# Cyclin‐dependent kinase 13 is indispensable for normal mouse heart development

## TL;DR

This study shows that CDK13 is crucial for normal heart development in mice, and its loss leads to congenital heart defects similar to those seen in humans.

## Contribution

The study reveals the essential role of CDK13 in mouse heart development and links its dysfunction to specific congenital heart defects.

## Key findings

- Cdk13 homozygous mutants were embryonically lethal and showed 100% incidence of CHD.
- Both homozygous and heterozygous Cdk13 mutants exhibited multiple types of CHD, including ventricular septal defects and bicuspid aortic valve.
- Reduced Cdk13 expression correlated with altered expression of genes critical for heart development.

## Abstract

Congenital heart disease (CHD) has an incidence of approximately 1%. Over the last decade, sequencing studies including large cohorts of individuals with CHD have begun to unravel the genetic mechanisms underpinning CHD. This includes the identification of variants in cyclin‐dependent kinase 13 (CDK13), in individuals with syndromic CHD. CDK13 encodes a serine/threonine protein kinase. The cyclin partner of CDK13 is cyclin K; this complex is thought to be important in transcription and RNA processing. Pathogenic variants in CDK13 cause CDK13‐related disorder in humans, characterised by intellectual disability and developmental delay, recognisable facial features, feeding difficulties and structural brain defects, with 35% of individuals having CHD. To obtain a greater understanding for the role that this essential protein kinase plays in embryonic heart development, we have analysed a presumed loss of function Cdk13 transgenic mouse model (Cdk13

tm1b
). The homozygous mutants were embryonically lethal in most cases by E15.5. X‐gal staining showed Cdk13 expression localised to developing facial regions, heart and surrounding areas at E10.5, whereas at E12.5, it was more widely present. In the E15.5 heart, staining was seen throughout. RT‐qPCR showed significant reduction in Cdk13 transcript expression in homozygous compared with WT and heterozygous hearts at E10.5 and E12.5. Detailed morphological 3D analysis of embryonic and postnatal hearts was performed using high‐resolution episcopic microscopy, which affords a more detailed analysis of structures such as cardiac valve leaflets and endocardial cushions, compared with more traditional histological techniques. We show that both the homozygous and heterozygous Cdk13

tm1b
mutants exhibit a range of CHD, including ventricular septal defects, bicuspid aortic valve, double outlet right ventricle and atrioventricular septal defects. 100% (n = 4) of homozygous hearts displayed CHD. Differential expression was seen in Cdk13

tm1b
homozygous mutants for two genes known to be necessary for normal heart development. The types of defects, and the presence of CHD in heterozygous mice (17.02%, n = 8/47), are consistent with the CDK13‐related disorder phenotype in humans. This study provides important insights into the effects of reduced function of CDK13 in the mouse heart and contributes to our understanding of the mechanism behind this disorder as a cause of CHD.

Congenital heart disease (CHD) is the most common defect in live births. The role of cyclin‐dependent kinase (CDK13) in cardiogenesis and CHD was studied using a transgenic mouse model (Cdk13

tm1b
) carrying deletion of exons 3 and 4, causing loss of function. High‐resolution episcopic microscopy (HREM) allows detailed morphological analysis to be performed. A range of CHDs were found in the embryonic day E15.5 mouse heart, in both Cdk13

tm1b
heterozygotes and homozygotes. Ao, aorta; IVS; interventricular septum; LA, left atrium; LV, left ventricle; PT, pulmonary trunk; RA, right atrium; RV, right ventricle.