss of cardioprotection by Ucn was also seen in whole hearts from PKCepsilon knockout mice. These findings indicate that the cardioprotective effect of Ucn is dependent upon PKCepsilon.

Reconstructing gene networks: what are the limits?

To fully realize the benefits of high-throughput post-genomic technologies it is necessary to reconstruct and analyse the complicated network of interactions through which most genes operate. We briefly summarize the mathematical frameworks that can be used to model such networks, and the types of algorithms available for their reconstruction. We then focus on dynamic models, typically described using differential equations, and explain the two main reconstruction approaches in current use. We discuss the data requirements of these algorithms and ask how well they correspond to current microarray data.

Urocortin protects cardiac myocytes from ischemia/reperfusion injury by attenuating calcium insensitive phospholipase A2gene expression

We have used Affymetrix gene chip technology to look for changes in gene expression caused by a 24 h exposure of rat primary neonatal cardiac myocytes to the cardioprotective agent urocortin. We observed a 2.5-fold down-regulation at both the mRNA and protein levels of a specific calcium-insensitive phospholipase A2 enzyme. Levels of lysophosphatidylcholine, a toxic metabolite of phospholipase A2, were lowered by 30% in myocytes treated with urocortin for 24 h and by 50% with the irreversible iPLA2 inhibitor bromoenol lactone compared with controls. Both 4 h ischemia and ischemia followed by 24 h reperfusion caused a significant increase in lysophosphatidylcholine concentration compared with controls. When these myocytes were pretreated with urocortin, the ischemia-induced increase in lysophosphatidylcholine concentration was significantly lowered. Moreover, co-incubation of cardiac myocytes with urocortin, or the specific phospholipase A2 inhibitor bromoenol lactone, reduces the cytotoxicity produced by lysophosphatidylcholine or ischemia/reperfusion. Similarly, in the intact heart ex vivo we found that cardiac damage measured by infarct size was significantly increased when lysophoshatidylcholine was applied during ischemia, compared with ischemia alone, and that pre-treatment with both urocortin and bromoenol lactone reversed the increase in infarct size. This, to our knowledge, is the first study linking the cardioprotective effect of urocortin to a decrease in a specific enzyme protein and a subsequent decrease in the concentration of its cardiotoxic metabolite.

K(ATP) channel gene expression is induced by urocortin and mediates its cardioprotective effect

BACKGROUND

Urocortin is a novel cardioprotective agent that can protect cardiac myocytes from the damaging effects of ischemia/reperfusion both in culture and in the intact heart and is effective when given at reperfusion.

METHODS AND RESULTS

We have analyzed global changes in gene expression in cardiac myocytes after urocortin treatment using gene chip technology. We report that urocortin specifically induces enhanced expression of the Kir 6.1 cardiac potassium channel subunit. On the basis of this finding, we showed that the cardioprotective effect of urocortin both in isolated cardiac cells and in the intact heart is specifically blocked by both generalized and mitochondrial-specific K(ATP) channel blockers, whereas the cardioprotective effect of cardiotrophin-1 is unaffected. Conversely, inhibiting the Kir 6.1 channel subunit greatly enhances cardiac cell death after ischemia.

CONCLUSIONS

This is, to our knowledge, the first report of the altered expression of a K(ATP) channel subunit induced by a cardioprotective agent and demonstrates that K(ATP) channel opening is essential for the effect of this novel cardioprotective agent.

