Genetic screening confirms heterozygous mutations in ACAN as a major cause of idiopathic short stature

Abstract

Short stature is a common pediatric disorder affecting 3% of the population. However, the clinical variability and genetic heterogeneity prevents the identification of the underlying cause in about 80% of the patients. Recently, heterozygous mutations in the ACAN gene coding for the proteoglycan aggrecan, a main component of the cartilage matrix, were associated with idiopathic short stature. To ascertain the prevalence of ACAN mutations and broaden the phenotypic spectrum in patients with idiopathic short stature we performed sequence analyses in 428 families. We identified heterozygous nonsense mutations in four and potentially disease-causing missense variants in two families (1.4%). These patients presented with a mean of -3.2 SDS and some suggestive clinical characteristics. The results suggest heterozygous mutations in ACAN as a common cause of isolated as well as inherited idiopathic short stature.

Figure 1

Mutations and protein structure of aggrecan. (a) Pedigrees of the six families with identified mutations in the ACAN gene. Both parents in families 2 und 3 presented with a height below/around −2 SDS. In both families the mother carrying the mutation was considered affected based on the phenotypic and radiographic evaluation (Table 1). (b) Model of the N-terminal Ig-domain of ACAN highlighting the site of the p.(Cys51Gly) mutation. In the wildtype (top model), cysteine at position 51 (C51) forms a disulfide bond with cysteine at position 133 (C133) that stabilizes the domain (cysteines are shown in space-filled presentation; the sulfur atoms of both cysteines that form the disulfide bond are shown in yellow). In the mutant variant (bottom model) this disulfide bond cannot be formed, which is expected to reduce domain stability of even cause unfolding of the entire domain. (c) Model of the third Link domain of ACAN highlighting the site of the p.(Asp568Asn) mutation. In the wildtype (top model), aspartate at position 568 (D568) forms two polar interactions (dotted lines) with the histidine at position 481 (H481) and the tyrosine at position 489 (Y489). In the mutant variant (bottom model) only one of these interactions can be formed, which is expected to reduce domain stability. (d) Aggrecan is a proteoglycan consisting of different functional domains (modified from Gkourogianni et al., 2016). The G1 domain (blue) consists of an immunglobulin-like repeat (oval) and two proteoglycan tandem repeats (orbital). The inter-globular domain separates it from the G2 domain (green) which consists of two proteoglycan tandem repeats (orbital). It is adjacent to the glycosaminoglycan attachment region (GAG) which carries keratan sulfate (dark blue) and chondroitin sulfate chains (orange). The C-terminal domain is the globular G3-domain which consists of two EGF-repeats (violet), a C-type lectin domain (grey) and a complement regulatory protein repeat (turquoise). In the upper part the localization of the identified novel heterozygous mutations (P1-6) is shown (green: missense mutation, red: nonsense mutation). In the lower part, all previously reported mutations are shown (see Supplementary Table 1) (green: heterozygous missense mutation, red: heterozygous nonsense mutation, blue: homozygous missense mutation).

Figure 2

Clinical and radiographic characteristics of two patients with heterozygous ACAN mutations. (a–e) Patient P2 was born at term with a weight of 3270 g and a body length of 48 cm. At the age of 10 years she had a height of 119.6 cm (−3.6 SDS), weight of 25.2 kg (BMI 17) and head circumference of 50 cm (−1.6 SDS) (f–j) Patient P3 was born at term with a weight of 3550 g and a body length of 50 cm. At the age of 11 years she presented with a height of 121 cm (−3.9 SDS), weight of 28.4 kg (BMI 19) and head circumference of 50 cm (−1.7 SDS). Psychomotor development was normal in both patients. Both presented with proportionate short stature, broad chest, short neck, brachydactyly with pronounced brachymetacarpaly V, broad thumb and great toe and an inhibition of extension of the elbow. (a,b,f,g) Facial anomalies include a broad nasal tip and a high forehead. (j) Radiographic evaluation revealed signs of Osteochondritis dissecans (arrows) in patient P3, her mother and maternal grandmother.