Upper limb muscle overgrowth with hypoplasia of the index finger: a new over-growth syndrome caused by the somatic PIK3CA mutation c.3140A>G

The term “PIK3CA – related overgrowth spectrum” (PROS) has gained popularity in recent literature to describe a group of overgrowth syndromes/disorders caused by somatic PIK3CA mutations [1]. In 2014, Al-Qattan described a new overgrowth syndrome in three unrelated Saudi patients and named it “upper limb muscle overgrowth in a proximo-distal gradient with hypoplasia of the index finger” (upper limb MO-hypoplasia IF). As the name implies, the two characteristic clinical features of the syndrome are:muscle overgrowth of one or both upper limbs which is manifested in a proximal-to-distal gradient, and congenital hypoplasia of the index finger. Other features of the syndrome are: flexion contractures of the fingers, hyper-extension deformity of the metacarpophalangeal joint of the thumb, ulnar deviation of the index finger, and a wide palm with abnormal palmar creases. In all previously reported cases, MRI confirmed that the limb overgrowth is due to enlarged/aberrant muscles with no fatty infiltration or vascular malformations. The most characteristic plain X-ray feature was widening of the space between the second and third metacarpals. No genetic analysis was done in Al-Qattan report [2].

In this paper, we describe a new case of upper limb MO-hypoplasia IF syndrome and document that it is caused by the somatic PIK3CA mutation c.3140A>G, p. His1047Arg. We also recruited one of the patients reported by Al-Qattan [2]; and confirmed the presence of the same mutation. A review of the literature was also done for similar cases. Finally, we offer a wider classification of “PIK3CA – related pathology spectrum”.

Clinically, both of our patients had all the characteristic features of theupper limb MO-hypoplasia IF syndrome.Histologically, muscle biopsies from both patientsshowed hypertrophic muscle fibers with no fatty infiltration or vascular malformations.Molecular genetic analysis of both patients showed a similar somatic mutation within PIK3CA gene at exon20 (c.3140A>G, p. His1047Arg) in the genomic DNA obtained from the muscle biopsies (Fig. 3), but not from blood leucocytes.

The upper limb MO-hypoplasia IF syndrome is not as rare as previously thought. Upon reviewing the English literature prior to Al-Qattan [2] report, several similar cases were found in the Hand Surgery literature [4‐9]. These hand surgeons commented on the muscle hypertrophy of the upper limbs as well as the aberrant muscles and the deviation of the fingers. However, when we examined the illustrations of these reports, we also found that hypoplasia of the index finger was a constant feature. We also found two similar cases reported in the Genetics literature. Castiglioni et al. [10] reported on a 6-year old girl with unilateral isolated upper limb muscle overgrowth. The authors only commented on the ulnar drift of the index finger; although the illustrations also showed hypoplasia of the index finger. An open biopsy of the interosseous muscle was done and revealed the same somatic mutation in PIK3CA (p.3140A>G, p. His1047Arg). Rasmussen et al. [11] reported on a 20-year old female with bilateral upper limb muscle hypertrophy and concurrent upper back lipoma. The authors only commented on the overlapping of the index fingers over the middle fingers; although the illustrations also showed hypoplasia of both index fingers. A muscle biopsy also revealed the same somatic mutation in PIK3CA. Our review clearly confirms that the “upper limb MO-hypoplasia IF” syndrome is a unique entity that should be added to the PROS. The fact that all cases that underwent genetic analysis showed the exact same somatic mutation in PIK3CA is interesting. However, this does not mean that a genotype-phenotype correlation exists; since 70% of patients with PROS have one of three somatic mutations in PIK3CA: p. Glu542Lys, p. Glu545Lys, and p. His1047Arg [12]. These three hotspot mutations also make about 70% of PIK3CA somatic mutations associated with cancer [13].

We believe that a wider classification of “PIK3CA-related pathology spectrum” (PRPS) should be offered (Table 1) and should include somatic and germline mutations. This pathology spectrum should include cancer, benign skin lesions/ tumors, Cowden syndrome, isolated vascular malformations as well as the PROS [1, 12‐25].The PROS should also be extended to include the “upper limb MO-hypoplasia IF” as well as the Klipple-Trenaunay syndrome as described by Yeung et al. [12]. We also recommend dividing the PROS into three sub-groups: PROS with brain involvement, PROS with multiple lipomatosis, and PROS without brain involvement / multiple lipomatosis. Our PRPS is summarized (Table 1). Note that our classification is based on pathological entities (which is more clinically oriented) and not based on genotype-phenotype correlations proposed in Mirzaaet al. [26].

The PIK3CA gene encodes the p110 α catalytic sub-unit of PI3K (phosphatidylinositol 3-kinase). All somatic and germline mutations of the PRPS are activating (gain-of-function) mutations of the PI3K pathway leading to enhanced cellular proliferation. The PI3K pathway (Fig. 4) is initiated by growth factor/cytokine stimulation of a receptor tyrosine kinase in the plasma membrane [27]. This activates PI3K (which is heterodimer composed of a regulatory sub- unit known as p85α and a catalytic sub-unit known as p110α). The activated PI3K will then add a phosphate group to PIP2 to become PIP3 (phosphatidyl inositol bis-phosphate to phosphatidyl inositol tris-phosphate) at the plasma membrane lipids. The PIP3 will then recruit Akt1 (protein kinase B) to the plasma membrane, allowing PDK1 (phosphoinositide- dependent kinase 1) to phosphorylate and activate Akt1 at its activation loop site (Thr 308). Further activation of Akt1 requires its phosphorylation at another site (Ser 473) by mTORC2 (mechanistic Target of Rapamycin Complex-2). The fully activated Akt1 will then induce several cellular processes including stimulation of cell proliferation and suppression of apoptosis. The PI3K pathway is negatively regulated by PTEN (Phosphate and tensin homolog) and this is done by converting PIP3 into PIP2. Hence gain-of-function of PIK3CA, AKT1, and MTOR; or loss-of-function of PTEN will result in cancer, benign tumors, and overgrowth syndromes. For example, proteus syndrome (sporadic, mosaic, asymmetric and progressive overgrowth of body parts with cerebriform connective tissue nevi being characteristic of the syndrome) may be caused by gain-of-function somatic mutations of AKT-1 [28], or loss-of-function germline mutations of PTEN [29].Similarly, Cowden and Cowden-like syndromes are caused by loss-of-function germline PTEN mutations as well as gain-of-function germline mutations of PIK3CA and AKT1 [16]. Finally, megalencephalysyndromes maybe caused by gain-of-function somatic mutations of PIK3CA (Table 1) or gain-of-function germline mutations of MTOR [30].

In conclusion, we describe a unique form of overgrowth syndrome caused by the PIK3CA somatic mutation: p. His1047Arg. We also review similar cases in the literature and offer a wider classification of PIK3CA – related pathology spectrum.