Intellectual disability is common in the US (2% of school-aged children). Despite this, almost no treatment options are available for this heterogeneous group of disorders. In part, treatment development has been halted by the general notion that intellectual disability is not treatable. However, if one could find a subgroup of treatable causes of intellectual disability and demonstrate the ability to treat these in postnaal life, one would help break this general dogma. Preliminary data suggest that two Mendelian disorders of the histone machinery, Kabuki syndrome and Rubinstein-Taybi syndrome, might be treatable causes of intellectual disability in postnatal life. We have demonstrated that a mouse model of Kabuki syndrome has postnatal hippocampal memory defects and these deficiencies appear to improve when the mice are treated in postnatal life with a histone deacetylase inhibitor. Here we propose to create a tetracycline responsive conditional mouse model of Kabuki syndrome. By altering Mll2 gene expression in postnatal life we will test the hypothesis that the hippocampal memory defects we observed in Mll2 (+/-) mice relate to an ongoing postnatal imbalance between the systems that remove and add H3K4 trimethylation. This model will allow us to test the hypothesis that treatments at different time points in postnatal life are equally effective at treating the hippocampal memory defect found in these syndromes. Together, these experiments would help establish Kabuki syndrome as an additional treatable cause of intellectual disability. Since Kabuki syndrome and Rubinstein-Taybi both have hippocampal memory deficiency caused by an abnormality of the histone modification systems that is associated with open chromatin, we hypothesize that they have a shared mechanism.
We aim to test this hypothesis by: (a) determining whether both disorders have a deficiency of either biochemical marker of open chromatin (i.e. histone H3K4 trimethylation and histone acetylation); (b) exploring whether the double heterozygote state (Mll2+/- and Cbp+/-) increases the severity of the abnormalities found in Mll2 (+/-); (c) testing the hypothesis that the gene families found to be down-regulated in these two diseases cluster in several critical pathways shared by the two disorders. By understanding the pathogenesis of these disorders one might be able to identify additional treatable causes of intellectual disability, biomarkers of therapeutc efficiency, and novel therapeutic targets. In an effort to expand the repertoire of agents for treatment of these two disorders we have developed two reporter alleles that encode proteins that inform the activity of a particular histone modification systems: one that fluoresces in response to activity of the H3K4 trimethylation system (deficient in Kabuki syndrome), and one that fluoresces in response to activity of the histone acetylation system (deficient in Rubinstein-Taybi syndrome). We intend to use these alleles for the development of assays for high-throughput discovery of therapeutic agents using both patient cells and/or mouse embryonic stem cells from our mice.

Public Health Relevance

Intellectual disability is common in the US and there are few treatments available. Our pilot studies suggest that Kabuki syndrome, a rare Mendelian disorder of the histone machinery, might be a treatable cause of intellectual disability. We would like to verify this finding, explore the pathogenesis of this and related diseases and seek additional less toxic therapeutic agents.

Agency
National Institute of Health (NIH)
Institute
Office of The Director, National Institutes of Health (OD)
Type
Early Independence Award (DP5)
Project #
5DP5OD017877-05
Application #
9349380
Study Section
Special Emphasis Panel (ZRG1)
Program Officer
Basavappa, Ravi
Project Start
2013-09-21
Project End
2019-08-31
Budget Start
2017-09-01
Budget End
2019-08-31
Support Year
5
Fiscal Year
2017
Total Cost
Indirect Cost
Name
Johns Hopkins University
Department
Genetics
Type
Schools of Medicine
DUNS #
001910777
City
Baltimore
State
MD
Country
United States
Zip Code
21205
Pilarowski, Genay O; Vernon, Hilary J; Applegate, Carolyn D et al. (2018) Missense variants in the chromatin remodeler CHD1 are associated with neurodevelopmental disability. J Med Genet 55:561-566
Migeon, Barbara R; Beer, Michael A; Bjornsson, Hans T (2017) Embryonic loss of human females with partial trisomy 19 identifies region critical for the single active X. PLoS One 12:e0170403
Benjamin, Joel S; Pilarowski, Genay O; Carosso, Giovanni A et al. (2017) A ketogenic diet rescues hippocampal memory defects in a mouse model of Kabuki syndrome. Proc Natl Acad Sci U S A 114:125-130
Bowdin, Sarah; Gilbert, Adel; Bedoukian, Emma et al. (2016) Recommendations for the integration of genomics into clinical practice. Genet Med 18:1075-1084
Bjornsson, Hans Tomas (2015) The Mendelian disorders of the epigenetic machinery. Genome Res 25:1473-81
Bjornsson, Hans T; Benjamin, Joel S; Zhang, Li et al. (2014) Histone deacetylase inhibition rescues structural and functional brain deficits in a mouse model of Kabuki syndrome. Sci Transl Med 6:256ra135
Fahrner, Jill A; Bjornsson, Hans T (2014) Mendelian disorders of the epigenetic machinery: tipping the balance of chromatin states. Annu Rev Genomics Hum Genet 15:269-93
Weissman, Jacqueline; Naidu, Sakkubai; Bjornsson, Hans T (2014) Abnormalities of the DNA methylation mark and its machinery: an emerging cause of neurologic dysfunction. Semin Neurol 34:249-57