Analysis of gene transcription in cells lacking DNA-PK activity

The DNA-dependent protein kinase (DNA-PK), comprised of the Ku70/Ku80 (now known as G22p1/Xrcc5) heterodimer and the catalytic subunit DNA-PKcs (now known as Prkdc), is required for the nonhomologous end joining (NHEJ) pathway of DNA double-strand break repair. The mechanism of action of DNA-PK remains unclear. We have investigated whether DNA-PK regulates gene transcription in vivo after DNA damage using the subtractive hybridization technique of cDNA representational difference analysis (cDNA RDA). Differential transcription, both radiation-dependent and independent, was detected and confirmed in primary mouse embryo fibroblasts from DNA-PKcs(-/-) and DNA-PKcs(+/+) mice. We present evidence that transcription of the extracellular matrix gene laminin alpha 4 (Lama4) is regulated by DNA-PK in a radiation-independent manner. However, screening of both primary and immortalized DNA-PKcs-deficient cell lines demonstrates that the majority of differences were not consistently dependent on DNA-PK status. Similar results were obtained in experiments using KU mutant hamster cell lines, indicating heterogeneity of transcription between closely related cell lines. Our results suggest that while DNA-PK may be involved in limited gene-specific transcription, it does not play a major role in the transcriptional response to DNA damage.

DNA-dependent protein kinase is not required for the p53-dependent response to DNA damage

Damage to DNA in the cell activates the tumour-suppressor protein p53, and failure of this activation leads to genetic instability and a predisposition to cancer. It is therefore crucial to understand the signal transduction mechanisms that connect DNA damage with p53 activation. The enzyme known as DNA-dependent protein kinase (DNA-PK) has been proposed to be an essential activator of p53, but the evidence for its involvement in this pathway is controversial. We now show that the p53 response is fully functional in primary mouse embryonic fibroblasts lacking DNA-PK: irradiation-induced DNA damage in these defective fibroblasts induces a normal response of p53 accumulation, phosphorylation of a p53 serine residue at position 15, nuclear localization and binding to DNA of p53. The upregulation of p53-target genes and cell-cycle arrest also occur normally. The DNA-PK-deficient cell line SCGR11 contains a homozygous mutation in the DNA-binding domain of p53, which may explain the defective response by p53 reported in this line. Our results indicate that DNA-PK activity is not required for cells to mount a p53-dependent response to DNA damage.

Identification of androgen-regulated genes in mouse kidney by representational difference analysis and random arbitrarily primed polymerase chain reaction

The molecular nature of tissue-specific gene regulation by androgens has not been well defined, partly as a result of the variable expression and incomplete regulation of currently available gene models. We have therefore aimed to establish more informative models by identifying alternative genes whose expression is tightly and coordinately regulated by androgens. Female C57BL/6 mice were dosed with dihydrotestosterone- or sham-treated for 8 days, after which kidneys were removed and complementary DNA (cDNA) prepared. We then applied the subtractive hybridization techniques of random arbitrarily primed-PCR and PCR-coupled subtractive hybridization method of cDNA representational difference analysis to the isolated cDNA. In addition to well characterized androgen-regulated genes [e.g. KAP (kidney androgen-regulated protein)], we demonstrate the differential expression of six genes previously not known to be under androgen control. RNA levels of SA, Cytochrome P450 4B1, IL-6ST (interleukin-6 signal transducer), OATP (organic anion transporter), and a newly identified gene, MJAM, were up-regulated by androgen, while 16-alpha-hydroxylase was decreased. Expression of these transcripts was inhibited in dihydrotestosterone-treated females by flutamide and in males by castration, confirming their dependence on androgens. Although all the genes demonstrate tissue-specific regulation by androgen, SA showed both kidney specificity and absolute requirement for androgen for its expression. These newly identified androgen-regulated genes will constitute very useful models for studying the nature of tissue-specific gene regulation by androgens.

