Clinical Description
Sphingosine phosphate lyase insufficiency syndrome (SPLIS) is characterized by varying combinations of steroid-resistant nephrotic syndrome, primary adrenal insufficiency (with or without mineralocorticoid deficiency), testicular insufficiency, immunodeficiency, and neurologic abnormalities, and may also include primary hypothyroidism and ichthyosis (Table 2).
To date, 46 individuals with sphingosine phosphate lyase insufficiency syndrome (SPLIS) have been reported [Atkinson et al 2017, Janecke et al 2017, Lovric et al 2017, Prasad et al 2017, Bamborschke et al 2018, Linhares et al 2018, Saygili et al 2019a, Settas et al 2019, Taylor et al 2019, Maharaj et al 2020, Zhao et al 2020]. Of note, the individual reported by Taylor et al [2019] is also included in the report by Zhao et al [2020]. The following description of the phenotypic features of SPLIS is based on these reports.
Table 2.
Features of Sphingosine Phosphate Lyase Insufficiency Syndrome
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Feature | # of Persons | Comment |
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Steroid-resistant nephrotic syndrome | 37/46 | |
Endocrine | Primary adrenal insufficiency | 31/46 | 5 who also had mineralocorticoid deficiency 1 |
Testicular insufficiency | | 8/26 w/cryptorchidism &/or micropenis |
Hypothyroidism | 6/46 | |
Immunodeficiency | 31/46 | |
Neurologic abnormalities | 22/46 | Cranial nerve deficits (11/46) Strabismus (6/46) Ptosis (2/46) Developmental delay (9/46) Regression/progressive neurologic involvement (6/46) Peripheral motor & sensory neuropathy (5/46) Spasticity
|
Sensorineural hearing loss | 8/45 | |
Ichthyosis/acanthosis | 13/46 | |
Nephrotic Syndrome
The range of renal involvement extends from nonimmune fetal hydrops at the severe end to delayed evidence of nephrosis for many years after diagnosis or no renal involvement after years of follow up, as observed in two sibs in their twenties and thirties [Atkinson et al 2017]. Typically, the nephrotic syndrome is congenital or occurs during infancy, is unresponsive to steroids, and progresses rapidly to end-stage kidney disease within one year. The oldest age of diagnosis of nephrotic syndrome among the 46 reported individuals is 18 years [Lovric et al 2017].
Six affected individuals underwent kidney transplantation: two at age five years; one at age five years and again at age 12 years; and one at age eight years. Age at transplant of the other two individuals was not provided; however, at time of last update one was age 8.4 years and the other 17.5 years.
Pathology of renal biopsies is usually consistent with glomerulosclerosis, especially with focal segmental glomerulosclerosis (FSGS) and ultrastructural finding of podocyte foot-process effacement. Three affected individuals had collapsing variant FSGS, a subclassification associated with rapid disease progression [Zhao et al 2020]. Some individuals had a pathologic diagnosis of diffuse mesangial sclerosis. Focal tubular dilatation, diffuse IgM staining, foci of calcification, lipid or hyaline droplets, perivascular sclerosis, and hypertrophic blood vessel walls have been reported in some renal biopsies.
Endocrine Involvement
Primary adrenal insufficiency may occur with or without adrenal calcifications, and may present as an Addisonian crisis requiring emergent treatment with corticosteroid and electrolyte replacement therapy. All individuals with primary adrenal insufficiency have glucocorticoid deficiency; some also have mineralocorticoid deficiency.
Most individuals with adrenal insufficiency have become symptomatic in the first decade of life. The oldest reported age of onset was 11 years [Lovric et al 2017].
Adrenal calcifications or enlargement, which may be seen prenatally, are likely a risk factor for adrenal insufficiency [Janecke et al 2017, Zhao et al 2020].
Testicular insufficiency is suspected in newborns with micropenis, cryptorchidism, or microorchidism. Hormone studies show low baseline levels of testosterone, no increase in testosterone levels in response to human chorionic gonadotropin (HCG), exaggerated gonadotropin response to luteinizing hormone-releasing hormone test in early infancy, low müllerian inhibitory factor, and low serum levels of inhibin B.
Hypothyroidism. The age of onset is unknown. Endocrine studies show low or normal T4, high TSH. Thyroxine replacement is necessary
Lymphopenia. Among individuals with SPLIS, the lower incidence of lymphopenia compared to nephrotic syndrome, adrenal insufficiency, and neurologic defects may be due to failure to recognize and report the presence of asymptomatic lymphopenia in the earliest descriptions of this disorder.
