A Genomically and Clinically Annotated Patient-Derived Xenograft Resource for Preclinical Research in Non-Small Cell Lung Cancer

Patient-derived xenograft (PDX) models are an effective preclinical in vivo platform for testing the efficacy of novel drugs and drug combinations for cancer therapeutics. Here we describe a repository of 79 genomically and clinically annotated lung cancer PDXs available from The Jackson Laboratory that have been extensively characterized for histopathologic features, mutational profiles, gene expression, and copy-number aberrations. Most of the PDXs are models of non-small cell lung cancer (NSCLC), including 37 lung adenocarcinoma (LUAD) and 33 lung squamous cell carcinoma (LUSC) models. Other lung cancer models in the repository include four small cell carcinomas, two large cell neuroendocrine carcinomas, two adenosquamous carcinomas, and one pleomorphic carcinoma. Models with both de novo and acquired resistance to targeted therapies with tyrosine kinase inhibitors are available in the collection. The genomic profiles of the LUAD and LUSC PDX models are consistent with those observed in patient tumors from The Cancer Genome Atlas and previously characterized gene expression-based molecular subtypes. Clinically relevant mutations identified in the original patient tumors were confirmed in engrafted PDX tumors. Treatment studies performed in a subset of the models recapitulated the responses expected on the basis of the observed genomic profiles. These models therefore serve as a valuable preclinical platform for translational cancer research.

SIGNIFICANCE

Patient-derived xenografts of lung cancer retain key features observed in the originating patient tumors and show expected responses to treatment with standard-of-care agents, providing experimentally tractable and reproducible models for preclinical investigations.

Genomic data analysis workflows for tumors from patient-derived xenografts (PDXs): challenges and guidelines

BACKGROUND

Patient-derived xenograft (PDX) models are in vivo models of human cancer that have been used for translational cancer research and therapy selection for individual patients. The Jackson Laboratory (JAX) PDX resource comprises 455 models originating from 34 different primary sites (as of 05/08/2019). The models undergo rigorous quality control and are genomically characterized to identify somatic mutations, copy number alterations, and transcriptional profiles. Bioinformatics workflows for analyzing genomic data obtained from human tumors engrafted in a mouse host (i.e., Patient-Derived Xenografts; PDXs) must address challenges such as discriminating between mouse and human sequence reads and accurately identifying somatic mutations and copy number alterations when paired non-tumor DNA from the patient is not available for comparison.

RESULTS

We report here data analysis workflows and guidelines that address these challenges and achieve reliable identification of somatic mutations, copy number alterations, and transcriptomic profiles of tumors from PDX models that lack genomic data from paired non-tumor tissue for comparison. Our workflows incorporate commonly used software and public databases but are tailored to address the specific challenges of PDX genomics data analysis through parameter tuning and customized data filters and result in improved accuracy for the detection of somatic alterations in PDX models. We also report a gene expression-based classifier that can identify EBV-transformed tumors. We validated our analytical approaches using data simulations and demonstrated the overall concordance of the genomic properties of xenograft tumors with data from primary human tumors in The Cancer Genome Atlas (TCGA).

CONCLUSIONS

The analysis workflows that we have developed to accurately predict somatic profiles of tumors from PDX models that lack normal tissue for comparison enable the identification of the key oncogenic genomic and expression signatures to support model selection and/or biomarker development in therapeutic studies. A reference implementation of our analysis recommendations is available at https://github.com/TheJacksonLaboratory/PDX-Analysis-Workflows .

Abstract 1075: Genomic data analysis workflows for tumors from patient-derived xenografts (PDXs): Challenges and guidelines

Patient-derived xenograft (PDX) models are in vivo models of human cancer that have been used for translational cancer research and therapy selection for individual patients. The Jackson Laboratory (JAX) PDX resource has over 450 models representing more than 20 different types of cancer. The models undergo rigorous quality control and are genomically characterized to identify somatic mutations, copy number alterations, and transcriptional profiles. Bioinformatics workflows for analyzing genomic data obtained from human tumors engrafted in a mouse host (i.e., Patient-Derived Xenografts; PDXs) must address challenges such as discriminating between mouse and human sequence reads and accurately identifying somatic mutations and copy number alterations when paired non-tumor DNA from the patient is not available for comparison.

