Floating–Harbor syndrome

Lore & Background

The first identified instances occurred in 1973 at the Boston Floating Hospital, and in 1975 at Harbor General Hospital in Torrance, California. The name Floating–Harbor syndrome was coined by Robinson et al. in 1988. Since then approximately 40 more cases have been described. The first case recorded was that of a five-year-old boy. In a 32-year follow up done in 2006, the patient was in good overall health, had never been hospitalized, and had been employed for the past 15 years. His mother stated he had a very good memory, was gregarious, had a temper, and at times was stubborn. Changes in the patient's facial configuration and body could be attributed to age and familial history (i.e., the patient shows signs of arthritis and hypertension). Still present were the low hairline, broad tip of the nose, short nasal labial distance, depressed columella, thin lips, and posteriorly positioned ears, as well as short stature and mild to moderate retardation.

Reader's Guide

Floating–Harbor syndrome is significant as a rare genetic disorder that illustrates the complexity of linking genotype to phenotype. The identification of heterozygous truncating mutations in SRCAP in 2012 provided a molecular basis for diagnosis, though some individuals with classic features carry no mutation, indicating that molecular diagnosis is not always successful. The syndrome shares clinical overlap with Rubinstein–Taybi syndrome due to the interaction of SRCAP with CBP, but distinct features such as absence of broad thumbs and microcephaly help differentiate them. Management focuses on monitoring growth, bone age, hearing, vision, dental health, and behavioral issues, with no cure available. The long-term follow-up of the first patient shows that individuals can lead productive lives despite mild to moderate intellectual disability and persistent facial features. The syndrome's variable expression and uncertain inheritance pattern underscore the need for further research and genetic counseling for families.

Did You Know?

• Fewer than 50 cases have been described in the literature.
• The first case was a five-year-old boy; a 32-year follow-up found him employed and in good health.
• Mutations in SRCAP are truncating and occur between codons 2407 and 2517 in exon 34.
• Some individuals with classic FHS features carry no SRCAP mutation.

Clinical Portrait: The Defining Triad

Floating–Harbor syndrome, alternatively called Pelletier–Leisti syndrome, is an exceedingly rare condition with fewer than fifty documented cases in the medical literature. It typically becomes apparent in early childhood and is defined by a recognizable triad: proportionate short stature accompanied by delayed bone age, a distinctive facial gestalt, and delayed speech development. The facial features are the most immediately striking hallmark. Affected individuals commonly present with a triangular face shape, deeply set eyes, a short philtrum, a wide mouth framed by a thin upper lip border, a long nose with a narrow bridge but broad and bulbous base, and low-set ears. Beyond the face, skeletal findings include brachydactyly, broad fingertips or clubbing, clinodactyly, short thumbs, prominent joints, and clavicle irregularities. Bone age is typically delayed by two standard deviations or more, though it tends to normalize between ages six and twelve. Adult height remains limited, with females capping around the twentieth percentile and males around the twenty-fifth. Dental anomalies such as caries, malocclusion, and small dysplastic teeth, along with visual impairment, round out the physical picture.

Unraveling the Genetic Basis

For years the etiology of Floating–Harbor syndrome remained a mystery, though familial clustering in a few reported pairs—a mother and son, a mother and daughter—hinted at an inherited genetic component. The breakthrough came in 2012 when a team published in the American Journal of Human Genetics used exome sequencing on a group of unrelated individuals displaying classic FHS features. They identified heterozygous truncating mutations, either nonsense or frameshift, clustered between codons 2,407 and 2,517 in exon 34 of the SRCAP gene. These mutations eliminate three C-terminal AT-hook DNA-binding motifs, crippling the protein's transactivation capacity. SRCAP encodes an SNF2-related chromatin-remodeling ATPase that serves as a coactivator for CREB-binding protein, the very gene whose dysfunction underlies Rubinstein–Taybi syndrome. This disrupted interaction plausibly accounts for the clinical overlap between the two conditions. The proposed mechanism is dominant-negative: the mutant protein interferes with the wild-type allele's function. SRCAP also transduces signals from nuclear steroid hormone receptors and Notch pathways, underscoring its broad role in gene regulation. Notably, three individuals whose phenotype most closely mimicked FHS carried no SRCAP mutation, reminding researchers that molecular testing, while powerful, is not infallible.

Speech, Cognition, and Behavioral Landscape

The developmental and behavioral footprint of Floating–Harbor syndrome extends well beyond the physical. Speech and language are profoundly affected: affected children commonly exhibit dysarthria, verbal dyspraxia with imprecise phoneme production, hypernasality, and a high-pitched voice quality. Receptive and expressive language impairment is severe and spans all functional domains. Intellectual disability is universal among documented cases, though its severity ranges from borderline-normal functioning to moderate impairment, and learning difficulties accompany every individual. Behaviorally, infancy is often marked by intense tantrums, while school-age children frequently display attention deficit-hyperactivity disorder with impulsivity, inattention, and restlessness. Unpredictable aggressive outbursts have been reported. In the small case series, one individual met criteria for autistic spectrum disorder, another for Asperger syndrome, and two for obsessive-compulsive disorder. Some girls experience early entry into puberty, which can lead to menorrhagia and irregular menstrual cycles. The heterogeneity of these manifestations means that no two patients present identically, and the full spectrum of symptoms may shift as the individual matures.

Diagnostic Pathways and the Rubinstein–Taybi Overlap

Historically, clinicians diagnosed Floating–Harbor syndrome purely on clinical observation, typically in early childhood, matching the patient's features against the published description. Today, molecular genetic testing has become a critical adjunct: sequence analysis targeting exon 34 of the SRCAP gene can detect the characteristic truncating mutations, which have been confirmed in nineteen patients to date. In most cases where classic facial features are present, the molecular test corroborates the clinical impression. A major diagnostic challenge is distinguishing FHS from Rubinstein–Taybi syndrome, given the shared SRCAP-CBP pathway involvement. However, several features help separate them: broad thumbs, narrow palate, and microcephaly are hallmarks of Rubinstein–Taybi but are characteristically absent in FHS, with the rare exception of one UK child diagnosed with both microcephaly and FHS. Inheritance patterns remain uncertain; the two familial pairs suggest possible autosomal dominant transmission with variable expressivity, while another report proposed autosomal recessive inheritance. Because no cure exists, management centers on close growth monitoring in the earliest years, annual general health screenings, and ongoing support for the individual and their caregivers.