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Toxic levels of phenylalanine in blood and brain is a characteristic of (untreated) phenylketonuria (PKU), leading to cognitive deficits in PKU mice. In addition, our recent findings showed that PKU mice (as well as PKU patients) have a disturbed sleep/wake cycle. As a consequence, sleep loss may contribute to cognitive deficits in PKU. Sleep loss has been linked to increased activation of microglia in the hippocampus. In this study, we set out to examine morphological features of the microglia population in the hippocampus of the mouse PKU model, using both the C57Bl/6 and the BTBR strain and their wild-type controls (age 5.3 ± 0.5 months; n = 16 per group, both males and females; n = 8 each). Microglial activation is reflected by retraction and thickening of the dendritic branches and an increase in cell body size of a microglial cell. Such morphological changes of microglia were studied by way of immunohistochemical staining for Iba-1, a microglia-specific calcium binding protein. We measured the number of microglia in seven subregions of the dorsal hippocampus. The level of microglial activation was determined, based on the ratio between the soma size and total cell size (soma size plus the area covered by the dendritic branches). Results showed subtle but statistical significant activation of hippocampal microglia in the C57Bl6, but not in the BTBR, PKU mice when compared with their wild-type controls. Also the total number of microglia was higher in the C57Bl/6 PKU (compared to the wild-type) mouse, but not in the BTBR PKU mouse. It is concluded that the C57Bl/6 PKU mouse has mildly higher microglia activity, which may support rather than hamper hippocampal homeostasis. The results further indicate that high levels of phenylalanine or disturbed sleep patterns do not consequently cause hippocampal microglial activation in the PKU mouse. It is currently unknown why the two PKU mouse strains show these differences in number and activation level of their hippocampal microglia, and to what extent it influences hippocampal functioning. Further scrutinizing the role of microglia functioning in the context of PKU is therefore warranted.

Toxic levels of phenylalanine in blood and brain is a characteristic of (untreated) phenylketonuria (PKU), leading to cognitive deficits in PKU mice. In addition, our recent findings showed that PKU mice (as well as PKU patients) have a disturbed sleep/wake cycle. As a consequence, sleep loss may contribute to cognitive deficits in PKU. Sleep loss has been linked to increased activation of microglia in the hippocampus. In this study, we set out to examine morphological features of the microglia population in the hippocampus of the mouse PKU model, using both the C57Bl/6 and the BTBR strain and their wild-type controls (age 5.3 ± 0.5 months; n = 16 per group, both males and females; n = 8 each). Microglial activation is reflected by retraction and thickening of the dendritic branches and an increase in cell body size of a microglial cell. Such morphological changes of microglia were studied by way of immunohistochemical staining for Iba-1, a microglia-specific calcium binding protein. We measured the number of microglia in seven subregions of the dorsal hippocampus. The level of microglial activation was determined, based on the ratio between the soma size and total cell size (soma size plus the area covered by the dendritic branches). Results showed subtle but statistical significant activation of hippocampal microglia in the C57Bl6, but not in the BTBR, PKU mice when compared with their wild-type controls. Also the total number of microglia was higher in the C57Bl/6 PKU (compared to the wild-type) mouse, but not in the BTBR PKU mouse. It is concluded that the C57Bl/6 PKU mouse has mildly higher microglia activity, which may support rather than hamper hippocampal homeostasis. The results further indicate that high levels of phenylalanine or disturbed sleep patterns do not consequently cause hippocampal microglial activation in the PKU mouse. It is currently unknown why the two PKU mouse strains show these differences in number and activation level of their hippocampal microglia, and to what extent it influences hippocampal functioning. Further scrutinizing the role of microglia functioning in the context of PKU is therefore warranted.