Here we describe bioinformatics analysis workflows and guidelines (https://github.com/TheJacksonLaboratory/PDX-Analysis-Workflows) that we developed specifically for the analysis of genomic data generated from PDX tumors. Our workflows incorporate commonly used software and public databases but are tailored to address the specific challenges of PDX genomics data analysis through parameter tuning and customized data filters and result in improved accuracy for the detection of somatic alterations in PDX models. We also report a gene expression-based classifier that can identify EBV-transformed tumors. Finally, to demonstrate the effectiveness of our workflows, we show the overall concordance of the genomic and transcriptomic profiles of the PDX models in the JAX PDX resource with relevant tumor types from The Cancer Genome Atlas (TCGA).

Using the reliable results obtained from the PDX genomics data analysis, we are able to compare the patient tumor with different PDX passages, perform classification analysis to verify the annotations of PDX tumors, as well as associate genomic signatures of each PDX tumor with results from dosing studies.

Acknowledgements

The data analysis workflows reported in this publication were partially supported by the National Cancer Institute of the National Institutes of Health under Award Number P30CA034196. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The public portal for JAX PDX data is supported by R01CA089713.

Citation Format: Xing Yi Woo, Anuj Srivastava, Joel H. Graber, Vinod Yadav, Vishal Kumar Sarsani, Al Simons, Glen Beane, Stephen Grubb, Guruprasad Ananda, Grace Stafford, Jeffrey H. Chuang, Susan D. Airhart, R. Krishna Karuturi, Joshy George, Carol J. Bult. Genomic data analysis workflows for tumors from patient-derived xenografts (PDXs): Challenges and guidelines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1075.

Utilizing data from patient-derived xenograft mouse models of human tumors to inform clinical decision making in Molecular Tumor Boards (MTB) deliberations

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Background: Molecular Tumor Boards (MTB) are often the critical decision-making step in identifying genome-guided treatments for patients with difficult-to-treat cancers, e.g. BRAF mutated metastatic colon cancers. A common challenge for MTBs is prioritizing between two or more actionable variants in a tumor. A potential solution to this challenge is to incorporate drug response in Patient-derived xenografts (PDX) models into MTB deliberations. The goal of this study was to evaluate the feasibility of using PDX models to elucidate drug-effectiveness in BRAF-mutated cancer as an example of a common clinical scenario. Methods: We selected BRAF-mutated PDX models from the JAX PDX resource based on the presence of an activating BRAF mutation and a second actionable variant from the JAX Cancer Treatment Profile (CTP). Somatic mutation data from PDX tumors were presented to members of the Intermountain MTB. PDX models were then treated with drugs recommended by the MTB; outcomes based on tumor growth inhibition (TGI) were shared with the MTB. Results: Gene/variant targets, associated drugs for the 2nd mutation and responses are described in the table. The MTB members determined that PDX data presented in TGI format is helpful in MTB deliberations. Activity of BRAF-targeted therapy was expected while the low activity of olaparib in the BRCA1-mutated colon cancer model was unexpected. The MTB then discussed molecular mechanisms that contributed to these outcomes. Conclusions: The pilot study demonstrated that utilizing PDX drug response data as an additional molecular annotation for MTB deliberations is feasible. Future studies will further optimize this process. [Table: see text]

Factors that influence response classifications in chemotherapy treated patient-derived xenografts (PDX)

