Coach Syndrome

A number sign (#) is used with this entry because COACH syndrome, which classically comprises cerebellar vermis hypo/aplasia, oligophrenia (mental retardation), ataxia, ocular coloboma, and hepatic fibrosis, is most commonly associated with compound heterozygous mutation in the TMEM67 gene (609884) on chromosome 8q22.

Joubert syndrome-6 (JBTS6; 610688) and Meckel syndrome type 3 (MKS3; 607361) are allelic disorders with overlapping phenotypes.

Less commonly, COACH syndrome is caused by mutation in other Joubert-associated genes, including CC2D2A (612013) and RPGRIP1L (610937).

Description

COACH syndrome is an autosomal recessive disorder characterized by mental retardation, ataxia due to cerebellar hypoplasia, and hepatic fibrosis. Other features, such as coloboma and renal cysts, may be variable. COACH syndrome is considered by some to be a subtype of Joubert syndrome (JBTS; see 213300) with congenital hepatic fibrosis. Identification of liver disease in these patients is critical because some may develop complications such as portal hypertension with fatal variceal bleeding (Brancati et al., 2009; Doherty et al., 2010).

Clinical Features

Verloes and Lambotte (1989) described 3 affected children in 2 sibships. In 1 of the families with an affected boy and girl, the parents were consanguineous. The features were early-onset ataxia with hypo/aplastic vermis, hepatic fibrocirrhosis, slender skeleton, peculiar face, and moderate mental retardation. Verloes and Lambotte (1989) concluded that the disorder is different from Joubert syndrome (see 213300), including the form associated with chorioretinal coloboma (243910), due to the lack of hepatic involvement in those disorders. In 2 sibs reported by Verloes and Lambotte (1989), Brancati et al. (2009) identified compound heterozygosity for 2 mutations in the TMEM67 gene (609884.0013-609884.0014). Additional clinical features included hypotonia, nystagmus, coloboma, and nephronophthisis with renal failure.

Wiesner et al. (1992) described adult sibs with this disorder. Postmortem in the sister, who died at age 46, showed distorted biliary ducts and multiple small medullary renal cysts. Because changes in the liver suggested biliary ductal proliferation, a trial of ursodiol was initiated in the younger brother with benefit. Wiesner et al. (1992) suggested that biliary proliferation is a manifestation of hepatic fibrosis in this disorder.

Gentile et al. (1996) reported 2 brothers of Italian descent with COACH syndrome. Clinical features included mental retardation, cerebellar ataxia with hypoplastia of the inferior vermis, visual impairment with nystagmus and oculomotor apraxia, and congenital hepatic fibrosis, with abundant bile ductules. From birth, both children showed developmental delay and hypotonia. One child required liver transplantation. Mild dysmorphic facial feature were also described, including hypertelorism, frontal bossing, anteverted nostrils, and downslanting palpebral fissures. Gentile et al. (1996) noted the phenotypic overlap with Joubert and Meckel syndromes. In the patients reported by Gentile et al. (1996), Brancati et al. (2009) identified compound heterozygosity for 2 mutations in the TMEM67 gene (609884.0016-609884.0017).

Kumar and Rankin (1996) described 2 sisters, aged 23 years and 6 years, who were found to have congenital ataxia, bilateral coloboma, of the optic nerves, mental retardation, and abnormal liver function. Magnetic resonance imaging showed cerebellar vermis hypoplasia in the younger girl and liver biopsy showed hepatic fibrosis in the older sister. In addition to previously described findings typical of COACH syndrome, the older of the patients showed progressive renal insufficiency with fibrocystic changes on renal biopsy. They suggested that COACH syndrome may have been the diagnosis in the family reported by Dieterich and Straub (1980).

Foell et al. (2002) described a child with profound cholestatic liver disease and COACH syndrome. At 2.5 months of age, the child was found to have cerebellar vermis hypoplasia and a unilateral optic nerve coloboma. Routine liver function testing at 5 months of age showed elevated liver enzymes. Liver biopsy at 16 months of age showed an early stage of cirrhosis with septal fibrosis and pseudolobules, inflammatory infiltrates, signs of cholestasis, and reduced numbers of intrahepatic bile ducts. There were no signs of kidney involvement.