Identifying differences in mRNA expression by representational difference analysis of cDNA

Detection of differentially regulated genes has been severely hampered by technical limitations. In an effort to overcome these problems, the PCR-coupled subtractive process of representational difference analysis (RDA) [Lisitsyn, N. et al. (1993) Science 259, 946-951] has been adapted for use with cDNA. In a model system, RAG-1 and RAG-2, the genes responsible for activating V(D)J recombination, were identified in a genomic transfectant by cDNA RDA in a small fraction of the time taken by conventional means. The system was also modified to eliminate expected difference products to facilitate the identification of novel genes. Additional alterations to the conditions allowed isolation of differentially expressed fragments. Several caffeine up-regulated clones were obtained from the pre-B cell line 1-8, including IGF-1B, and a predicted homologue of the natural killer cell antigen, NKR-P1. The approach was found to be fast, extremely sensitive, reproducible, and predominantly lacked false positives. cDNA RDA has the capacity and adaptability to be applied to a wide range of biological problems, including the study of single gene disorders, characterization of mutant and complemented cell types, developmental or post-event expression time courses, and examination of pathogen-host interactions.

Expression of the excision repair gene, ERCC3 (excision repair cross-complementing), during mouse development

Expression of the human ERCC3 (excision repair cross-complementing) gene in cells from patients with xeroderma pigmentosum (XP) group B (XP-B) corrects the defect in repair of UV light-induced DNA damage. XP-B is one of three groups of XP which exhibit the clinical symptoms of both XP and Cockayne's Syndrome (CS). CS and XP-B/CS patients develop severe neurological dysfunction during development. In order to explore the link between the defective gene and the neurological deficits in XP/CS, we have studied the expression of ERCC3 mRNA in developing mice by in situ hybridisation. ERCC3 was found to be ubiquitously expressed in cells from all regions and all developmental stages, from 9 day post-coitum embryo, to 15 day post-natal brain. In post-natal brain, regional differences in expression correlated with cell density and there was no evidence of cell specific or developmental alterations in levels of expression. These results indicate that the constitutively expressed gene does not perform a discrete developmental function. The neurological defects apparent in XP-B are likely to arise pleiotypically from the participation of ERCC3 in interactions with other elements involved in particular aspects of neurodevelopmental control. These results emphasise the developmental importance of genes whose primary functions are apparently unconnected with development.

Nuclear tri-iodothyronine (T3) binding in neonatal rat brain suggests a direct glial requirement for T3 during development

Tri-iodothyronine (T3) binding studies were performed on neuronal and glial nuclei prepared from developing rats brain by discontinuous sucrose gradient centrifugation. Maximum binding capacities (MBC) and dissociation constants (Kd) were obtained from Eadie-Hofstee plots of transformed data. An ontogenic study on nuclei prepared from whole brain revealed that on day 5 after birth, glial nuclear MBC was 1774 +/- 201 (S.E.M.) fmol/mg DNA compared with 974 +/- 117 fmol/mg DNA for the neurones (P less than 0.01). Although diminishing to 667 +/- 112 fmol/mg DNA by day 21, alterations in neuronal MBC over the neonatal period were not statistically significant, whereas glial MBC diminished steadily to 557 +/- 133 fmol/mg DNA in glial nuclei (P less than 0.05). Over the same period, a significant reduction in Kd was noted only in the glia, from 3.17 +/- 0.40 to 1.83 +/- 0.34 nmol/l (P less than 0.03). Ligand specificity of the receptor in both nuclear types on day 21 was tri-iodoacetic acid greater than T3 greater than thyroxine greater than 3,3',5'-T3, but this was less clearly demonstrated at day 5. Regional studies on days 15 and 21 demonstrated that for both neuronal and glial nuclei, receptors are concentrated in the cerebral cortex and diminish in a cranio-caudal direction. Cerebral glial MBC on day 21 was 2215 +/- 147 fmol/mg DNA, at this stage still exceeding the cerebral neuronal capacity of 1111 +/- 207 fmol/mg DNA. The results indicate that neonatal glia may respond directly to thyroid hormones via nuclear receptor binding, and that receptors are predominantly located in the cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of maternal hypothyroxinaemia during fetal life on the calmodulin-regulated phosphatase activity in the brain of the adult progeny in the rat

Calmodulin-regulated phosphatase activity was measured in the brain of 2-month-old rats born from hypothyroid and normal dams, using a fluorometric enzyme assay developed for this purpose. Calmodulin content was measured in the same brain regions by radioimmunoassay. Significant differences between groups in weight and protein content, basal phosphatase and calmodulin-regulated phosphatase activity were found. The brain region most affected was the cerebellum, where basal and calmodulin-regulated phosphatase activities, and protein content were increased. The data point towards a lasting effect of maternal hypothyroxinaemia on the brain function of the progeny.

