Myoclonus dystonia plus syndrome due to a novel 7q21 microdeletion^†

INTRODUCTION

Myoclonus dystonia (M-D) is a rare movement disorder characterized by a combination of myoclonic jerks and dystonia (DYT11; OMIM#159900). Clinical manifestations generally occur in the first or second decade of life, and myoclonus, predominating in the arms and axial muscles, is usually the main and most disabling feature [Roze et al., 2008]. M-D is an autosomal dominant disorder that can be caused by mutations in the epsilon-sarcoglycan (SGCE) gene, located in chromosome region 7q21 [Zimprich et al., 2001; Kinugawa et al., 2009]. Penetrance is strongly dependent on the parental origin of the disease allele, maternal imprinting being associated with lower penetrance of the phenotype upon maternal inheritance of the mutated allele [Zimprich et al., 2001]. Mutations or large deletions of the SGCE gene are detected in about 40% of patients with the typical phenotype, suggesting that the disorder is genetically heterogeneous [Tezenas du Montcel et al., 2006]. Mutations or intragenic deletions within the SGCE gene account for almost all cases of M-D syndrome due to SGCE deficiency. In the remaining SGCE-deficient patients, large chromosomal microdeletions encompassing the entire SGCE gene of paternal origin, and adjacent genes, can result in M-D associated with additional clinical features: the so-called “M-D plus syndrome” [Asmus et al., 2007; Bonnet et al., 2008]. We report on a new case of M-D due to a microdeletion in chromosome region 7q21 and discuss the need to take into account associated clinical features when designing molecular analyses.

PATIENTS AND METHODS

RESULTS

Sequencing analysis of the 11 exons encoding the major SGCE transcript (NM_003919.2) revealed no mutations. We then used QMPSF to screen for deletions and duplications and thus to ensure accurate molecular diagnosis of the M-D syndrome. We identified a partial deletion of the gene, removing exons 2–11 but respecting exon 1 (Fig. 1A), showing that the distal breakpoint is located in intron 1. Using array-CGH to determine the extent of the deletion, we showed that the proximal breakpoint was located in intron 3 of the CDK6 gene (NM_001259.5) (Fig. 1B), giving an estimated deletion size of 1.88 Mb (arr 7q21(92,241,997–94,120,153)×1 pat). The boy's father shared the same chromosomal defect (data not shown).

DISCUSSION

This patient had the typical clinical features of M-D syndrome, but also short stature, microcephaly, and mental retardation. Molecular analysis showed no mutations within the SGCE gene but identified a microdeletion encompassing the SGCE gene in the 7q21 chromosome region. Such cases of M-D with complex phenotypes may be under diagnosed, as additional features are classically exclusion criteria for the M-D syndrome [Kinugawa et al., 2009]. We therefore insist that patients with M-D as well as mental retardation, microcephaly, dysmorphism or short stature should be screened for 7q21 microdeletion. This is an important issue, as methods for detecting mutations and deletions are different. Although he shares the same genetic defect, the propositus' father has a similar but milder phenotype. Intrafamilial phenotypic variability has already been described in families with classical M-D syndrome due to SGCE deficiency [Kinugawa et al., 2009] and in other forms of dominant primary dystonia, including dystonia due to torsin A deficiency or GTP cyclohydrolase 1 deficiency [Robison et al., 1999; Opal et al., 2002]. Such intrafamilial variability suggests the influence of genetic or epigenetic modifying factors on disease progression.

Microdeletions involving the SGCE gene have been described as a cause of the M-D syndrome [DeBerardinis et al., 2003; Asmus et al., 2005, 2007; Bonnet et al., 2008; Grünewald et al., 2008; Han et al., 2008]. Two types of deletion can be distinguished: (i) partial deletions of the SGCE gene corresponding to exonic deletions and resulting in the typical M-D syndrome; and (ii) large deletions encompassing the flanking genes, resulting in complex phenotypes including M-D. At the present time, seven reports of such large 7q21 deletions involving the SGCE gene have been reported [DeBerardinis et al., 2003; Asmus et al., 2007; Bonnet et al., 2008; Grünewald et al., 2008]. It is worth noting that no recurrent breakpoint has so far been found. Therefore, these large deletions may arise either by nonhomologous recombination with homologous end-joining or by fork stalling and template switching [Lee et al., 2007].

