Heimdal Ketil, Sanchez-Guixé Monica, Aukrust Ingvild, Bollerslev Jens, Bruland Ove, Jablonski Greg Eigner, Erichsen Anne Kjersti, Gude Einar, Koht Jeanette A, Erdal Sigrid, Fiskerstrand Torunn, Haukanes Bjørn Ivar, Boman Helge, Bjørkhaug Lise, Tallaksen Chantal M E, Knappskog Per M, Johansson Stefan
Orphanet J Rare Dis. 2014 Sep 26;9:146. doi: 10.1186/s13023-014-0146-0.
A subset of hereditary cerebellar ataxias is inherited as autosomal recessive traits (ARCAs). Classification of recessive ataxias due to phenotypic differences in the cerebellum and cerebellar structures is constantly evolving due to new identified disease genes. Recently, reports have linked mutations in genes involved in ubiquitination (RNF216, OTUD4, STUB1) to ARCA with hypogonadism.
With a combination of homozygozity mapping and exome sequencing, we identified three mutations in STUB1 in two families with ARCA and cognitive impairment; a homozygous missense variant (c.194A > G, p.Asn65Ser) that segregated in three affected siblings, and a missense change (c.82G > A, p.Glu28Lys) which was inherited in trans with a nonsense mutation (c.430A > T, p.Lys144Ter) in another patient. STUB1 encodes CHIP (C-terminus of Heat shock protein 70 - Interacting Protein), a dual function protein with a role in ubiquitination as a co-chaperone with heat shock proteins, and as an E3 ligase. We show that the p.Asn65Ser substitution impairs CHIP's ability to ubiquitinate HSC70 in vitro, despite being able to self-ubiquitinate. These results are consistent with previous studies highlighting this as a critical residue for the interaction between CHIP and its co-chaperones. Furthermore, we show that the levels of CHIP are strongly reduced in vivo in patients' fibroblasts compared to controls.
These results suggest that STUB1 mutations might cause disease by impacting not only the E3 ligase function, but also its protein interaction properties and protein amount. Whether the clinical heterogeneity seen in STUB1 ARCA can be related to the location of the mutations remains to be understood, but interestingly, all siblings with the p.Asn65Ser substitution showed a marked appearance of accelerated aging not previously described in STUB1 related ARCA, none display hormonal aberrations/clinical hypogonadism while some affected family members had diabetes, alopecia, uveitis and ulcerative colitis, further refining the spectrum of STUB1 related disease.
一部分遗传性小脑共济失调以常染色体隐性遗传特征（ARCA）的形式遗传。由于新发现的疾病基因，基于小脑和小脑结构表型差异对隐性共济失调进行的分类也在不断演变。最近，有报道将参与泛素化过程的基因（RNF216、OTUD4、STUB1）中的突变与伴有性腺功能减退的ARCA联系起来。
通过纯合子定位和外显子组测序相结合的方法，我们在两个患有ARCA和认知障碍的家族中鉴定出STUB1基因的三个突变；一个纯合错义变体（c.194A＞G，p.Asn65Ser）在三个患病同胞中分离，另一个错义变化（c.82G＞A，p.Glu28Lys）与另一名患者的一个无义突变（c.430A＞T，p.Lys144Ter）呈反式遗传。STUB1编码CHIP（热休克蛋白70相互作用蛋白的C末端），这是一种具有双重功能的蛋白质，作为热休克蛋白的共伴侣参与泛素化过程，并作为E3连接酶发挥作用。我们发现，尽管p.Asn65Ser替代能够自我泛素化，但在体外会损害CHIP对HSC70进行泛素化的能力。这些结果与之前的研究一致，这些研究强调这是CHIP与其共伴侣之间相互作用的关键残基。此外，我们发现与对照组相比，患者成纤维细胞中CHIP的水平在体内显著降低。
这些结果表明，STUB1突变可能不仅通过影响E3连接酶功能，还通过影响其蛋白质相互作用特性和蛋白量来导致疾病。STUB1相关ARCA中观察到的临床异质性是否与突变位置有关仍有待了解，但有趣的是，所有携带p.Asn65Ser替代的同胞都出现了明显的加速衰老现象，这在之前的STUB1相关ARCA中未曾描述过，没有人表现出激素异常/临床性腺功能减退，而一些受影响的家庭成员患有糖尿病、脱发、葡萄膜炎和溃疡性结肠炎，进一步明确了STUB1相关疾病的范围。