The effect of 3,5,3'-triiodothyronine on leucine uptake and incorporation into protein in cultured neurons and subcellular fractions of rat central nervous system

Previous studies have shown that nuclear thyroid hormone receptors in rat brain are preferentially localized within neurons. These cells also synthesize protein at a high rate, and the aim of the present study was to investigate any relationship between these two characteristics. In this paper we have shown that T3 stimulates leucine uptake and incorporation into protein in primary cell cultures of neurons. Stimulation was apparent with concentrations of hormone as low as 1.25 nM and increased in a dose-dependent manner up to 10 nM T3. However, the rapidity of the effect (evident at 25 min, and significant at 40 min) suggests that protein synthesis is stimulated at the level of translation, rather than transcription. More detailed study with 5 nM T3, revealed that incorporation into both soluble (cytoplasmic) and insoluble (membrane-associated) protein fractions was stimulated to similar degrees, and therefore the effect on protein synthesis was general. Furthermore, T3-mediated stimulation of leucine uptake into neurons was completely abolished in the presence of the protein synthesis inhibitors, actinomycin D and cycloheximide, and therefore the effect on leucine uptake was attributed to an increased requirement for the amino acid in protein synthesis (pleiotrophic effect). Parallel studies conducted with synaptosomes and mitochondria isolated from the central nervous system of adult euthyroid animals revealed that 5 nM T3 was without effect on leucine uptake and incorporation into protein. Possible reasons for this lack of effect are discussed.

Differences in nuclear triiodothyronine binding in rat brain cells suggest phylogenetic specialization of neuronal functions

The central nervous system depends on thyroid hormones (TH) in regard to its development, maturation, and maintenance of normal functions. As there is much evidence to suggest that the effects of TH are mainly mediated through specific nuclear binding sites, we have studied the anatomical distribution of T3 nuclear receptors in different regions of adult rat brain, and the localization of receptors in the fractionated neuronal and glial nuclei of neocortex, paleocortex, and cerebellum. Purified nuclei from the various brain regions were prepared by ultracentrifugation in 2.2 M sucrose. Purified neuronal and glial fractions were obtained by discontinuous sucrose gradient centrifugation in 2.2 and 2.4 M sucrose. The washed nuclear fractions were used for T3 binding assay at 37 C for 30 min and the data analyzed by least squares nonlinear regression analysis. Nonfractionated nuclei from all regions studied were found to have similar dissociation constant (Kd) values (1.04-1.38 nM) and Eadie-Hofstee plots indicated the presence of an apparently ubiquitous single class of high affinity, low capacity binding sites. The increase in binding from cerebellum (54 +/- 24 fmol/mg DNA; mean +/- SE) to neocortex (666 +/- 89 fmol/mg DNA) showed a caudo-cranial pattern. In fractionated neuronal nuclei, the same trend was observed, only to a greater degree (1628 +/- 266, 994 +/- 76 and 212 +/- 29 fmol/mg DNA in neocortex, paleocortex, and cerebellum, respectively); the difference between corresponding values for glial nuclei of neocortex and paleocortex (357 +/- 139 and 250 +/- 92 fmol/mg DNA, respectively) was not statistically significant, and no specific T3 binding was found in cerebellar glial nuclei. These data suggest that TH may have an important role in neurons from phylogenetically newer regions, concerned with higher mental functions. The caudo-rostral distribution pattern may also indicate a gradient of TH actions in central nervous system regions.