In this study, we investigated the impact of initial tumor volume, rate of tumor growth, cohort size, study duration, and data analysis method on chemotherapy treatment response classifications in patient-derived xenografts (PDXs). The analyses were conducted on cisplatin treatment response data for 70 PDX models representing ten cancer types with up to 28-day study duration and cohort sizes of 3-10 tumor-bearing mice. The results demonstrated that a 21-day dosing study using a cohort size of eight was necessary to reliably detect responsive models (i.e., tumor volume ratio of treated animals to control between 0.1 and 0.42)-independent of analysis method. A cohort of three tumor-bearing animals led to a reliable classification of models that were both highly responsive and highly nonresponsive to cisplatin (i.e., tumor volume ratio of treated animals to control animals less than 0.10). In our set of PDXs, we found that tumor growth rate in the control group impacted treatment response classification more than initial tumor volume. We repeated the study design factors using docetaxel treated PDXs with consistent results. Our results highlight the importance of defining endpoints for PDX dosing studies when deciding the size of cohorts to use in dosing studies and illustrate that response classifications for a study do not differ significantly across the commonly used analysis methods that are based on tumor volume changes in treatment versus control groups.

Humanized mice (humice) carrying patient-derived xenograft (PDX) as a platform to develop immunotherapy in bladder cancer (BCa)

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Background: Immunotherapy with anti-programmed cell death 1 (PD1) or PD ligand 1 (PD-L1) antibody has emerged as a promising therapeutic modality, but has a response rate of approximately 20% in BCa. There are various drawbacks associated with current animal models. The objective of this study is to establish and characterize humice carrying PDXs in which both the immune cells and BCa cells are derived from humans. Methods: NOD-scid IL2Rgammanull or NSG, mice received CD34+ hematopoietic progenitor cells (HPC) cells i.v. after whole body radiation. PDXs were established through direct implantation of human BCa clinical specimens into NSG mice. Immune cell subpopulations were analyzed through flow cytometry analysis. Humice carrying HLA-unmatched PDXs were treated with an anti-PD1 antibody pemborlizumab (pembro) or in combination with a BKT/ITK inhibitor ibrutinib to determine the anti-tumor efficacy and toxicity. Results: PDXs retained the morphology fidelity and 92-97% of genetic alterations of parental patient cancers. Of the first 8 PDXs tested, 3 had high PD-L1 ( > 10 FPKM) as determined by RNA-seq which was further confirmed with flow cytometry analysis. Major human immune cell sub-populations were reconstituted in humice. No xenograft versus host disease was observed before pembro treatment. In humice with HPC donor 6466, pembro significantly inhibited tumor growth (p = 0.0016 at Day 29) of PDX BL293, but had no effect in another PDX BL440 with the same HPC donor 6466, or with the same PDX BL293 but with a different HPC donor 912. In another set of humice (HPC donor 710) carrying PDX BL293, pembro alone inhibited tumor growth. However, addition of ibrutinib did not potentiate the efficacy of pembro, but increased toxicity. Tumor regression with pembro treatment was associated with decrease of CD4+PD1+, CD8+PD1+ cells at peripheral blood and increased CD45+ and CD8+ cells in PDXs. Conclusions: Humice carrying PDXs reconstitute with human immune system, and can potentially be used to screen for effective immunotherapeutic agents or combinations, and to study resistant mechanisms.

Abstract P6-06-02: Molecular characterization of a patient-derived xenograft (PDX) resource for triple negative breast cancer

The Jackson Laboratory (JAX) has developed a resource of human tumors implanted into immune deficient mice (patient derived xenografts; PDX) as a platform for testing standard of care and novel therapeutic options for Triple Negative Breast Cancer. PDX models provide an advantage over cell culture based models for testing therapeutic interventions because they retain properties such as tumor cell heterogeneity that are critical to the biological properties of a patient’s tumor and response to treatment.

Tumor material acquired from biopsy or surgical resection was implanted subcutaneously into the flank of immune deficient NOD-scid IL2r gamma-chain null (NSG) mice. The PDX resource currently contains 21 established breast cancer PDX models (12 TNBC) with 24 additional models currently in development. Two of the established TNBC PDX models have BRCA1 mutations. The median age of the patients from whom tumor material was obtained for all breast models is 53 (45-89).