Multiple individuals with SPLIS have experienced frequent infections including several whose cause of death was related to infection [Lovric et al 2017, Bamborschke et al 2018, Saygili et al 2019b, Zhao et al 2020]. Most individuals who died of sepsis had experienced prolonged hospitalizations, complex courses, and other risk factors for infection.
To date, two individuals with SPLIS have had abnormal TREC (T-cell receptor excision circle) on newborn screening. In one, absolute lymphocyte count was low with distorted distribution of naive to memory cells and low B and NK cell counts; IgG levels were not determined; immune response to vaccinations was protective. In the other, absolute lymphocyte count was low with low absolute CD3 T cells and normal B and NK cell counts; IgG levels were low; immune response to vaccinations was not determined [Zhao et al 2020].
Neurologic Abnormalities
Cranial nerve deficits can affect cranial nerves III, IV, VI manifesting as ptosis, strabismus, esotropia, and/or amblyopia.
Cranial nerve VIII involvement manifests as sensorineural hearing loss. The loss may be congenital or diagnosed later in the first decade; it can be progressive and severe, and unilateral or bilateral (e.g., bilateral, upward sloping with air-bone gap at 500 Hz).
Developmental delay. Some children demonstrate normal development for a period of time and achieve expected milestones as indicated by Denver Developmental Screening Test, followed by impaired acquisition of new skills. For the majority of reported individuals, detailed information about developmental progression is not available.
Regression / progressive neurologic changes. Some individuals demonstrate normal development for a period of time without signs of neurologic impairment, followed by delay in gross motor, language, and social skill development, and subsequently by a loss of skills and function (gait, language, and social interaction). This regression is often associated with progressive MRI changes and can progress to generalized hypotonia, seizures, and death.
Age of onset of deterioration and type of first manifestation are variable, in some cases as young as 12 months (case 4 in Zhao et al [2020]) and as old as 25 years (ptosis [Lovric et al 2017]). Some individuals have no reported neurologic impairment.
Peripheral neuropathy manifestations can be any of the following:
Acute or subacute onset
Mononeuropathy or polyneuropathy involving upper or lower limbs, often distal
Median or ulnar paralysis
Absent reflexes
Sensory neuropathy, transient pain, loss of vibration sense
Spontaneous resolution that is complete or with residual deficits
Progression leading to muscle wasting, contractures, scoliosis, hemiparesis
In two sisters who had no other manifestations of SPLIS, the following were observed [Atkinson et al 2017]:
Nerve conduction studies showed undetectable compound muscle and sensory nerve action potentials;
EMG showed spontaneous activity and a neuropathy pattern;
Axonal neuropathy was demonstrated by axonal disintegration on sural nerve biopsy in one sib.
Seizures. Generalized and complex partial seizures may be associated with adrenal insufficiency, hypoglycemia, or progressive neurologic disease.
Microcephaly (n=4). Usually reported without details or neuroradiologic measurements of brain size. In one individual with additional brain developmental defects, occipitofrontal head circumference was recorded at 31.5 cm at age two weeks (i.e., <3rd centile) [Bamborschke et al 2018].
Other
One individual was hospitalized on numerous occasions due to gastrointestinal symptoms with no identified infectious etiology.
In some instances, affected infants were below normal weight, often in association with severe illness requiring hospitalization. Failure to thrive may be the presenting complaint, and may be associated with adrenal insufficiency, nephrotic syndrome, or poor feeding.
Rare skeletal abnormalities have been observed (craniotabes, short stature, rachitic rosary sign, scoliosis, asymmetric skull). Scoliosis may be secondary to neurologic defects.
Rarely: intestinal malrotation; pericardial effusion, dilated cardiomyopathy, dysmorphic features (hypertelorism, down-slanting palpebral fissures).
Ichthyosis/acanthosis. Ichthyosis may present at birth or later. Hyperpigmentation has been a presenting manifestation in numerous individuals usually as a consequence of primary adrenal insufficiency. Skin biopsies have shown thinned epidermis with hyperkeratosis and decreased granular layer of skin.
Abnormalities noted prenatally in 14/15 pregnancies included nonimmune hydrops, adrenal calcifications, and increased nuchal translucency. There have been several instances of intrauterine fetal demise.