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Smith-Magenis syndrome (SMS, MIM 182290) is a multiple congenital anomalies and mental retardation syndrome associated with an interstitial deletion within chromosome 17p11.2. More than 100 patients with SMS have been described. 1 Clinical characteristics include minor craniofacial and skeletal anomalies such as brachycephaly, frontal bossing, synophrys, midfacial hypoplasia, short stature and brachydactyly, neurobehavioral abnormalities such as aggressive and self-injurious behavior and sleep disturbances, ophthalmic, otolaryngological, cardiac, and renal anomalies. 1,2 In the majority of patients, a common deletion size is observed. 3–5 This common deletion is derived from nonallelic homologous recombination (NAHR) between low-copy repeat (LCR) gene clusters (distal and proximal SMS-REPs) during either maternal or paternal gametogenesis. 5,6 Approximately 20% to 25% of SMS patients reported have either smaller or larger sized deletions. 7–9 In the patients with unusual sized (smaller or larger) deletions, the precise mechanism of deletion remains to be determined; however, other LCRs predominate at breakpoints. 10 Three additional SMS patients without a microdeletion have been reported recently, each harbors a mutation in RAI1, which maps within the SMS critical region. 11

Fifty-eight persons with SMS were evaluated through an International Review Board (IRB) approved multidisciplinary clinical protocol in the General Clinical Research Center (GCRC) at the Texas Children's Hospital in Houston. Clinical finding on a subset of these patients (N = 27) were reported previously, but without molecular studies to define the extent of the deletion. 2 For this study, 51 of the 58 patients underwent molecular analysis to determine deletion size. Thirty-nine of 51 persons (76%) analyzed by PFGE and/or FISH were found to have the common SMS deletion. Although there are common physical features among these patients, the variability between individual patients with molecularly identical sized deletions is marked. Multiple genes have been identified within the SMS common deletion interval. 8 Recently, mutations in RAI1 have been identified in three patients with many SMS characteristics, yet these individuals are not affected with cardiac or renal abnormalities, short stature, midfacial hypoplasia, or brachydactyly. 11 Only MYO15 has been clearly associated with a specific phenotypic feature (sensorineural hearing impairment) in a patient with SMS (1123) who harbored a recessive mutation on the nondeleted allele. 12

We examined the variability of phenotype of SMS patients with the common deletion and compared that observed for patients with alternatively sized deletions. Remarkably, despite identical deletion in 76% of patients, substantial clinical variability can be observed.

METHODS

Patient ascertainment and evaluation

From December 1990 to September 1999, 58 persons with SMS [del(17)(p11.2p11.2)], (43% male; mean age 9 years; age range 1 year 6 months to 31 years) were enrolled in the multidisciplinary clinical study of SMS through the GCRC at the Texas Children's Hospital in Houston, under a protocol approved by the Baylor College of Medicine IRB. All 58 patients were ascertained by abnormal chromosome analysis with del(17)(p11.2p11.2). Informed consent was obtained from the patient's parent or legal guardian. Clinical evaluations performed in these patients include: physical examination, developmental and cognitive profiles, ophthalmological and otolaryngological examinations, audiological assessment, 12-hour or 24-hour polysomnography, echocardiogram, renal ultrasound, scoliosis survey, radiographs of forearms and hands, lipid profile, and thyroid function studies.

Cytogenetic analysis

Each patient had an interstitial deletion in 17p11.2 as evidenced by G-banded chromosome analysis. The presence of a deletion was confirmed by FISH analysis using probes specific for FLII13 and ZFP179 (ZNF179), 14 both mapping within the SMS common deletion region, with the peripheral myelin protein 22 gene, PMP22, mapping within the commonly duplicated CMT1A region, 15 as a control. All patients were deleted for FISH probe FLII. ZFP179 was deleted in all patients with common deletions, two patients with smaller deletions (1195 and 1354), and three patients with larger deletions (147, 536, and 1153). Large deletion patients (200 and 484) were deleted for all three probes and are likely affected with hereditary neuropathy with liability to pressure palsies (HNPP) due to the deletion of PMP22. 16 The patient with the complex rearrangement (1221) was reported previously. 17

PFGE analysis

PFGE was performed on the patient samples as described 5,18 to identify whether the deletions represented the LCR-mediated common rearrangements. A deletion is considered common if the breakpoints map within the proximal and distal SMS-REPs and is distinguished by a unique de novo ≈1.1-Mb band, corresponding to the SMS rearrangement-specific common junction fragment. The molecular extent of the deletion was not determined in 7 of the 58 patients as cell lines were not available for culture.

Genotyping

Parental origin of the deleted chromosome was determined in 30 of the 58 cases by microsatellite marker analysis using genomic DNA purified from peripheral blood as described. 6 The remaining patient samples were either not analyzed for parental origin or were uninformative. The parent of origin was identified by lack of transmission of an allele to the patient for two or more of loci D17S805, D17S2256, D17S2257, D17S2259, and D17S52258.