Symptoms associated with M-D in this setting include cerebral cavernous malformations, mild bone and joint disorders, split hand/split foot malformation, hearing loss, short stature, microcephaly, mental retardation, and facial dysmorphism. Cerebral cavernous malformations are due to deletion of the KRIT1 gene, bone and joint disorders to deletion of the COL1A2 gene, split hand/split foot malformations to deletion of the SHFM1 gene, and sensorineural hearing loss to deletion of the DLX6-DLX5 genes. The genes involved in short stature, mental retardation, microcephaly, and facial dysmorphism have yet to be identified. Here, the detection of a SGCE deletion involving exons 2–11 in a patient with a complex phenotype characterized by M-D and short stature, microcephaly, and mental retardation led us to perform fine mapping of the rearrangement. We found that this deletion spanned a 1.88-Mb genomic region corresponding to one of the smallest reported 7q21 deletions. KRIT1, SHFM, and DLX6-DLX5 were not deleted. The COL1A2 gene was deleted but the patient had no manifestations of osteogenesis imperfecta, except for finger and ankle hyperlaxity and scoliosis, although the latter could have been due to long-standing axial dystonia. In addition, array-CGH studies allowed us to exclude another associated chromosomal anomaly as the cause of short stature and mental retardation.

Deletion of the 7q21 region is frequently associated with mental retardation and short stature. In view of previous observations of a Silver-Russell patient with uniparental (maternal) disomy of chromosome 7 and no mental retardation, we infer that distinct genes are responsible for short stature and mental retardation [Guettard et al., 2008]. In our patient the deleted genomic segment encompasses 19 genes, none of which have previously been implicated in mental retardation or growth retardation. By analyzing previously published cases of mental retardation associated with pure 7q21 deletion [Scherer et al., 1994; DeBerardinis et al., 2003; Fukushima et al., 2003; Wieland et al., 2004; Courtens et al., 2005; Asmus et al., 2007; Tzschach et al., 2007] and, more precisely, by comparing the 1.63-Mb deletion in Patient #1 of Asmus et al. 2007 and the 1.88-Mb deletion in our patient, we identified a critical 455-kb region (92,241–92,696K) including only six genes, namely CDK6, LOC100128994, RN7SLP4, SAMD9, SAMD9L, and LOC253012/HEPACAM2 (Fig. 2). CDK6 and LOC253012/HEPACAM2 (recently named Miki for mitotic kinetics regulator), both of which are involved in the regulation of cell proliferation, are candidate genes for mental retardation. Miki has been shown to localize to centrosomes/spindles during mitosis [Asou et al., 2009]. Interestingly, five genes encoding centrosomal proteins have been implicated in primary microcephaly, which is always associated with mild to severe mental retardation [Kumar et al., 2009]. LOC253012/HEPACAM2 could thus be a good candidate gene for mental retardation and microcephaly. Alternatively, this 455-kb region could contain a regulatory element involved in long-range control of gene expression [Kleinjan and van Heyningen, 2005]. However, another patient described by Asmus et al. 2007 (Patient #2), who had a deletion encompassing the critical region, had no reported mental retardation. The reason for this is unclear. Comprehensive analysis of pure 7q21 deletions in patients with and without short stature [Scherer et al., 1994; DeBerardinis et al., 2003; Fukushima et al., 2003; Wieland et al., 2004; Courtens et al., 2005; Asmus et al., 2007; Tzschach et al., 2007; Grünewald et al., 2008] allowed us to identify a distinct critical region of 496 kb (92,696–93,192K), which contains five genes, namely CCDC132, MIRN653, MIRN489, CALCR, and LOC392748 (Fig. 2). CALCR, encoding the calcitonin receptor, that plays a role in bone mineralization, is a possible candidate for growth retardation. Two miRNA genes, MIRN653 and MIRN489, are other possible candidates. MicroRNAs (miRNAs) are small, noncoding RNAs which bind to complementary sites on the 3′ UTR of dozens of target mRNAs to modulate their expression, most often by inhibiting protein translation. They are important in the regulation of cell proliferation and differentiation, organogenesis, and progenitor cell maintenance [Kloosterman and Plasterk, 2006]. Fine mapping of novel 7q21 deletions should help to identify new genes involved in mental retardation, microcephaly, and short stature.

ELECTRONIC DATABASE INFORMATION

Accession numbers and URLs for data presented herein are as follows:

GenBank, http://ncbi.nih.gov/GenBank/ (for SGCE cDNA, accession number NM_003919.2); Online Mendelian Inheritance in Man (OMIM), http://www.ncbi.nlm.nih.gov/Omim/.