Tumors that successfully engrafted were characterized for somatic mutations using the new JAX Clinical Cancer Panel, Copy Number Variants using the Affymetrix human 6.0 SNP array, and gene expression using both Affymetrix U133 plus v2 and RNA-Seq. Normalized gene expression was analyzed for characteristic patterns in a pan-cancer approach across all PDX models and further compared with the previously identified TNBC molecular subtypes (Lehmann et al. 2011. JCI 121:2750-2767). The combination of principal components analysis and classification via expression pattern resulted in putative matches of models to most of the known molecularly defined subtypes of TNBC tumors.

Tumor bearing mice for the TNBC PDX models have been treated with docetaxel, cisplatin, cyclophosphamide and doxorubicin. Preliminary studies of tumor response to these treatment regimes revealed systematic differences that can be correlated with features of the genomic analysis, including expression subtype characterization.

The JAX collection of TNBC cancer PDX models is a well-annotated, publically available resource of models with deep genomic characterization and standard of care therapy response data for use in the development of advanced therapeutic options. Genomically defined subgroups within the collection suggest strategies to refine patient selection and treatment algorithms. Information about the models along with summarized genomic data is publicly available at the Mouse Tumor Biology database PDX web portal (http://tumor.informatics.jax.org).

Citation Format: Joel H Graber, James G Keck, Susan D Airhart, Carol J Bult, Edison T Liu. Molecular characterization of a patient-derived xenograft (PDX) resource for triple negative breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-06-02.

Cancer-specific nanotheranostics to improve the diagnosis and treatment of bladder cancer

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Background: We recently developed a bladder cancer-specific targeting ligand named PLZ4, nanometer-scale micelles and nanoporphyrin. Here we report the diagnostic and therapeutic applications of these nanotheranostics coated with PLZ4. Methods: PLZ4 was synthesized through solid phase synthesis. Bladder cancer-specific PLZ4-coated nanomicelles (PNM) and nanoporphyrin (PNP) were developed through conjugating the nanotheranostics with PLZ4 on the surface, and loaded with therapeutic and/or imaging agents in the core. Bladder cancer cell lines and patient-derived xenografts (PDXs) were used to determine the diagnostic and therapeutic applications. Results: In vitro studies with cell lines revealed that both PNM and PNP could specifically deliver the drug load to bladder cancer cells, but not to adjacent confounding cells. After intravenous injection, PNM loaded with paclitaxel could specifically deliver the drug load to xenografts developed from a human and a dog bladder cancer cell line, and a PDX, but not to lung cancer xenografts in the same mice. These paclitaxel-loaded PNM could overcome cisplatin resistance, and prolong the overall survival of mice carrying PDXs from 27 days with free paclitaxel to 76 days (p<0.0001). PNP can be used for photodynamic diagnosis and therapy while being able to chelate gadolinium for cancer-specific magnetic resonance imaging (MRI), and load chemotherapeutic drugs for cancer-specific targeted chemotherapy. It is over 50 times more potent that 5-aminolevulinic acid in photodynamic therapy (p<0.0001). After intravesical instillation into the bladder cavity of an orthotopic PDX model, PNP could specifically target bladder cancer cells, but not adjacent normal urothelial cells in the same bladder. Conclusions: PNM and PNP can potentially be used for diagnosis, imaging detection and cancer-specific targeted delivery of therapeutic agents of both non-myoinvasive and advanced bladder cancer. A phase I clinical trial of PNM for intravesical instillation in human patients with non-myoinvasive bladder cancer, and another phase I trial with PNP for photodynamic diagnosis and therapy in dog bladder cancer patients have been funded.