Brancati et al. (2009) defined COACH syndrome as a subtype of Joubert syndrome with congenital hepatic fibrosis. They identified 14 families with this constellation of features, including 1 of the original families with COACH syndrome reported by Verloes and Lambotte (1989). Clinical features of all families included moderate to severe mental retardation and liver disease, which varied from hepatomegaly and fluctuating liver enzymes to severe fibrosis with portal hypertension and esophageal variceal bleeding. All also had cerebellar vermis hypo- or aplasia, and all with brain MRI showed the molar tooth sign. Other features included developmental delay, hypotonia, oculomotor apraxia (75%), coloboma (42%), nystagmus, ataxia, and hyperreflexia (42%). Three patients (25%) had a severe malformation of the posterior fossa, with global cerebellar hypoplasia associated with cystic dilatation of the cisterna magna communicating with the fourth ventricle. Less common features included seizures (17%), choreodystonic movements (17%), nephronophthisis (33%), and breathing abnormalities (33%). In 1 family, a second pregnancy was terminated after prenatal ultrasound showed features consistent with Meckel syndrome, including occipital encephalocele and polydactyly.

Doherty et al. (2010) reported 23 families with COACH syndrome, defined as Joubert syndrome with clinically apparent liver disease. The mean age of examination was 9 years (range 0 to 22 years). Invariant features included developmental delay, intellectual disability, hypotonia, and abnormal eye movements. Congenital hepatic fibrosis as ascertained by histology was confirmed in 18 (63%) of 26 cases. Although there were no deaths from liver disease, 4 (17%) had portal hypertension and 2 (8%) needed a liver transplant. Colobomata were present in 17 (71%) and renal disease was present in 10 (42%), including 5 with nephronophthisis and 6 with macrocystic kidney disease. Three (13%) had chronic renal insufficiency or end stage renal disease and 2 (8%) needed renal transplantation. Additional features included encephalocele (4%), abnormal respiratory control (80%), hypoplasia/agenesis of the corpus callosum (8%), ptosis (25%), and intestinal malrotation (8%). None had polydactyly or retinal dystrophy. Importantly, coloboma was not an invariant feature of COACH syndrome.

Molecular Genetics

In 8 (57%) of 14 families with COACH syndrome, defined as Joubert syndrome with congenital liver fibrosis, Brancati et al. (2009) identified compound heterozygous mutations in the TMEM67 gene (see, e.g., 609884.0013-609884.0017). The clinical variability of the disorder, relating to the extent and severity of liver and neurologic dysfunction as well as to the presence or absence of ocular and renal findings, was hypothesized to be due to genetic modifiers, similar to other ciliopathies, including Bardet-Biedl syndrome (BBS; 209900). The findings confirmed that COACH syndrome can be considered a distinct subtype of Joubert syndrome with congenital hepatic fibrosis.

Based on the clinical features of a patient with Joubert syndrome-9 (JBTS9; 612285) associated with compound heterozygous mutations in the CC2D2A gene (612013.0004 and 612013.0006), Gorden et al. (2008) postulated that COACH syndrome, in some cases, may be a variant representing a transitional phenotype between Joubert syndrome and Meckel syndrome. The patient was a 22-year-old woman with agenesis of the corpus callosum, hydrocephalus, cerebellar vermis hypoplasia, abnormal eye movements, coloboma, mild renal disease, and hepatic fibrosis requiring liver transplant at age 10 years.

Doherty et al. (2010) identified mutations in the TMEM67 gene in 19 (83%) of 23 families with COACH syndrome, defined as Joubert syndrome with liver disease. In contrast, TMEM67 mutations were only found in 2 (1%) of 209 families with Joubert syndrome without liver involvement. One patient with COACH syndrome had mutations in the CC2D2A gene (612013.0007 and 612013.0008), and 1 family had mutations in the RPGRIP1L gene (610937.0011 and 610937.0012). The findings further supported the concept that COACH syndrome is a form of Joubert syndrome with hepatic fibrosis. The proposed ciliary function for the TMEM67, CC2D2A, and RPGRIP1L genes supported a unifying underlying pathophysiology for liver disease in these disorders.