Treatment of immune thrombocytopenia with hetrombopag olamine tablets in a Kabuki syndrome patient with new KMT2D mutations

Abstract Kabuki syndrome (KS) is a rare multisystem-affecting genetic disorder, and usually accompanied with autoimmune disorders such as immune thrombocytopenic purpura (ITP). Here, we report a 16-year-old patient with Kabuki syndrome with ITP and observe the therapeutic effect of TPO agonist hetrombopag olamine tablets. The duration of maintenance therapy and follow up were both 17 months. Whole exon sequencing (WES) of the patient’s peripheral blood showed c.5775_5778del (p. Leu1926LysfsTer120) heterozygous mutation in the KMT2D gene, which was not reported before.


Introduction
Kabuki syndrome (KS; OMIM 147 920 and 300 867) is a group of rare diseases characterized by special facial features, skeletal dysplasia, abnormal skin texture and mental retardation, and abnormal gastrointestinal development.It is mostly accompanied by endocrine and immune dysfunction related autoimmune disease, especially immune thrombocytopenia (ITP).It is named by patient's physical characteristics that resemble the costumes of Japanese Kabuki actors.The syndrome was first reported in 1981 by two Japanese scholars, Niikawa and Kuroki. 1,2In this paper, a case of hetrombopag olamine tablets treated with ITP with a KS type I patient was reported, and a new mutation of the KMT2D gene c.5775_5778del (p.Leu1926LysfsTer120) was detected, which was a code-shifting variant that expanded the gene spectrum of KS mutations.

Case report
A 16-year-old male patient was admitted to the Department of the First Affiliated Hospital of the University of Science and Technology of China, with "Platelet reduced for more than 9 years, repeated gingival bleeding for 2 days" on 17 November 2021 (Platelet count:11 × 10 9 /L).The patient had previously visited the local hospital for gingival bleeding without obvious causes in 2013.The platelet count was 36 × 10 9 /L and was then transferred to the Department of Pediatrics of this hospital with platelet count decreased to 6.4 × 10 9 /L.He was clinically diagnosed with ITP and was administrated with dexamethasone for four consecutive days (20 mg/day) and then changed to prednisone acetate tablets 20 mg/day.Seven days later, the platelet count was 225 × 10 9 /L and he was discharged from the hospital with prednisone acetate tablets 20 mg per day as maintenance treatment.In July 2020, the patient had bleeding gums and platelet count was 6 × 10 9 /L, and he was hospitalized again.Bone marrow puncture suggested the impairment of megakaryocyte maturation and thrombocytopenia, so he was diagnosed with ITP.After treatment with dexamethasone and high-dose gamma globulin, his platelets rose to the normal range, and he was discharged.This time, the patient was hospitalized again due to severe thrombocytopenia.
After admission, the patient was given dexamethasone intravenous infusion and the patient was discharged from the hospital after dexamethasone therapy with platelet count of 34 × 10 9 /L and continued oral prednisone 10 mg Tid.However, the platelet count dropped to 20 × 10 9 /L, and admitted to the hospital again on 25 December 2021.Dexamethasone 20 mg intravenous infusion of d 1-4 was administered, and he was discharged when his platelet recovered to 29 × 10 9 /L.On 3 January 2022, the platelet count was normal (166 × 10 9 /L).This time hetrombopag olamine tablets 2.5 mg per day were given as maintenance treatment.Three weeks later, the platelet count increased to 329 × 10 9 /L.During the adjustment of the hetrombopag dose, once the number of platelets is below 20 × 10 9 /L, we will give dexamethasone 20 mg d 1-4 intravenously.After regular follow-up and follow-up of outpatients at intervals every 1-2 weeks, platelets were basically maintained at more than 100 × 10 9 /  L, and the last time was 8 June 2023, continued to give hetrombopag olamine tablets 2.5 mg qd.orally (Figure 3).The duration of maintenance therapy and follow up were both 17 months.