Patient-derived xenograft platform to guide precision medicine in bladder cancer

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Background: The prognosis for bladder cancer has not changed in 30 years. No targeted agents have been approved even though reproducible genetic abnormalities have been identified. The goal of this project was to develop and characterize a patient-derived xenograft (PDX) platform to determine the efficacy of molecularly guided targeted and chemotherapy therapy, drug re-purpose, study resistance mechanisms, and design novel therapy to overcome resistance. Methods: PDXs were developed from direct implantation of uncultured patient bladder cancer specimens into immunodeficient NSG mice. Deep sequencing in combination with computational biology was performed to characterize PDXs and identify druggable genetic aberrations that guided efficacy screening and mechanistic studies. Results: Nineteen PDXs have been established with annotated clinical information. PDXs retained morphology and 92-97% genetic aberrations of parental patient cancers. Deep sequencing revealed multiple druggable genetic aberrations, including the fibroblast growth factor receptor 3 (FGFR3) and other tyrosine kinase receptor pathways. Compared to the progression-free survival (PFS) of 9.5 days in the control arm, matched therapy with an FGFR3 inhibitor BGJ398 prolonged PFS to 18.5 days (p=2.61 X 10-6) in PDXs overexpressing FGFR3. Serial biopsies during treatment revealed reactivation of the downstream pathways coincided with development of resistance while targeting these downstream effectors reversed resistance (12 vs. 22 days, p=0.001). Efficacy studies also revealed that PDXs had differential response to chemotherapeutic drugs that could potentially guide selection of chemotherapeutic drugs for first- and second-line therapies. To determine the clinical applicability of non-myoinvasive bladder cancer, we further developed an orthotopic PDX model that mimiced disease progression to invasive and metastatic bladder cancer. Conclusions: The PDX platform allows screening for multiple targeted therapy, chemotherapy or combinations simultaneously for the most efficacious drugs or combination, and serial biopsies during treatment to study drug resistance, a task not possibly replicable at the clinical setting.

Caffeine enhances fragile (X) expression in somatic cell hybrids

Yunis and Soreng recently demonstrated enhanced expression of common fragile sites as and of fra(X) when 2.2 mM caffeine is added to FUdR treated lymphocyte cultures 6 hours before harvest. We failed to replicate this finding for fra(X) expression in lymphocytes. However, we do find a consistent increase in expression levels in somatic cell hybrids between a Chinese hamster cell line and 3 unrelated individuals with the fra(X) mutation when caffeine is present for the last 2 hours before harvest. This was particularly true for "low-expressing" cell lines, in which a 4-6 fold increase was observed. Using a thymidylate synthase deficient hybrid which could be blocked in the S phase by thymidine deprivation, we found that caffeine significantly reduced the recovery time from thymidine release to mitosis. This produced the highest level of fra(X) expression (48%) only one hour after release from thymidine deprivation. These results show that caffeine does enhance fra(X) expression in at least some cell types. The effect is probably indirect, inhibiting the mitotic delay usually associated with DNA damage.

Somatic cell hybrid studies of fragile (X) expression in a carrier female and transmitting male

We have extended our previous studies of fra(X) expression in somatic cell hybrids to the analysis of a carrier female with a low level of expression and to an unaffected, transmitting male who shows no expression in lymphocytes or lymphoblasts. Optimum conditions for fra(X) expression was treatment with 10(-8) M FUdR for 16 hours. In recent experiments, addition of 2.2 mM caffeine 2 hours before harvest was found to increase expression consistently. Two clones from the carrier female containing the fra(X) chromosome but not the normal X showed expression of 2-4%, indicating that expression in heterozygous females is not influenced by the presence or absence of the normal X. Expression rate was increased to 20% by exposure to FUdR plus caffeine. Analysis of hybrids containing only the fra(X) in an inactive state, and after reactivation by 5-azacytidine, showed no change in the frequency of expression. A hybrid clone from the nonexpressing, transmitting male containing only the X and chromosome 13, showed expression ranging from 2% without caffeine to 12% with caffeine in three different experiments. The ability to induce fra(X) expression in hybrid from this nonexpressing male may be explained in one of several ways: 1) a second mutation has occurred, 2) an autosomal suppressor locus was lost, or 3) the hamster genome or unusually short cell cycle lowers the threshold for expression, particularly in the presence of caffeine.