RESULTS

A recombination-specific junction fragment of ≈1.1 Mb was identified in 39 of 51 patients (≈76%), suggesting the common SMS deletion (Fig. 1). The ≈1.1-Mb junction fragment was not evident in 12 patients, six of whom (12%) have smaller deletions (540, 641, 1190, 1195, 1354, and 1456) and five of whom (10%) have larger deletions (147, 200, 484, 536, and 1153), as defined by FISH analyses. 10 One patient (1221) has a complex chromosomal rearrangement leading to deletion of 17p11.2. 17 The deletion was of paternal origin in 17/30 (56.6%) cases and maternal in 13/30 (43.3%).

Fig. 1
figure 1

The common 17p11.2 deletion in SMS. A, Map of one normal chromosome 17p11.2 depicting the SMS-REPs, placement of the FISH probes (small red and green rectangles), and Not I restriction digest sites (vertical lines) and expected fragment sizes on PFGE is shown at the top of the figure. The common SMS deletion results from NAHR between the distal and proximal SMS-REPs. After digestion by Not I and PFGE, a ≈1.1 Mb junction fragment is seen in a patient with SMS, but not his parents (left). Likewise, in SMS patients with the common deletion, a specific pattern is evidenced by FISH analysis 10 using unique probes flanking the distal and proximal SMS-REPs (right). B, PFGE analysis of 25 representative patients with the ≈1.1 Mb junction fragment. This junction fragment is not detected in parents (data not shown), or in patients with smaller (641, 1190, 1354, and 1456) or larger (266, 536, and 1153) sized deletions, nor the patient with the complex rearrangement (1221).

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The patients exhibited the characteristic physical features of SMS including one or more of the following: brachycephaly, frontal bossing, synophrys, midfacial hypoplasia, down-turned mouth, and brachydactyly. A history of developmental delay and/or cognitive impairment was ascertained, and all patients had cognitive impairment and low adaptive functioning by objective psychometric analysis. 19 The majority of patients had full-scale IQ scores in the moderate to mild range of mental retardation. 2,19 Low adaptive functioning (by the Vineland Adaptive Scales) was seen in all patients, regardless of IQ score. 19 All patients showed sleep disturbances when assessed by objective criteria using the Epworth Sleepiness Scale, total sleep time and sleep stage distribution (by polysomnography), and the Multiple Sleep Latency Test. 20 Variable features in SMS patients with del(17)(p11.2p11.2) included otolaryngological and ophthalmological abnormalities, hearing impairment, short stature, scoliosis, electroencephalogram (EEG) abnormalities, cardiac, and renal or urinary tract anomalies. Abnormalities in lipid profiles and metacarpal phalangeal profile are also variable but have been previously reported in this cohort. 21,22

Table 1 summarizes the variable clinical findings in this cohort of 58 patients. Tables 2 and 3 summarize the clinical features in 51 patients in whom deletionsize was determined, and stratifies these findings based on the molecular size of the 17p11.2 deletion. Patients in this study are considered to have otolaryngological abnormalities if findings were evident on physical examination (by an otolaryngologist) at the time of the GCRC admission. Thus, a past medical history of otitis media does not qualify, but findings of tympanic scarring due to chronic otitis media, or acute otitis media do. Findings in this category include the following: cleft palate or submucous cleft palate; velopharyngeal incompetence; vocal cord polyps, nodules, paralysis, thickening, or edema; tympanic membrane perforation, scaring or active otitis media; sinusitis; cholesteatoma; laryngomalacia; and adenoid hypertrophy. Ophthalmic abnormalities are similar to those previously reported 23 and include myopia, myopic astigmatism, hyperopia, microcornea, iris dysplasia, coloboma, posterior embryotoxon, esotropia, and strabismus.

Table 1 Phenotypic features in patients with del(17)(p11.2p11.2)
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Table 2 Phenotypic comparison in patients with variable sized deletions
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Table 3 Clinical variance in patients with the common SMS deletion
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Hearing impairment in SMS may be conductive, sensorineural, or mixed. 2 Fifty-seven patients underwent formal audiological evaluation. Of the patients with the common deletion, 28% had normal hearing sensitivity, 46% had conductive impairment, 13% sensorineural, and 13% mixed. Of the patients harboring a small deletion, 2 had normal sensitivity, 1 had conductive impairment, 1 sensorineural, and 2 mixed. Two patients with larger deletions had conductive impairment, 1 had mixed, and 2 had normal hearing sensitivity.