Discussion
Kabuki syndrome is a rare genetic disorder with multisystem effects.Genetically, KMT2D gene mutations cause KS disease, accounting for 75% of all patients, are autosomal dominant, 5% of cases are caused by KDM6A mutations, are X-linked dominant, and about 20% of cases have unknown etiology.The main clinical manifestations of the syndrome are postnatal growth delay, skeletal anomalies, distinctive facial features, congenital visceral development deformities, abnormal skin texture, and from mild to moderate intellectual disability, of which the special facial features are defined by long palpebral fissures with eversion of the lateral third of the lower eyelids, short columella with a broad and depressed nasal tip and a cleft or high-arched palate.The ears are large and prominent, the teeth erupt and are abnormally arranged, the jaw is small, the posterior hairline is low, etc.Most patients with KS are scattered, with a few familial cases without regional clustering. 3The mechanism of thrombocytopenia in patients with KS is currently unclear and has been rarely reported, which may be associated with immunodeficiency.MARGOT et al. reported that nearly half of 177 KS patients had immunodeficiency characteristics, while ITP accounted for 7.3%, which was the most common complication of autoimmune diseases.At present, the pathogenesis of KS is mainly related to two gene mutations, namely, KMT2D (lysine-specific methyltransferase 2D) and KDM6A (lysine-specific demethylase 6A), which are, respectively, called KS I and KS II. 4,5The KMT2D protein (histone-3-lysine-4 methyltransferase protein) encoded by the KMT2D gene is responsible for turning on chromatin and activating homeobox and NESTIN genes during cell differentiation, while histone demethylase encoded by the KDM6A gene demethylates histone-3-lysine-27, turning off chromatin.Both genes affect gene expression by influencing the opening or closing of chromatin. 6Toshiki Mushino et al. showed that identifying pathogenic mutations in the KMT2D gene in patients with Kabuki syndrome is important in predicting potential complications of autoimmune diseases such as ITP.These two genes play a role in various signaling pathways during embryogenesis as epigenetic regulators, and these mechanisms explain the characteristic phenotypes of patients with KS.The codeshift variant detected on the KMT2D gene, in this case, was c.5775_5778del (p.Leu1926LysfsTer120), which was verified to be a germline variant associated with symptoms, was not recorded in the ClinVar, HGMD, and OMIM databases and was judged to be a new pathogenic mutation according to ACMG guidelines.
Though there have been many related studies, the pathogenesis of KS combined with ITP is not clear.In 2016, Lindsley et al. found that more than 80% of KS patients with KMT2D mutation had hypogammaglobulinemia and B-cytopenia, leading to fluid immunodeficiency and autoimmune diseases in some cases. 7Proteins encoded by KMT2D and KDM6A are involved in the epigenetic regulation of FOXP3, a key gene for the differentiation of naïve CD4 + T cells into Treg, which helps maintain peripheral immune function.Pathogenic variants in KMT2D or KDM6A may cause FOPX3 dysfunction and a decrease in T-cell function, which may be one of the reasons why patients with KS often have ITP.In our case, we detected the patient's peripheral blood lymphocyte subsets, it showed a decrease in the ratio of CD4 + /CD8 + , Treg/CD4 + , and Breg/CD19 + compared with the control in our hospital (Figure S1).
As with ITP, steroids are effective in ITP patients with KS, whether prednisone or dexamethasone can be used, but there is also steroid dependence and inevitable complications of long-term use.We chose hetrombopag olamine tablets as a maintenance agent for the treatment of thrombocytopenia in patients with KS and ITP.Hetrombopag olamine tablets are an oral small molecule synthetic nonpeptide thrombopoietin receptor (TPO-R) agonist obtained by structural modification of eltrombopag, reducing the dose of action and reducing liver toxicity. 8In 2021, the use of hetrombopag olamine tablets in China for the treatment of primary ITP in adult patients was first approved.Since then, a series of studies have shown that hetrombopag is considered to be effective, welltolerated, safe, and controllable, and no major adverse effects have been reported.
Autoreactive antibodies produced by immune dysregulation of T and B cells are a major feature of the pathogenesis of ITP, leading to defective clearance of platelets and their production.Treg cells play an important role in maintaining peripheral immune tolerance.In 2010, Bao et al. demonstrated for the first time that TPO-R agonists improve Treg cell activity, which explains the persistence of platelet responses in 7% of ITP patients treated with TPO-R agonists in clinical trials. 9ince Treg cells play an important role in controlling immune balance and immunopathology in vivo, in order to further improve the treatment effect of ITP and shorten the time of remission, the key is to restore Treg cell function and inhibit the occurrence of autoimmune dysfunction. 10After our patients used hetrombopag, the number of platelets increased significantly and stabilized at normal levels, and it can be said that hetrombopag, as a new TPO-R agonist produced in China, can also increase the proportion of Treg cells.
Alternative treatments for KS and refractory ITP can be rituximab, steroids, or eltrombopag.Our study shows that hetrombopag olamine tablets can also be one of the drug options for the treatment of patients with KS and ITP, and its advantage is that hormone-related complications are avoided and no adverse effects have been reported.Moreover, the patient was not hospitalized again for thrombocytopenia for 17 months after taking therapy hetrombopag olamine tablets.
KS is rare worldwide and often does not receive correct diagnosis and treatment due to insufficient clinical awareness.In this article, we report a new case of KS combined with ITP with a KMT2D mutation, which was relieved by treatment with hetrombopag olamine tablets.Hopefully, it will give clinicians new inspiration.However, we did not conduct in-depth research on the effects of the new mutation on immune function in patients and the mechanism of thrombocytopenia.