Recombination and amplification of pyrimidine-rich sequences may be responsible for initiation and progression of the Xq27 fragile site: an hypothesis

HYPOTHESIS

A pyrimidine-rich sequence (PRS) of DNA is present as a normal sequence in the q27 band of the human X chromosome. Under conditions of pyrimidine nucleotide triphosphate deprivation during S phase, deoxyuridine monophosphate is misincorporated and has to be excised during G2 by DNA repair mechanisms. When a simple PRS is present on both homologous X chromosomes during oogenesis, PRS may undergo amplification through non-homologous crossing-over to produce the initial lesion of the fragile (X). Carriers of such initial lesions will be unaffected transmitting females or males. When an X chromosome bearing such an initial lesion is itself paired with a homologous X carrying a simple PRS during oogenesis, a much higher rate of non-homologous crossing-over may occur resulting in progression to an even longer stretch of pyrimidine rich DNA in this region; the increased length of PRS through amplification makes the region too long to be repaired during G2 and allows it to be seen as a fragile site in metaphase chromosome preparations. Furthermore, this amplified lesion may interfere with transcription of one or more genes in this region and produce the phenotype of the Martin-Bell syndrome.

A rodent-human hybrid containing Xq24-qter translocated to a hamster chromosome expresses the Xq27 folate-sensitive fragile site

A somatic cell hybrid containing a single human X chromosome bearing the Xq27 fragile site was lethally irradiated and re-hybridized to its HPRT- Chinese hamster parent. One of 24 colonies surviving selection for HPRT was found to have retained human G6PD but not PGK. This line, X3000-11, which shows Xq24-qter translocated to a hamster chromosome by trypsin G-banding and a single human chromatin fragment corresponding to this segment of the X by G-11 staining, expresses the fragile site on exposure to 5-fluorodeoxyuridine. Dot blots using total human DNA suggest that X3000-11 retains approximately 0.2% of the human genome. By Southern blotting, X3000-11 retains Factor IX, DXS11 and DXS42 but lacks DXYS1, DXS3 and DXS17. This hybrid is being used to construct a cosmid library in the vector pCOS2 from which a sub-library of 500-1000 clones of human origin will be isolated using in vivo recombination with cloned Alu and Kpn family repeats. Such a sub-library will greatly facilitate chromosome walking to the fragile site as well as the testing of individual clones for their ability to create a folate-sensitive fragile site by DNA transfer into permissive Chinese hamster recipient cells.

Thymidylate synthase-deficient Chinese hamster cells: a selection system for human chromosome 18 and experimental system for the study of thymidylate synthase regulation and fragile X expression

Chinese hamster lung (CHL) V79 cells already deficient in hypoxanthine phosphoribosyltransferase were exposed to uv light and selected for mutations causing deficiency of thymidylate synthase (TS) by their resistance to aminopterin in the presence of thymidine and limiting amounts of methyl tetrahydrofolate. Three of seven colonies chosen for initial study were shown to be thymidylate synthase deficient (TS-) by enzyme assay, thymidine auxotrophy, and their inability to incorporate labeled deoxyuridine into their DNA in vivo. Complementation analysis of human X TS- hamster hybrids revealed that TS activity segregated with human chromosome 18. Southern analysis of a panel of 14 human X hamster hybrids probed with complementary DNA from mouse TS confirmed the chromosome assignment of TS to human chromosome 18; quantitative Southern blotting using unbalanced human cell lines further localized the gene to 18q21.31----qter. Another hybrid was generated that contained a human X chromosome with the Xq28 folate-dependent fragile site as its only human chromosome in a hamster TS- background. The fragile site could be easily and reproducibly expressed in this hybrid without the use of antimetabolites simply by removing exogenous thymidine from the medium. These TS-deficient cells are useful for: somatic cell genetics as a unique selectable marker for human chromosome 18, studies on regulation of the TS gene, and analysis of the fragile (X) chromosome and other folate-dependent fragile sites.