INTRODUCTION In 20-25% of the pediatric B cell precursor acute lymphoblastic leukemia (BCP-ALL) patients, the driving cytogenetic aberration is unknown. It is important to identify more primary lesions in this remaining B-other group to provide better risk stratification and identify possible treatment options. In this study, we aimed to identify novel recurrent fusion genes in BCP-ALL through RNA sequencing. METHODS We used paired-end total RNA Illumina sequencing to detect fusion genes with STAR-fusion and FusionCatcher in a population-based ALL cohort (n=71). We used Affymetrix U133 Plus2 expression arrays in a larger population-based ALL cohort (n=661) and an infant ALL cohort (n=70) to compare gene expression levels. Fluorescent in situ hybridization (FISH) was performed using Cytocell NUTM1 break-apart probe set MPH4800. RESULTS We identified an in-frame SLC12A6-NUTM1 fusion transcript composed of exons 1-2 of SLC12A6 fused to exons 3 to 8 of NUTM1 by RNA sequencing. Both genes are located on 15q14 within 5.3 Kb distance on opposite strands, and the fusion could result from an inversion. The fusion transcript is predicted to encode almost the total NUTM1 protein including the acidic binding domain for the histone acetyltransferase EP300. The SLC12A6-NUTM1 fusion case showed high NUTM1 expression, while NUTM1 expression was absent in the remaining cases. Using gene expression profiling, we identified four additional pediatric and two non-KMT2A-rearranged infant BCP-ALL cases with high NUTM1 expression. In the population-based cohort reflecting all different cytogenetic subtypes, these cases were restricted to the B-other group without known sentinel cytogenetic abnormalities. FISH showed a NUTM1 break apart pattern in all four tested NUTM1-positive cases indicative of a balanced translocation. RNA sequencing confirmed an ACIN1-NUTM1 fusion in one of the infant cases. We conclude that NUTM1 is normally not expressed in leukemic lymphoblasts, and that its expression can be induced by a gene fusion. The karyotypes of the predicted NUTM1 fusion cases combined with RNA sequencing data suggest that different chromosomal rearrangements are involved, likely resulting in different NUTM1 fusion partners. In literature, BRD9-NUTM1, IKZF1-NUTM1, and CUX1-NUTM1 fusions were reported in pediatric B-other cases, and BRD9-NUTM1 and ACIN1-NUTM1 fusions were reported in non-KMT2A-rearranged infants. Our combined aberrant gene expression and FISH results indicate that NUTM1 fusions occur in 2.4% (5/210) of pediatric and in 28% (2/7) of infant BCP-ALL cases without a sentinel cytogenetic aberration. The recurrence of NUTM1 aberrations in BCP-ALL cases without a known driver and the resulting expression of NUTM1 suggests that this fusion could be a new oncogenic driver in leukemia. All seven patients with a NUTM1 fusion achieved continuous complete remission with a median follow-up time of 8.3 years (range 4.8-13.8 years), suggesting that NUTM1 fusions in BCP-ALL have a favorable prognosis. To get an insight in the underlying biology, we compared gene expression between NUTM1-positive and NUTM1-negative pediatric B-other cases. We identified 130 differentially expressed probe sets (FDR ≤0.01) with a peculiar enrichment of those located on chromosome band 10p12.31 (Bonferroni adjusted p=4.05E-04). The genes in cytoband 10p12.31, including BMI1, were variably upregulated in 6/7 NUTM1-positive cases and positively correlated to NUTM1 expression levels. The NUTM1 protein is capable of binding and hereby stimulating the histone acetyltransferase activity of the EP300 protein. The EP300 protein preferentially binds a risk allele of BMI1 associated with increased risk for BCP-ALL. The BMI1 protein has been shown to convert BCR-ABL1-positive progenitor cells into BCR-ABL1-positive BCP-ALL cells. Hence, we postulate that NUTM1 fusion proteins contribute to leukemogenesis by stimulating EP300, leading to upregulation of BMI1 and other 10p12.31 genes in BCP-ALL. CONCLUSION NUTM1 fusions are a rare but recurrent event in BCP-ALL that seems to have a good prognosis. The NUTM1 fusions result in expression of the normally silent NUTM1 gene and are associated with upregulation of a cluster of genes on 10p12.31 including the leukemogenic BMI1 gene. Disclosures No relevant conflicts of interest to declare.