An EEG was performed as part of the polysomnography (sleep study). Seizures occur in 10% to 20% of SMS patients, 1 although a higher percentage of patients were recognized to have EEG abnormalities in the absence of clinically evident seizures. 2 In this study, a clinical history of seizures was associated with either normal or abnormal EEGs. Only patients with abnormal EEGs are noted in the tables. Abnormal findings include focal spike discharge activity or generalized activity, including bursts of generalized spike and wave activity. These findings are considered epileptogenic, although were not associated with clinically evident seizures. Of the 6 patients studied with smaller deletions (not all with the same breakpoints), 5 have abnormal EEGs, yet only 3 have a clinical history of seizures.

The presence of cardiac anomalies was determined by 2D echocardiography performed on 44 of the 58 patients. Valvular abnormalities (e.g., pulmonary regurgitation and tricuspid regurgitation) with or without associated ventricular changes (e.g., right ventricular enlargement) were more common than structural defects such as patent foramen ovale, cleft mitral valve, atrial or ventricular septal defects, or (in one case) tetralogy of Fallot. These structural defects were found in only 21% of patients (9/44), and seen in patients with common, small, large, and undetermined sized deletions.

Renal ultrasound was performed on 57 patients. Genitourinary abnormalities include the following: duplication of ureters; ureteropelvic junction obstruction; renal pelvic ectasia; ectopic kidney; abnormally small kidneys, and unilateral renal agenesis. Of the 39 patients with common deletion, 6 had an abnormality including 3 patients with duplication of the ureters, 2 with abnormally small kidneys, and 1 with ureteropelvic junction obstruction.

DISCUSSION

Smith-Magenis syndrome (SMS) is a multiple congenital anomalies, mental retardation syndrome associated with a heterozygous deletion of chromosome 17p11.2. Characteristic physical features include minor craniofacial 24 and skeletal anomalies, 22 ophthalmological 23 and otolaryngological abnormalities, and malformations of the heart, including tetralogy of Fallot, 25,26 and kidney. 1,2 The neurobehavioral features of SMS are perhaps the most distinctive characteristic of this microdeletion syndrome and include self-injurious, aggressive, and maladaptive behavior 3,19,27,28 and significant sleep disturbances. 3,20,29

We have previously described the clinical phenotype of SMS in 27 patients with del(17)(p11.2p11.2), 2 and herein have extensively characterized the clinical phenotype in another 31 SMS patients. Whereas common phenotypic features can be defined, a wide range of clinical variability exists among patients. Interestingly, although the majority (76%) of the evaluated SMS patients have the same-sized deletion by molecular analysis, the clinical spectrum is variable even among these patients. Furthermore, as previously described in SMS patients with different sized deletions, there is no correlation between the size of the deletion and the major clinical features of SMS. 7 With the exception of abnormalities found on EEG and renal ultrasound, the patients with the smaller-sized deletions have features similar to those seen in the common deletion patients. Although the presence of EEG abnormalities does not seem to correlate with other clinical features, there does seem to be a correlation with deletion size in that the patients with the smaller deletions are more likely to have an abnormal EEG; however, the difference was not statistically significant (Fisher's Exact, P = 0.18). The explanation for this observation is elusive, because genes implicated in epilepsy do not map to the breakpoints of these deletions; however, this finding may be reflective of a position effect, 30 or could merely be due to problems inherent in analyzing data from a small sample size. Our patients with smaller-sized deletions were more often affected with genitourinary anomalies than patients with the common or large deletion; however the difference was not statistically significant (Fisher's Exact, P = 0.06).

SMS has been considered to be a contiguous gene deletion syndrome (CGDS), and it had been postulated that the phenotypic variation among SMS patients could be due to variations in the size of the deletion, although a common deletion region was previously assigned by polymorphic marker analysis. 3,4 Recently, three patients with physical, developmental, and behavioral features of SMS were reported to have mutations of RAI1. 11 Although these patients have ocular and otolaryngological abnormalities, and one is affected with seizures, they have no malformations of the cardiovascular or genitourinary systems, nor are they affected with short stature, midfacial hypoplasia, or brachydactyly. 11 With the recent data establishing RAI1 mutations in persons with characteristics of SMS, it is plausible that the major features of this syndrome are due to RAI1 haploinsufficiency. This phenomenon has been observed in other multiple congenital anomaly syndromes such as Rubinstein-Taybi (RTS; MIM 180849) and Alagille (AGS; MIM 118450) syndromes resulting from haploinsufficiency of a gene encoding a transcription factor (CBP and JAG1, respectively). Although the vast majority of SMS patients reported harbor an interstitial deletion of 17p11.2, the proportion of patients with features of SMS and a heterozygous RAI1 mutation remains to be determined, and indeed, the full phenotypic spectrum of SMS may be evident only in persons harboring the chromosomal deletion.

Clinical evaluation and management of newly diagnosed SMS patients may eventually depend on the results of cytogenetic and molecular analyses; however, given the data presented herein, and until more data are collected on patients with RAI1 mutations, current management guidelines (www.geneclinics.org) should be followed for assessment.