Fragile (X) expression induced by FUdR is transient and inversely related to levels of thymidylate synthase activity

Thymidylate synthase (TS) activity was monitored in fluorodeoxyuridine (FUdR)-treated lymphoblasts from individuals carrying the fragile (X) [fra(X)] chromosome. Fra(X) expression and levels of TS activity were measured over a 72-hr period at different cell densities. TS activity was 80%-90% inhibited immediately after exposure to FUdR and remained suppressed for the first 24 hrs. Fra(X) expression was not found until 6-8 hrs after FUdR treatment, and at 24 hrs, reached a maximum expression of approximately 50%. At 48 and 72 hrs, however, increasing levels of TS activity paralleled a dramatic drop in fra(X) expression. High fra(X) expression at 48 and 72 hrs could be maintained by rechallenging cultures with increasing doses of FUdR. At low cell densities, fra(X) expression was maintained at high levels for a much longer period of time. In two lymphoblastoid cell lines from obligate carriers, which either expressed at very low levels or did not express the fra(X) in routine cultures, TS activity was also 90% inhibited but with no corresponding fra(X) expression 12 or 24 hrs after FUdR treatment. We conclude that: FUdR inhibits TS activity immediately and induces fra(X) expression 6-8 hrs later, FUdR-induced fra(X) expression and TS activity are inversely related, the FUdR effect on fra(X) expression and TS activity is time and cell-density dependent, and inhibition of TS activity is a necessary but not sufficient condition for fra(X) expression.

New chromosomal syndrome: Miller-Dieker syndrome and monosomy 17p13

The Miller-Dieker Syndrome (MDS) consists of lissencephaly, characteristic facies, pre- and postnatal growth retardation, plus various other birth defects. Autosomal recessive inheritance has been presumed based on four reported families with two or more affected siblings. We present substantial evidence that monosomy 17p13.3 causes the MDS phenotype. This includes two patients with ring chromosome 17, one patient with a de novo 17p13 deletion, and one patient with monosomy 17p due to an unbalanced 7p; 17p translocation. We report the first prenatal diagnosis of MDS in a 20-week fetus from this latter family. Additionally, we report a balanced translocation between chromosome 17 and different autosomes (8, 12, and 15) in three of the four familial cases of lissencephaly. The finding of a chromosomal basis for this presumed autosomal recessive disorder significantly alters genetic counseling and makes prenatal diagnosis possible in some families.

Expression of the fragile (X) chromosome in an interspecific somatic cell hybrid

Interspecific somatic cell hybrids were constructed between a Chinese hamster lung cell line deficient in hypoxanthine phosphoribosyltransferase and two lymphoblastoid cultures (GM 4025 and GM 3200) from unrelated males affected with the fragile (X) syndrome. Thirteen independent colonies survived selection in hypoxanthine-azaserine, while only one colony survived selection in hypoxanthine-aminopterin-thymidine. One hybrid formed from GM 4025 was found to contain a human X chromosome as the only detectable human chromosome in the majority of cells analyzed. Induction of fragile (X) expression in this hybrid at frequencies up to 20% was achieved by treatments with 5-fluoro-2'-deoxyuridine (5 X 10(-8) M or 1 X 10(-7) M) or methotrexate (5 X 10(-6) or 1 X 10(-5) for 12 h. Use of the somatic cell hybrid system may allow study of the fragile (X) from different patients on a homogeneous xenogeneic background and may provide a better system for characterization of the fragile (X) at the biochemical and molecular level.

Chromosome 15 abnormalities and the Prader-Willi syndrome: a follow-up report of 40 cases

High-resolution chromosome analysis and multiple banding techniques were performed on blood samples from 40 patients with Prader-Willi syndrome (PWS) as a follow-up to our recent report in which we found interstitial deletions of 15q in four of five patients with this syndrome. Of the 40 new patients, 19 had interstitial del(15q), one had an apparently balanced 15;15 translocation, and one was mos46,XX/47,XX+idic(15) (pter leads to q11::q11 leads to pter). These data confirm our previous report and demonstrate that half of all patients with the clinical diagnosis of PWS have chromosome abnormalities involving chromosome 15 detectable by high-resolution methods. Although the majority of these involve a specific deletion of bands 15q11-q12, other alterations of chromosome 15 may be present.