**8. Ciliary dysfunction: Human phenotypes**

Given the multiple roles of cilia in development and physiology, it is not surprising that defects in cilia cause multiple human diseases (reviews see Afzelius, 2004 [73]; Badano et al., 2006 [74]; Fliegauf et al., 2007 [75]; Quinlan RJ et al, 2008 [2]; D'Angelo A. et al, 2009 [76])

Perhaps the most puzzling aspect of ciliopathies is that different ciliary diseases involve different, often partially overlapping sets of symptoms. For instance, Bardet-Biedl patients suffer from obesity, retinal degeneration, and cystic kidneys, whereas Oral-Facial-Digital syndrome patients suffer from polydactyly and cystic kidneys. However, both types of diseases result from defects in genes whose protein products localize to the ciliary basal body. Why don´t all ciliary defects produce the same set of symptoms? The key to this question is to realize that despite the growing wealth of genomic and proteomic data on cilia composition [77], cilia are not just lists of genes but complex organelles with variable ultrastructures that must assemble and function in different tissue contexts. Why might defects in two cilia-related genes lead to distinct diseases? There are several ways this can happen: (1) some ciliary genes may have additional cilia-unrelated gene functions; (2) some mutations may affect the ciliogenesis of only a subset of all cilia in the body; and (3) genetic defects may affect different ultrastructural modules of cilia and, thus, only influence a subset of ciliary functions [78].

#### **8.1 Cilia motility dysfunction**

	- Early embryo death.
	- Respiratory dysfunction (bronchiectasis, sinusitis).
	- Reproductive sterility.
	- Hydrocephalus

#### **8.2 Sensory cilia dysfunction (cilium)**

Given the many roles of cilia in physiology and development, it is not surprising that its defects cause multiple human diseases. Perhaps cilia´s most enigmatic aspect is that only one defect involves different diseases, often with a partially overlapping set of symptoms [2]. High ranked phenotypes by: 1º the necessary ubiquitous presence in each of the cell types of the human body, and 2º the emerging role in morphogenetic signal transduction. (TABLE 2).


134 Neuroimaging for Clinicians – Combining Research and Practice

ventral patterning is achieved through coordination of Wnt and Shh signaling.

Sets the polarization of the cells along the plane of a tissue membrane. This pathway is important for neural tube closure and cochlear extension of the inner ear. Is involved in cell

Regulates cell adhesion and motility mediated by Wnt-5a, triggers intracellular Ca2+ release to activate Ca2+-sensitive enzymes such as protein kinase C (PKC ), calmodulin-dependent

Given the multiple roles of cilia in development and physiology, it is not surprising that defects in cilia cause multiple human diseases (reviews see Afzelius, 2004 [73]; Badano et al., 2006 [74]; Fliegauf et al., 2007 [75]; Quinlan RJ et al, 2008 [2]; D'Angelo A. et al, 2009 [76]) Perhaps the most puzzling aspect of ciliopathies is that different ciliary diseases involve different, often partially overlapping sets of symptoms. For instance, Bardet-Biedl patients suffer from obesity, retinal degeneration, and cystic kidneys, whereas Oral-Facial-Digital syndrome patients suffer from polydactyly and cystic kidneys. However, both types of diseases result from defects in genes whose protein products localize to the ciliary basal body. Why don´t all ciliary defects produce the same set of symptoms? The key to this question is to realize that despite the growing wealth of genomic and proteomic data on cilia composition [77], cilia are not just lists of genes but complex organelles with variable ultrastructures that must assemble and function in different tissue contexts. Why might defects in two cilia-related genes lead to distinct diseases? There are several ways this can happen: (1) some ciliary genes may have additional cilia-unrelated gene functions; (2) some mutations may affect the ciliogenesis of only a subset of all cilia in the body; and (3) genetic defects may affect different ultrastructural modules of cilia and, thus, only influence a

**7.2 The planar cell polarity pathway (PCP)** 

polarity, tissue and cell movement processes.

**7.3 WNT/CA+2 OR "Non-canonical" pathway** 

**8. Ciliary dysfunction: Human phenotypes** 

subset of ciliary functions [78].

**8.1 Cilia motility dysfunction**  Major clinical features: Early embryo death.

> Reproductive sterility. Hydrocephalus

kinase II and Ca2+ (CaMKII) without ß-catenin pathway activation.

Respiratory dysfunction (bronchiectasis, sinusitis).

cord. Wnt signaling is active in the mouse ventral spinal cord at the time when ventral cell types are specified. Furthermore, using an approach that stabilizes beta-catenin protein in small patches of ventral spinal cord cells at different stages, Wnt signaling activates different subsets of target genes depending on the time when Wnt signaling is amplified. Moreover, disruption of Wnt signaling results in the expansion of ventrally located progenitors. Finally, Wnt signaling interacts with Hh signaling at least in part through regulating the transcription of Gli3. Yu W. et al [72] reveal a novel mechanism by which

> Retina: photo-receptors: have a primary cilium (9+0) that connects the outer segment (rhodopsin disks) to the inner segment where rhodopsin is synthesized. Molecules that detect light are synthesized in the inner segment and must be transported to the outer segment by intraciliar transport. Mutations in a gene involved in transport of these molecules needed for seeing or outer segment maintenance result in degeneration of photoreceptors leading to blindness such as in retinitis pigmentosa. Dysfunction of primary cilia due to mutations in cilia-centrosomal proteins is associated with pleiotropic disorders. The primary (or sensory) cilium of photoreceptors mediates polarized trafficking of proteins for efficient phototransduction. Retinitis pigmentosa GTPase regulator (RPGR) is a cilia-centrosomal protein mutated in >70% of X-linked RP cases and 10%-20% of simplex RP males. Accumulating evidence indicates that RPGR may facilitate the orchestration of

