Ezh1 and Ezh2 are histone methyltransferases that are required for normal skeletal growth. Mice with a complete knockout of Ezh1 and cartilage-specific knockout of Ezh2 exhibited postnatal growth retardation and impaired longitudinal bone growth. The knockout mice showed decreased proliferation and size of growth plate chondrocytes as well as premature differentiation into hypertrophic chondrocytes. This indicates that loss of Ezh1 and Ezh2 inhibits chondrocyte proliferation and causes premature differentiation, impairing skeletal growth.

1. Histone methyltransferases Ezh1 and 2 are required for normal
skeletal growth
Authors: Julian Lui, Kevin Barnes, Presley Garrison, Abigail Paulson, Vijay Shimoga, Ola
Nilsson, Jeffrey Baron
Abstract:
EZH2 (Enhancer of zeste 2) is a key member of the Polycomb1 Repressor Complex 2
(PRC2), which is responsible for methylation of histone 3 lysine 27 (H3K27), a histone
modification important for chromatin condensation and gene silencing. In humans,
heterozygous mutations in EZH2 cause Weaver syndrome (1), an overgrowth disorder
with skeletal abnormalities and advanced bone age. In addition, the EZH2 gene lies in a
locus associated with adult human height variation (2), suggesting that EZH2, and
perhaps H3K27 methylation, play important roles in skeletal and overall body growth. We
therefore sought to investigate the regulation of skeletal growth by EZH2. Because Ezh1
and 2 have partially redundant functions in the PRC2 histone methylase complex, we
generated a mouse model with a complete homozygous knockout of Ezh1 and cartilage-
specific homozygous knockout of Ezh2 (Ezh1-/-
, Ezh2fl/fl
, Col2a1-cre). The knockout mice
exhibited postnatal growth retardation that gradually becomes more prominent from
birth to weaning (body weight at 2-wks old, KO 3.70±0.61 vs WT 8.16±0.47 g, P<0.01).
Immunostaining showed that dimethyl- and trimethyl-H3K27 were absent in the cartilage
of Col2a1-cre Ezh1-/-
Ezh2fl/fl
mice, indicating loss of PRC2 function in cartilage.
Longitudinal bone growth of Col2a1-cre Ezh1-/-
Ezh2fl/fl
mice was significantly impaired by
3 days of age, with decreased tibial length (5.44±0.08 vs WT 6.00±0.11mm, P<0.01) and
decreased proliferation in the proliferative columns (5.07±0.26 vs WT 6.19±0.19 BrdU-
labeled cells/column, P<0.01). Knockout mice also exhibited decreased proliferative zone
height (291±8m vs WT 395±11m, P<0.01), decreased number of proliferative
chondrocytes per column (56±2 cells vs WT 73±3 cells, P<0.01), decreased terminal
hypertrophic cell size (15.4±0.4m vs WT 20.5±0.5m, P<0.01), but increased number of
hypertrophic cells per column (13.9±0.5 cells vs WT 11.3±0.5 cells, P<0.01). Collectively,
these data indicate that loss of histone methyltransferases Ezh1 and 2 in the growth plate
impairs longitudinal skeletal growth by inhibiting chondrocyte proliferation and causing
premature hypertrophic differentiation and therefore suggest that H3K27 methylation by
the PRC2 complex is essential for normal mammalian skeletal growth.