Ciliopathies: Primary Cilia and Signaling Pathways in Mammalian Development 137

pathways, the data support a role for the primary cilium in modulating neurogenesis, cell


INHERITANCE: RA. Genes:12 BBS genes. Locus:11q13; 16q21; 3p12-q13; 15q22.3; 2q31;

PHENOTYPE: Obesity (83%). Polydactyly, syndactyly, or both (75%). Mental Retardation (80%), spinocerebellar degeneration. Retinal degeneration: Retinitis pigmentosa (68%) with night vision problems, loss of peripheric-central vision. By the age of 20, 73% are blind. Genital hypoplasia, hypogonadism (60%). Renal cystic disease. Other: nystagmus, anosmia,

INHERITANCE: RA; GENE: ALMS1; Locus:2P13. Protein: Alström syndrome protein 1. PHENOTYPE: Obesity. Type 2 diabetes mellitus. Sensorineural hearing loss (cochlear neuronal degeneration). Photophobia and nystagmus (degeneration of photoreceptor cone

INHERITANCE: RA. Genetic heterogeneity. 3 loci: M*KS1* on 17q23, *MKS2* on 11q13, *MKS3* 

PHENOTYPE: A fatal disease, characterized by: occipital encephalocele, bilateral renal cystic

INHERITANCE: X-linked Dominant (fatal in males). GENE: OFD1; Locus: Xp22.3-p22.2.

PHENOTYPE: ORAL: Membrane between oral mucosa and alveolar bone. Partial clefts: upper lip, tongue, alveolar, palate. FACIAL: alar cartilage hypoplasia, short filter, hypertelorism with lateral location of inner edges. DIGITAL: asymmetric shortening of fingers (polydactyly / syndactyly). RENAL: renal microcysts. OTHER: Mental retardation (IQ 70), agenesis of the corpus callosum, cerebellar abnormalities and hydrocephalus,

INHERITANCE: RA. 3genes: (AHI1, NPHP1, CEP290). Incidence: 1x100.000 NB. Affects the cerebellum (vermis hypoplasia) and brainstem; appears in neonatal period with a

cells). Others: short stature, cardiomyopathy, liver and renal failure, hypogonadism.

polarity, axonal guidance and possibly adult neuronal function [97,98].

**9. Global ciliary dysfunction in pleiotropic human diseases** 

**Bardet-Biedl syndrome (BBS) or Laurence Moon-Biedl syndrome [99].** 

asthma, diabetes insipidus, cardiac malformations and situs inversus.

dysplasia, hepatic ductal dysplasia and cysts, and polydactyly.

PROTEIN: Oral-facial-digital syndrome 1 protein.

characteristic breathing pattern of tachypnea/apnea.


Retinal dystrophy, obesity, and polydactyly.

20p12; 4q27; 14q32.11; 7p14; 12q; 9q33.1; 4q27.

**Alström (Alms) Syndrome** 

**Meckel-Gruber Syndrome** 

**Orofaciodigital syndrome 1** 

**Joubert syndrome (JBTS)** 

on 8q21.13-q22.1

alopecia ...



multiple ciliary protein complexes. Disruption of these complexes due to mutations in component proteins is an underlying cause of associated photoreceptor degeneration. Here, we highlight the recent developments in understanding the mechanism of cilia-dependent photoreceptor degeneration due to mutations in RPGR and PGR-interacting proteins in severe genetic diseases, including retinitis pigmentosa, Leber congenital amaurosis (LCA), Joubert syndrome, and Senior-Loken syndrome. Additionally, we explore the physiological relevance of photoreceptor ciliary protein complexes [88, 89].


Table 2. Clinical features that may predict cilia involvement (listed by relevance) From: Badano JL, et al [74]

RENAL: Polycystic kidney disease (PKD) [90], and Nephronophthisis [91]. PKD is the most common inherited disease in the United States. Current estimates are that 600 000 patients have PKD in the US, with 12.5 million cases worldwide (for a recent, comprehensive review on PKD see [90]. The inherited PKDs include autosomal dominant type (ADPKD), autosomal recessive (ARPKD), and nephronophthisis. ADPKD, the most common form, occurs in 1 amongst 600–800 live births and affects 500 000 persons in the US. The disease occurs during adult life and is characterized by extensive cystic enlargement of both kidneys. Of the two types of ADPKD, type I is caused by a mutation in the PKD1 gene, and type II by a mutation in the PKD2 gene [92]. The proteins encoded by both genes are transmembrane proteins. Polycystin 1 is proposed to be a cell–cell and cell– matrix adhesion receptor [93] and polycystin 2 is thought to act as a calcium-permeable membrane channel [94]. Polycystic kidney is the result of altered intraciliar transport of non-motile primary cilia of the renal tubule epithelial cells [95]. In PKD, cyst formation is associated with increased numbers of cells in the circumference of renal tubules. In mice with renal-specific inactivation of Tcf2, and in the *pck*  rat, which has reduced expression of *Pkd2* and/or *Pkhd1*, mitotic alignments along the axis of the tubules are significantly distorted, indicating a loss of PCP [96].

#### **Cognitive impairment**

Patients with ciliary dysfunction disorders display variably expressive brain dysgenesis as well as neurocognitive impairments. Joubert syndrome is a ciliopathy defined by cerebellar vermis hypoplasia, oculomotor apraxia, intermittent hyperventilation, and mental retardation. Recent evidence suggests important roles for the primary cilium in mediating a host of extracellular signaling events such as morphogen, mitogen, homeostatic and polarity signals. Based upon the clinical features of ciliopathies and cilia mediated signaling pathways, the data support a role for the primary cilium in modulating neurogenesis, cell polarity, axonal guidance and possibly adult neuronal function [97,98].
